Polytunnels, Greenhouses and Protective Cropping E-Book

Thady Barrett

• Herausgeber: Crowood
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 378

Veröffentlichungsjahr: 2016

Beschreibung

This comprehensive book, written by an acknowledged expert, is packed with useful information and is an invaluable reference work that covers all aspects of protected horticulture. It discusses the appropriate siting for a greenhouse enterprise, and covers greenhouse design principles and commercial glasshouses. It also considers cladding materials, the development and use of polythene-clad tunnel structures, and greenhouse energy sources. The greenhouse environment, growing rooms, irrigation, composts and other growing media are examined as well as plant nutrients, fertilizers, pest and disease control, nursery hygiene and much more. This is essential reading for keen amateur gardeners with an interest in growing plants under glass, and an invaluable reference work for undergraduate and post-graduate horticultural students, consultants, commercial horticultural growers and for all those involved in the protected horticultural sector. Fully illustrated with 86 colour photographs, graphs and drawings.

Leseprobe

Polytunnels, Greenhousesand Protective Cropping

A GUIDE TO GROWING TECHNIQUES

Thady Barrett

THE CROWOOD PRESS

First published in 2016 by

The Crowood Press Ltd

Ramsbury, Marlborough

Wiltshire SN8 2HR

www.crowood.com

This e-book first published in 2016

© Thady Barrett 2016

All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publishers.

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library.

ISBN 978 1 78500 186 4

Disclaimer

The author and the publisher do not accept any responsibility in any manner whatsoever for any error or omission, or any loss, damage, injury, adverse outcome, or liability of any kind incurred as a result of the use of any of the information contained in this book, or reliance upon it. If in doubt about any aspect of protected horticultural cultivation, growing and cropping, readers are advised to seek professional advice.

Dedication

To George Allen, OBE, director for the Lea Valley Experimental Horticultural Station 1963–1981, who provided me with the opportunity to witness the development of alternative clad greenhouses for wider use in horticulture. And to the grower/managers who directed my practical experience and education in such a wide range of greenhouse crops (1980 and 1982): Wolfgang Kruger, David Kemp and John Hopkins.

Contents

Preface

Acknowledgements

1 – Greenhouse Design Principles

Identifying Loads Placed on Structures

General Overview

Height of Structure

Light Interception

Reference Link

2 – Commercial Glasshouses

A-Frame Glasshouses

The Venlo Greenhouse

The Cabrio Greenhouse

Widespan Greenhouse

Newer Structures

Reference Links

3 – Cladding Materials: Overview

Manufacturer Claims!

Horticultural Glass Options

Alternatives to Cladding with Glass

Reference Links

4 – The Development and Use of Polythene-Clad Tunnel Structures

Giant Cloches

Spanish Tunnels

Single-Span Polytunnels

Multispan Polytunnels

Keder Greenhouses using Polydress

Retractable Roof Structures

Airstream Greenhouse

Current Examples of Polythene Films

Summary

Reference Links

5 – Greenhouse Energy Sources

Alternative Fuel Energy Sources

Reducing Heat Losses

Reference Links

6 – The Greenhouse Environment

Artificial Greenhouse Lighting

CO2 Enrichment

Heat Requirements and Distribution

Greenhouse Ventilation

Greenhouse Cooling

Humidity Control and Air Mixing

Screen Technology and Materials

Monitoring Equipment for the Greenhouse Aerial Environment

Reference Links

7 – Growing Rooms

Ultimate Control of the Growing Environment

Germination in Growing Cabinets

The Future of Growing Rooms

Reference Link

8 – Irrigation

Water Sources

Filtration and Water Treatment

Water Storage and Distribution

How Much Water Do Plants Need

Reference Links

9 – Composts and other Growing Media

Setting the Criteria for Grown Media Mixes

Growing Media

Alternative Growing Media

Grower Mixes

Reference Links

10 – Hydroponics

Typical Hydroponic Cleaning Operations

Delta-T Electronic Devices

Computer Programs and Devices for Automatic Control of Watering

Hydroponic Systems for Leafy Vegetables and Other Crops

Future Developments

Aquaponics

Reference Links

11 – Fertilizers for Greenhouse Crops

Plant Nutrients

Fertilizer Labelling

Liquid Feeding

Controlled Release Fertilizers

Organic Fertilizers

Reference Links

12 – Pest and Disease Control

Integrated Pest Management (IPM)

Biological Pest Control

Preventative Measures to Reduce Pest and Disease Pressures

The Use of Pesticides on Horticultural Crops

Biopesticides

Soil Management and the Control of Pests and Diseases

Reference Links

13 – Nursery Hygiene

Prevention

Conditions that Lead to Increased Pest and Disease Levels within Crops

Products that can Assist with Providing Sanitary Conditions

Biofilm Formation and its Management

Further Reading List

Index

Preface

As soon as one mentions the word ‘polytunnel’ – it being the first word in the title of this book – there are those who conjure up their own images of intensive horticultural cropping in many geographical areas of the world, as do those not directly involved in agriculture. Polytunnels can be seen as a blight on the landscape, while some folk in the industry see them as a cheap alternative for those who do not have the resources to build in glass. There are many misconceptions within the horticultural industry that do not reflect the application of the incredible scientific and technical developments around the world for numerous cladding materials. This aspect in itself continues to evolve to meet the needs of growers tasked with producing the crop qualities demanded by retailers, and the complex schedules necessary to meet continuity of production.

