Gardening Under Plastic - Bernard Salt - E-Book

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Bernard Salt

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Plastic materials have transformed gardening. Plastic gnomes, plastic hose, plastic cans, plastic tubs, plastic pots and now plastic protection in the form of low cost cloches, frames, mulches, fleece, Nets, windbreaks and polytunnels. The quality of plastic sheeting available for polytunnels can now give protection for year round cultivation even in cold, northern climates - greatly extending the Growing season. For gardens too small to Accommodate a complete polytunnel, cloches and cold frames can produce an abundance of plants and many hours of enjoyment. In Garden Under Plastic, Bernard Salt gives information on the practical use of polytunnels and other hardware, methods of cultivation for Flowers, fruit, vegetables and other crops, and potential problems from pests and diseases that may strike under these particular circumstances.

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Bernard Salt



Bernard Salt

Illustrations have been supplied as follows: Front cover, frontispiece and preface:Garden AnswersMagazine, Colin Leftley. Page 29: Garden Direct. Page 40: Traditional Garden Supply Co. All other photos, line drawings and diagrams: Bernard Salt.

The author would like to thank Garden Answers Magazine and Colin Leftley for help with photographic material; Northern Polytunnels; Defenders Ltd/ Wye College; Solar Tunnels; CLM Fabrications; Growth Technology; First Tunnels; Phostrogen Ltd; Miracle Care Garden; Jiffy Products; Armillatox Ltd; Link Stakes.

First published in the United Kingdom in 1999 by


1 Gower Street



An imprint of Pavilion Books Company Ltd

Copyright © Batsford

Text © Bernard Salt

Published in the United Kingdom as eBook in 2015

The moral rights of the authors have been asserted.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the copyright owner.

eISBN 9781849942232

This book can be ordered direct from the publisher at the website:, or try your local bookshop.

Distributed in the United States and Canada by Sterling Publishing Co.,

387 Park Avenue South, New York, NY10016, USA



Chapter 1 Protected Cultivation

THE POLYTUNNEL:Siting, Erecting a Polytunnel, Ventilation, Temperature, Inside Organisation, Plant Support, Choosing a Polytunnel, Planning Permission?, What to Grow?, Cropping Plan,CLOCHES:Cloches available, Floating Mulches, Horticultural Fleeces, Plastic Film, Opaque Mulches, Other Uses of Plastic in the Garden, Cold Frames

Chapter 2 Propagation and Growing

PLANT CARE IN A POLYTUNNEL:Watering the Polytunnel, The Use of a Capillary Matting, The Use of a Hosepipe, Composts, Plant Containers, Cuttings, Propagating by Softwood Cuttings, Ten Hints for Success with Softwood Cuttings, Propagating by Hardwood Cuttings, Propagation from Hardwood Cuttings, Propagation of Conifers, Feeding Plants in the Polytunnel

Chapter 3 Flowers

Bedding Plants, Biennials & Perennials, Hanging Baskets, Flowers for Cutting, Other Flowers Cut from Seed, Outside Work,

Chapter 4 Fruit

Aubergines, Cucumbers, Grapes, Melons, Peppers, Strawberries, Tomatoes, Growing Tomatoes by Different Methods,

Chapter 5 Vegetables

USING THE SOIL BEDS:Spinach, Carrots, Cabbage, Cauliflower, Peas, Potatoes, Sweetcorn, Courgettes, Parsley, SALAD CROPS:Lettuce, Beetroot, Salad Onions, Land Cress, Radish, Endive, Raising Plants for the Outside Production of Vegetables and Salads,

Chapter 6 Pests, Diseases and Disorders

Pest and Disease Control in a Polytunnel, Chemical Control, Biological Control, Integrated Garden Care, Problems Common to Most Plants, Aphids, Caterpillars, Cutworm, Leaf Miner, Mealy Bug, Red Spider Mite, Thrips, Slugs & Snails, Whitefly, Vine Weevil, Wireworm, Black Leg, Clubroot, Damping Off, Downy Mildew, Foot Rot, Powdery Mildew, Scab, Grey Mould, Stem Rot, Violet Root Rot, White Rot, Virus, Mice, Blossom End Rot, Magnesium Deficiency, Oedema, Drought, Cold, Bolting, Splitting, Pollination, Tip Burn, Rust, Too Hot, Boron Deficiency



Practical Points for Success



During my lifetime plastic materials have transformed gardening. Plastic gnomes, plastic hose, plastic cans, plastic tubs, plastic pots and plastic protection in the form of cloches, frames, mulches, fleece, nets, wind breaks and polytunnels.

As well as the large range of plastic sheets and fleeces, there is a wonderful variety of cloches and polytunnels available to increase gardening success and enhance enjoyment.

I bought my first polytunnel 14 years ago and it has improved my diet as well as my life. Apart from the two coldest winter months I