The objective in my mind is to open up the possibilities for many hybrid solutions to meet the varying needs of crops, and the geographical and climatic limitations that can be encountered. For those traditionalists of northern Europe, standard horticultural glass as a cladding material itself is not redundant, but is also evolving to meet energy conservation requirements and light interception properties.

Clearly as soon as any book hits the shelves, some sections are already going to be out of date – such is the pace of change, driven by an increasing demand for sustainable food supplies, to the developing economies requiring the more luxury items. The development of LEDs continues to march inextricably forwards, with the euphoria that it can solve many problems – if only the price could come down, I hear you say!

It’s fair to say that the chapters on structures and cladding materials are somewhat biased towards horticulture at more northerly latitudes; however, there is much to be learnt about composts, irrigation, feeding, and pest and disease control. These are all familiar topics to every geographical location, and solutions found in one locality often cross-fertilize to other regions, for all to benefit.

In compiling the research for this book, I had the opportunity to establish links with many of the research and commercial organizations out there servicing the needs of horticulture. Trade shows play an important part in this, as initial contacts can be made, and remembering a face helps open more doors and brings forward much of the camaraderie that exists in our industry.

Companies large and small, and research organizations, have all strongly supported this endeavour with the provision of technical data, editing descriptions and images, and some have even taken the time to give me guided tours of their own enterprises – a truly international spectrum of contributors. I am also grateful to those who have allowed me on to their enterprises to take photographs and to glean their personal experiences and approaches.

There are few books on this topic generally; readers may be aware of a very limited number of standard reference texts that contain significant scientific input and in-depth technical data: I have not sought to compete with these. It is perceived that the primary readers for this level of text will range from the undergraduate student, those in managerial positions within the industry, to owners of small to medium-sized enterprises and across the horticultural cropping spectrum.

Acknowledgements

It is impossible to identify and give thanks to everyone who contributed to this work, but I have endeavoured to ensure that their respective organizations are credited within the text via inclusion of website addresses.

The following have been instrumental in compiling this book:

Glenn Behrman of GreenTech, Hugo Plaisier of Ludvig Svensson, Richard GreatRex and Caroline Reid of Syngenta Bioline, Andrew Wilson and Adam Ferjani of ICL UK & Ireland, Catherine Dawson of Melcourt Industries, Dominic Cahalin of Delt-T Devices, Rudolf Van Looveren of Van Looveren NV, Henry Wainwright of the Real IPM Co, Hugo Paans and Peter Vos of Erfgoed Holding BV, Electra Hatcher of Keder Greenhouses, Krzysztof Hernick of Tomtech, Mike Abel of Agralan, Marianne Nieuwenhuijsen of Asking.nl/InnovatieNetwerk, Ruth Ashfield of Agriculture and Horticulture Development Board, Harel Dotan and Ronit Golovaty of Paskal Technologies, Seiji Matsuda of AGC Chemicals/f-clean, John Downey of Act Publishing, Jack Crooker of Ball DPF, Tobias Huni of Birchmeier, Michael Neff of American Society for Horticultural Science, Michelle Keyser of National Greenhouse Manufacturers Association, Walter van Esch of In Greenhouses magazine, Michael Seifert of Moeschle, Andy Barber and Aaron McIvor of BPI Visqueen Hort, Amy McCormack of Natures Source Plant Food, Philip Ronn of Argents Nurseries, Chris Batchelor of Wallings Nursery, Robert Farthing of Cornerways Nursery, Mike Smith of WD Smith, Mark Lever of UK Salads, Barry Robertson at John Innes Centre, Andrey Ivanov and Chris Newenham of Wilkins & Sons, Peter Hill of Hill Brothers and Rabbe Ringbom of Valoya.

1

Greenhouse Design Principles

In North America and in Europe there are standards set out for greenhouse designers, defined by codes to follow that consider the geographical and topographical siting and the proposed use of the project at the design stage. For example, in response to heavy storms and consequent damage in Holland in the 1980s, the ability to adequately insure glasshouses became tested. In response, the industry had to quickly come up with, and adopt, what is now the European standard (EN12021-1) to design, calculate and build in the correct glasshouse structural strength.

Greenhouse designs revolve around calculating loads that are placed on the proposed structure, but will also ensure that the structure has adequate stiffness to limit both vertical and transverse deflections. Lateral forces that consider the wind load on a structure will also cover seismic requirements. Readers wishing to know more can follow the American standards, for example via the National Greenhouse Manufacturers Association (http://www.ngma.com).

IDENTIFYING LOADS PLACED ON STRUCTURES

There is a range of ‘loads’ associated with greenhouse design; these include the following:

Dead loads: refer to structural weight with cladding

Live loads: refer to the roof

Collateral loads: refer to the weight of mechanical equipment, for example such as circulations fans, permanently mounted heaters and irrigation equipment, and will also include, for example, gulleys suspended or attached to the structure for crop culture

Plant live load: includes the weight of hanging plants at full productivity

External factors to consider include wind, snow and seismic events, which also provide a load on the proposed structure:

Single widespan greenhouse with structural components.

Wind load: takes into consideration basic wind speed, wind exposure, prevailing wind direction, and the internal and external pressures placed on the components

Snow load: encompasses the likelihood of snow level accumulating on the roof, mounted against the sides, and set against both the internal use and any heat requirements that can mitigate against snow load. However, in the event that the greenhouse is unoccupied for a number of weeks between crops in the winter months, the loading is now increased temporarily over the same area when the crop is being heated to 18–21°C. It may also be that temporary reduction in normal crop temperatures may be made in the worst case scenario to conserve heat where there is a possibility of an interrupted fuel supply.   Snow can be light and fluffy with a water equivalent of 12in equal to 1in of rain, or at the other extreme where 3in of wet snow equates to 1in of rain

There are generally different requirements for greenhouses that are used totally for production, and those that invite the public in (retail units), where health and safety is more stringent in response to an increased risk of injury or damage. In the latter case, building regulations governing the energy conservation aspect of the project can ultimately impact on being able to have a roof with sufficient light transmission for adequate plant growth.

GENERAL OVERVIEW

In a general overview there are features that are typical of most greenhouse designs; these are described under the following headings:

Primary roof system: Typically a truss, rigid frame, arch or something similar.

Secondary structural system: Bracing and support components, for example purlins, glazing bars, ridge beam and gutters. End-wall framing may have other roles such as supporting glazing, as well as bracing a structure and axial load.

Foundations: These may be spread footings, continuous concrete footings, or flagpole-type foundations placed directly on the earthen floor.

Cladding: This includes glass, polycarbonate, fibreglass or polythene, and other materials that have light transmission properties. For retail areas, various composite materials may be recommended to meet building regulations where a case cannot be made for ‘growing area’. These materials may also be incorporated in part of a new structure where an area is dedicated for crop preparation, or grading, packing and distribution requirements, and which is separated from the growing space.

Mobile or static structures: Early designs of mobile greenhouse were popular on market garden enterprises to extend continuity of cropping so as to extend the season. For example, early crops were established under cover, and then the entire structure was moved down to the next position for the next planting, leaving the original crop to continue growing and maturing in the open air. Crops might have been overwintered, or in the case of brassica production, young plants were sown and reared in pots ready for planting outside at the earliest opportunity in the spring. Cut flower enterprises also liked them. Early bedding plant production meant that the structure could be moved off to allow plants to harden off before marketing. The closest modern version of this is to use Spanish tunnels with their temporary covers – these are much easier to manage and cheaper to construct, but are less stable in high winds.

These mobile structures are based on galvanized frames and gutters; however, using wooden glazing bars they were designed to be moved in their entirety along ‘dollies’. The gable ends could be hinged up, the side skirts of rubber sheeting lifted, and then with the aid of a winch and a tractor, the whole structure would move down the site.

HEIGHT OF STRUCTURE

There continues to be an interest in building higher and higher greenhouses. The apprehension about increasing fuel costs pro rata does not appear to be founded on experiences that growers have from investing in higher gutter heights.

The current increase in heights reflects the following requirements:

To accommodate equipment such as up to two separate horizontal screens

To have suspended gutters for hydroponic crop cultivation or multi-tier baskets

To allow for the height of vines for improved crop management and ease of picking at fruiting level

To improve air movement generally, with the associated decrease in fungal problems

To encourage fewer energy spikes: the greater the air volume, the slower the temperature changes, resulting in less fluctuation

In the summer months there is a greater ‘pull’ through the higher roof vents, which creates more efficient cooling

LIGHT INTERCEPTION

During the winter months at northerly latitudes, the interception of as much light as possible determines early yield in salad crops, which commands the highest prices. Consequently much focus has been on improving light levels within greenhouses. However, in economic terms the balance has to be made between the levels of light intercepted and the heat-retaining properties of the cladding as energy costs continue to rise.

The reduction in light levels during short days at northerly latitudes is linked to the angle of the sun striking the corresponding angles used in the greenhouse structure. When light strikes the covering of a greenhouse, some of it is transmitted to benefit the plants inside, part of it is absorbed by the material itself and some is reflected. The least amount of reflection occurs at 90 degrees to sunlight, and the lower the angle the greater the reflection. Maximizing the amount of light transmitted is a matter of choice between cladding material, greenhouse design and the orientation of the greenhouse walls.

The cladding material needs to be assessed and compared for its light transmission and absorption properties, whilst reflection is a matter of angle to directional sunlight.

Glass manufacturing has advanced in leaps and bounds, with even an option now of having anti-reflective (AR) coatings built in at the manufacturing stage. AR coatings improve the transmission of diffuse light on a cloudy day. ETFE (ethylene tetrafluoroethylene copolymer) film, as used in the Eden Project in Cornwall UK, has the lowest refraction index and hence the highest transmission of light.

Traditional Dutch light greenhouse showing the sloping sides utilized to improve light interception in the winter months.

When it comes to light levels, the location and positioning of structural and ancillary support metalwork plays an important part. In mono-culture greenhouses with salad crops, growers will look at everything in order to reduce shade from greenhouse components and to maximize the diffusion of the light that enters the structure, even to putting white reflective polythene on the floor.

The orientation of crops to reduce the shadow effect requires them to run north/south, and this, along with structure design to support crop gutter systems, will decide the orientation of the greenhouse. Ironically for greenhouse operators in many northerly latitudes, the lowest light transmission rates occur when the operators need it most – that is, in winter and spring – and the highest rates occur when the operators need it least!

REFERENCE LINK

http://www.ngma.com

2

Commercial Glasshouses

A-FRAME GLASSHOUSES

Most glasshouse designs are based on the A frame as it is symmetrical and easily prefabricated before erection. However, in some climates growers may prefer a saw-tooth arrangement, and occasionally a lean-to, though the latter requires a back wall such as may be found in a traditional walled garden setting.

Whilst the following information is related to commercial horticultural enterprises raising crops, the needs of plant breeders and other research institutes are increasingly being recognized as having much higher standards of fittings and environmental control, often multi-compartmented with positive pressure atmospheres to keep out pests and diseases. Many greenhouse construction companies have experienced intense competition in the commercial greenhouse sector, so that tendering for specialist facilities such as for research, plant breeding, vertical farms projects, botanic gardens and garden centre buildings has become the norm. Also a wider view is taken, that the structure can be clad with anything that meets the client’s needs.

THE VENLO GREENHOUSE

The Venlo has a long history of providing a basic ‘standard’ greenhouse, widely adaptable and cheaper to build, resulting in it being the most popular model on which to base most greenhouse projects. The design has changed little, apart from, for example, widening the panes of glass further to reduce glazing bars and increase light interception.

The structure of the Venlo is multi-functional compared to conventional wide spans, mainly due to the trellis girder (lattice girder), which allows multiple spans based on the 3.2m width. The trellis girder allows easy installation of horizontal screens, suspended heating pipes, cropping gutters, spray booms and lighting. All the structure is made from galvanized steel. As so much of it can be prefabricated and hence standardized, the costs of production are kept very keen indeed and the same manufacturer will be supplying a number of greenhouse construction companies. Costs of steel do vary on the world market and of course may be subject to periods of volatile currency exchange rates: this needs to be considered in any agreement with the supplier.

Bay widths of 3.2m are standard, however some growers may wish to opt for 4m where maximum light levels are required. During the design stage it is important to determine the best width option to reflect the use of the greenhouse. Crop row layout and spacing or design of travelling benches, for example, need to be considered to ensure the optimum utilization of space. There can be up to four roof sections at a time supported using lattice girders between each row of load-bearing uprights, giving a working clear-floor area for crop culture using the 3.2m module of 6.4m (double span) through to 12.8m for the four-roof section.

Typical gutter height now starts at 3m, dependent on crop requirements. On newer projects an extended height on part of the facility – for example up to 7m in height – is used to accommodate centralized potting, grading and dispatch on the ground floor, along with an administrative office function and storage of packing materials on the upper floor. The higher the structure, the greater the wind factor that needs to be built in to the design specifications on loading.

Foundations consist of pre-sat concrete panels, block walls or concrete walls cast on site, typically 300mm high. Perimeter and internal upright posts are bolted on to concrete or steel foundation posts set into concrete.

New structure: the triple-bay Venlo being constructed.

Gutters are traditionally made of galvanized steel. However, gutters are increasingly made of aluminium because it allows complex profiles that, for example, accommodate a better seal for individual panes of glass, are more enclosed, and present a smoother profile, which further reduces heat loss from the structure. The modern gutter is quite complex in profile, and it is likely the grower will also want to specify a mechanism for collecting internal condensation, as well as directing rainwater into collecting vessels. With the increasing size of individual projects, rainfall patterns need to be considered in relation to the capacity of the guttering system to cope.

Ventilation consists of, typically, two or three pane ventilators fitted on one or both sides of the ridge, operated by motors using a push/pull mechanism. The layout of the vents allows independent opening on the windward and leeward sides of each ridge so that wind direction can be taken into consideration when ventilating. During installation these vents will alternate along the ridge, and may be further insect proofed with special netting.

For salad producers, live plant and associated hydroponic gulley loads need to be considered at the design stage, often associated with high-yielding crops such as tomatoes and cucumbers. For pot plant production there is now a need to incorporate mobile/travelling bench systems with the overhead capacity of being able to lift out individual benches when necessary.

Triple-span Venlo complete with travelling benches for seedling production.

Improvements in Venlo design over the years include better sealing of glass on to the glazing bars and overall structure in order to reduce heat loss. The lattice girder is a potential source of shadow, but other measures may be put in place to offset this, such as using diffuse glass, or by spacing aluminium glazing bars further apart and using wider panes of glass, or alternatively having a polythene clad roof system. Bridge Greenhouses in the UK now offer to run the lattice girder under the gutter (http://bridgegreenhouses.co.uk).

THE CABRIO GREENHOUSE

This style of greenhouse uses the Venlo dimensions and steel framework, but roof design is altered so that the entire roof opens to almost vertical from where it is hinged at the gutter. A continuous rack-and-pinion system is operated by axles supported by pillow block bearings, with each roof section having its own motor. Triple-strengthened glass is used to clad the roof, or alternatively polythene cladding can be used. The glass will be supported on all sides for maximum protection. This facility may be an option for growers who need to fully harden off their crop in situ.

As an alternative – though more for retail greenhouse environments – a one-in-three Cabrio may be considered, as this would lower costs and still provide good ventilation. A number of Dutch suppliers offer a range of alternatives (http://www.prinsgroup.co.uk).

WIDESPAN GREENHOUSE

An alternative double-skinned polythene greenhouse built by Vermako at Hill Brothers (UK), with a ventilation system that is hinged at the gutter and opens up to optimize ventilation.

Widespans of up to 16m are available, dependent on manufacturer and often used in retail where clutter-free floor space is considered to be of greater importance. Suppliers offer a range of cladding options from glass, composite cladding, polycarbonate and impact-resistant PVC. In climates where there is the risk of significant snow loading in winter months, this type of structure is popular as a single non-linked standalone facility that best avoids snow loads in any greenhouse valleys. They are often fitted with continuous roof-ridge ventilation on one or both sides, which uses a motor-driven rack-and-pinion opening system – traditionally this may have been a manual chain-and-drive wheel mechanism.

Traditional widespan with insect-proof side vents for fan ventilation in this research greenhouse.

Widespan with continuous ridge ventilation, and side ventilation via a rack-and-pinion system.

NEWER STRUCTURES

Curved Greenhouse Clad with Glass

There are a few examples where curved roof structures are clad with glass. The basis of the design consists of single roof sheets that extend from gutter to ridge, made from tempered glass that is ‘bent’ into position. Westbrook greenhouse systems in the USA have installed such a multi-bay, gutter-connected, commercial growing structure (http://westbrooksystems.com). In the UK, the Royal Horticultural Society (RHS) has also used large sheets of curved glass in their new display greenhouse.

The use of curved glass in this way adds strength to the structure. Large sheets improve light interception, and direct condensation to run down the inside towards the gutter, where it can be collected.

Fully Opening Structures

The current take-up for this type of greenhouse is mainly for areas of protected plant display in some of the larger garden centres, where the comfort of customers and the ability to continue shopping despite the weather appeals to plant area managers. Additional benefits include keeping the worst of the weather such as wind, rain and hail from damaging the instant impulse appeal/condition of plants. The heat-retaining ability is therefore not an issue in this environment, though frost protection is an option.

However, producers for hardy nursery crops, and even bedding plant growers who wish to harden off plants in situ, are taking an interest in these, recognizing the advantages they can have over a conventional glasshouse. For hardy perennial growth, the ability in the spring to bring on herbaceous perennials and alpines that have been overwintered under protection helps to kick-start the season earlier and maintain the quality of the new growth, as compared to crops that are traditionally based out of doors.

Air flow can still be provided via dual purpose – that is, side – ventilation from floor to gutter on dull, humid days where there is little natural air movement within the crop, and/or by opening the roof on clear, sunny days to dry off crops after watering and improve UV light levels, which leads to better plant colour.

Water management under such covers is more controllable, and the consequent losses of plants due to wet and cold conditions during the winter months is minimized as plants can be kept on the dry side.

Designs vary from modifying the more conventional glasshouse structures, to using polythene tunnels based on hoops (see Rovero tunnels featured in a later chapter).

The VDH greenhouse company uses standard glasshouse structural designs, Venlo or asymmetric, but has its own roof panel design, where the roof can be open to a maximum 85 degrees, and glazing options are for single or inflated polythene cladding. Various cladding options available for the side walls include inflated double polythene sides, polycarbonate and even glass. With less metal in the roof, and consequently a larger area of light-transmitting material with good diffuse qualities and heat-retention properties, this design has attracted its audience of supporters. Another company, Naturelight, also provides a fully opening flat roof greenhouse (http://naturelight-greenhouses.co.uk).

Closed and Semi-Closed Greenhouses

The closed greenhouse environment has been technically proven to work as a concept on experimental examples. There has been some commercial uptake, but its adoption as a fully closed environment is less likely to be adopted in commercial crop production; geographical location could be an important factor in the success of its use. The system does have merits for those research institutions and plant breeders who have an absolute need for isolation.

Original thinking was devised around providing an enclosed, completely controlled environment where even ventilation through the roof space is eliminated, and whereby all internal gases and humidity are somehow recycled. In essence, there would be no heat loss through the roof vent and its associated fittings, and other methods would be employed to combat excessive humidity or to reduce temperature, particularly leaf temperature.

The benefits could include a secure biological growing environment independent from external airborne problems, and lowering fuel costs from primarily the heat source. Also, any elements of the environment in excess are not wasted – that is, they are fully sustainable.

Diagram of the closed greenhouse concept.

Source: ASKING.NL/INNOVATIENETWERK

The following factors should be considered:

Mechanisms for summer cooling with little or no ventilation

Supply of continuous CO

2

enrichment: CO

2

depletion is a worry without adequate supplementary supply

Storing excessive heat gained in an aquifer and then turning this into a heat-pump facility during night time or colder periods when heating is required

How economic this could be is dependent on what financial benefits it brings in comparison to traditional glass through yield increases, and/or quality that is reflected in prices returned to the grower. It should also be considered in what type of climate the best advantages would be gained, and whether the increased costs in electricity consumption could be offset against the reduction in the heating energy bill; this might also factor in the ability to generate one’s own electricity or tap into a cheap local supply.

Some large-scale commercial projects have been completed for tomato production, and yield increases have been noted. However, the initial capital costs are currently beyond the reach of many small and medium-sized growers, whereas big business investment has capitalized on some of these early pioneering projects whilst also consolidating to dominate supplies to the supermarkets.

Comment

For Dutch growers with access to gas, the growth in combined heat and power (CHP) plants has currently met their ambitions to lower energy bills and thus remain in profit. The feed-in tariff for excess electricity generated and fed into the national grid is very much in their favour, and a useful source of subsidy for their production enterprises. The CHP can also be a major source of CO2.

Most of the current emphasis is on the next generation semi-closed greenhouse because it lowers construction costs and reduces the risk factor whilst making the environment easier to understand and manipulate, such that a small amount of natural ventilation is possible during peak cooling periods. A dehumidification process using a mechanical cooling system recovers latent heat that can be redistributed, or stored in a smaller aquifer used for heat pump extraction. There is no ventilating through windows at all during the winter months, especially at northerly latitudes, while there is increased use of energy screens.

REFERENCE LINKS

http://bridgegreenhouses.co.uk

http://www.prinsgroup.co.uk

http://westbrooksystems.com

http://naturelight-greenhouses.co.uk

3

Cladding Materials: Overview

The National Greenhouse Manufacturers Association (NGMA) in the USA has grouped cladding materials into the following types (http://www.ngma.com):

Type 1: Thin plastic films such as polyethylene, polyvinyl chloride (PVC) polycarbonate, ethylene vinyl acetate (EVA) and polyvinyl fluoride.

Type 2: Rigid plastic panels such as single-layered corrugated polycarbonate, fibreglass re-enforced plastic (FPR), multi-wall acrylic, and impact-modified acrylic and polycarbonate.

Type 3: Rigid glass materials.

In summary, the following characteristics have been identified in acrylic, polycarbonate, polyethylene films and glass:

Acrylic: Offers the highest light transmission of all multi-wall panels at 90 per cent; it is energy efficient, not affected by UV, is shatter resistant, and is the most weatherable clear thermoplastic; however, it has limited impact strength.

Impact-modified acrylic in multi-wall panels will deliver 85 per cent light transmission and high energy efficiency that is not affected by UV, and possesses ten times the strength of acrylic – it should be sufficient to protect against hail damage.

Polycarbonate: A clear thermoplastic polyester of carbonic acid with high impact resistance and the best fire resistance of the plastic glazing materials. It is usually used in multi-wall format with light transmissions of 80 per cent. It has long-term performance, and UV protection is usually added during manufacturing.

Polyethylene films: Offer flexible coverings and typically incorporate UV absorbers to extend the replacement cycle beyond a single year. High density materials offer greater strength but are less flexible.

Glass: Made from a combination of sodium, iron and lead silica, and can be tempered to increase impact strength. 3mm horticultural glass has approximately 88 per cent light transmission.

MANUFACTURER CLAIMS!

The NGMA site lists a range of test procedures that apply to many greenhouse glazing materials; these include measuring light transmission, tensile strength and impact strengths, amongst others.

Light measurement is measured in nanometres (nm). This will measure across the light spectrum including UV, visible, PAR and infra-red.

UV is the lower end of the spectrum with light in the region of 10–400nm, and only amounts to 3 per cent of total radiation, but its energy does cause chemical reactions that result in yellowing, brittleness and other fading characteristics of some materials, which reduces their lifespan – hence the use of UV inhibitors in many polymer materials: these are chemical compounds that are incorporated at manufacturing to reduce the degrading effects of UV.

Visible light is in the spectrum of 400nm to 700nm, the rainbow colours. Photosynthetically Active Region (PAR) looks at a narrower spectrum from blue to far red, and is seen as the primary area to provide plant response in terms of growth.

Near far red is the heat effect from sunlight that warms up the greenhouse and spans from 700nm to 2,500nm.

Far infra-red is what is produced by the mass inside the greenhouse. The transmission properties of cladding materials to far infra-red vary significantly, and each cladding material has to be analysed for this property during selection.

The energy efficiency of materials is expressed as a ‘U’ or ‘R’ value. ‘U’ determines the rate of heat transfer, and the lower the value the better the material. ‘R’ is a reciprocal value used in advertising. These figures are used to determine the heating requirements for a greenhouse.

Heat loss from structures is not confined solely to the glazing material, but is influenced by glazing systems; also better sealing by using rubber gaskets, for example, will reduce heat loss further.

Treatments to glazing materials are available that lower the co-efficient of friction on the surface: they do this by providing an oily, slippery, anti-drip surface, which causes water films to run down the side rather than form droplets. This further improves light quality and reduces disease risk.

Resistance to hail has been further improved in multi-wall panels in recent times and needs to be re-assessed.

Treatments to reduce incoming far-red energy are available for a range of glazing materials, and may also be applied on the inside to further retain energy in the greenhouse. These treatments are usually added at the manufacturing stage to improve stability. Shading treatments are also available, which can be applied separately once the greenhouse is in place, but they usually require annual renewal.

HORTICULTURAL GLASS OPTIONS

Traditional Horticultural Glass

Traditional drawn glass of 3mm thickness is based around two common sizes: 24 × 24in and 24 × 18in. It accommodated a large proportion of early commercial greenhouses, including the widespan that can still be seen today. The quality of the glass was not deemed to be as important for greenhouse purposes as compared to the domestic market, and the cost of manufacturing was kept low. The same pane of glass was used on both roof, sides and gable ends; consequently the structures were often based around these dimensions. Replacement was therefore easier, and many growers learned how to cut glass themselves. Most maintenance on greenhouses used to be carried out by nursery staff themselves, unlike today!

The disadvantage of using these smaller panes of glass was the number of overlaps between individual panes, which could quickly become green with trapped algae and therefore reduce light levels. Moss also began to grow between the panes and push them apart further, and this in itself increased the amount of ‘natural ventilation’, resulting in even greater heat loss. The extent of this potential heat loss became an increasingly important factor as energy prices began to rise in the 1970s. The solution then was to use a lap seal based on clear silicon. This conserved heat energy and reduced the need to replace cracked panes of glass; nurseries soon discovered how powerful this glue was, and simply applied it as a strip along the crack, without having to replace the actual pane.

The downside was that if a pane of glass did have to be replaced, lifting it off the silicon was difficult and often resulted in even more panes being broken than were originally damaged – that’s how strong it was. Excess humidity became more apparent without the natural ventilation that occurred through the overlaps, and this needed some new skills in managing the greenhouse environment.

It still remains a very popular model for domestic greenhouses today, however due to safety concerns, various forms of toughened and safety glass are increasingly being offered to this market. Traditional drawn glass is still available but is manufactured in Eastern Europe where the costs of production are lower.

The development of the Venlo greenhouse started with the ‘Dutch Light’ used for cold frames. It was based on using larger sheets of 4mm glass in a wooden frame, and these frames were then placed together to form a large commercial wooden greenhouse. As galvanized steel greenhouse frames came more into use, along with the development of aluminium glazing bars, these large sheets could be accommodated. Light transmission was better due to less structural support with bigger panes of glass, and few overlaps on the roof or side walls meant less accumulation of dirt.

The change in glasshouse design to further improve light levels in greenhouses pushed for even larger sheets of glass based around these standard 4mm panes. The drive to increase light levels in glasshouses served to push the boundaries in reducing the number of glazing bars by widening the standard widths between bars to accommodate panes of glass up to 1m wide. This is very much dictated by the type of glass being manufactured, which varies in weight and strength and with other added coating components. Cost is also clearly a factor as the significant increase in capital and replacement costs is associated with many newer alternatives.

The properties of horticultural glass began to change in the 1990s, as a result of the demand for better heat energy saving and a range of other properties. One manufacturer, van Looveren in Belgium, sits at the forefront of development with what is now widely used: float glass (http://www.vanlooveren.be). Float glass is stronger due to the manufacturing process where the molten mixture of silicon, lime and soda is premixed, then heated in a furnace at 1,550°C before being poured/measured on to a bath full of liquid tin at 1,000°C; this forms a floating ribbon of glass to the desired thickness. The result is a higher quality glass of visual perfection compared to traditional methods. The glass is allowed to cool while in a horizontal position that further improves its strength.

The Development of Coatings

Further refinements to this broad outline of the manufacturing process can add other properties to the glass. Coatings can be applied that alter energy transmission: these are built into the glass and become a permanent feature with the following properties:

Reduce the amount of infra-red radiation leaving the structure

Reflect sunlight, to reduce heat build-up in hot sunshine

Shading for crops with a lower light requirement, or more usually for dedicated work areas in a greenhouse complex

The brands of horticultural glass traded by Van Looveren in Belgium are set out in the table below. Hortiplus N® is 4mm horticultural float glass with a thin layer of metallic oxide that has an emissivity of 0.15: that is, only 15 per cent of long infra-red rays are reflected back to the outside. The U-factor is close to that of double glazing but without the added weight – see the comparative table below. Hortiplus N® can also be tempered so that it becomes safety glass.

Horticultural Glass Properties

Type of glass

U-factor(W/m2.K)

Light transmission

Horticultural glass Clear 4mm

5.9

89%

Horticultural glass Hortiplus N 4mm

3.7

81%

Double glazing 4-9-4 (2 × 4mm – cavity 9mm)

3.1

80%

Double glazing 4-12-4 (2 × 4mm – cavity 12mm)

2.8

80%

Source: Van Looveren

Sundim®: 4mm horticultural sun-reflective float glass with a bronze brown-like coating on one side, which faces upwards when put in position and reflects approximately 50 per cent of the sunlight. This may be considered for garden centres or for dedicated work areas such as corridors. Sundim® can also be tempered so it becomes safety glass.

Hortiwhite®: 4mm horticultural float glass with a white ceramic coating on one side that is baked into the glass during the tempering process. It is a safety glass (2.5 times stronger than the normal 4mm glass), which deflects 74 per cent of the sunlight and further absorbs 24 per cent. Again, this type of glass may be of interest where safety is judged to be more important and internal light levels are not critical.

Horticlear®: The trade name given to a frameless ventilation pane of tempered horticultural glass 5mm thick; the absence of aluminum further improves light levels in the greenhouse. They can be best described as hardened ventilation windows, and because they don’t have any raised fitting parts, this provides for smoother and more accurate roof cleaning with gantries.

Van Looveren also supply all other existing horticultural glass kinds (also diffuse glass without or with anti-reflex coating to one or both sides) and all horticultural synthetic panels (http://www.vanlooveren.be).

Information

Tempered horticultural glass (or safety glass) refers to any type of horticultural glass that when punctured will explode into tiny fragments that cannot cause substantial injuries. However, it is important to note that safety glass has to be custom ordered/manufactured to the sizes required – it cannot be cut like conventional glass.

Increasing Role for Diffuse Glass

The degree of scattering of incoming light influences the amount of light received by lower leaves in tall crops such as tomatoes and cucumbers; this is particularly significant during periods of naturally low light levels. The ‘haze’ factor is the measure of this scattering, and this is currently being researched in terms of plant growth and morphology. Current recommendations are for a haze factor of 50 per cent and a hemispherical transmission of 83 per cent.

For growers considering the use of diffuse glass, specifications need to be drawn up that suit the specific crop, as crop responses do vary. It would also be advisable to get some independent testing for what comes on site to ensure the quality from the manufacturer.

Combined with a specification for diffuse properties, an anti-reflective coating on upper and/or lower surfaces may be applied at the same time. The use of sand with a low iron content during the manufacturing process reduces the variability in the glass itself before it is ‘treated’ with other additives.

Current observations indicate that in a tall crop such as tomatoes, there is a greater active photosynthetic surface where light is being diffused and the leaf temperature is within 2 degrees of greenhouse air temperature. Where conventional glass is used and the crops are exposed to direct sunlight there is a significantly higher differential. There also appears to be a redistribution of temperature through the vertical profile of tall crops, which results in an overall evenness in leaf temperatures.

ALTERNATIVES TO CLADDING WITH GLASS

There have been many advances in the development of alternative cladding materials; these may reduce capital costs, offer climatic advantages or a greater flexibility in design to meet production requirements, increase light levels, provide greater insulation, and may even be portable (as in temporary structures). Polythene in its many forms is probably the best known, and this has its own chapter. However, do not discount what other materials have to offer; these include polycarbonate, PVC cladding, PMMA multiwall panels, ETFE film and Polydress SolaWrap.

Polycarbonate

Solid 4mm polycarbonate is used in domestic greenhouses as an alternative to or replacement for glass, for safety reasons. It has high mechanical strength and impact resistance, and can withstand hailstorms without shattering. It is often used for dome-like structures, as the flexible nature of the long sheets can be exploited, and it is ideal for research purposes where completely sealed growing units avoid contamination but alternative cooling mechanisms need to be built in at the design stage.

For commercial horticultural purposes, clear multiwalled insulating panels are specified. They come in a range of thicknesses, and may be double or triple walled, and may even be in quintuple forms. Thicknesses of 4mm up to 16mm are available, with 8mm found to be the most popular.

Polycarbonate as a material is flame resistant, chemically inert, low in weight, and easy to cut and bend into shape. During manufacturing, a UV protection layer is commonly included, but there are also options for heat reflecting and condensation limiting layers, and there is a range of colour possibilities, though these are more for use in the general building industry.

Generally UV light protection is provided on one side – this needs to be noted at construction. The UV protection stops the material becoming brittle and yellowing.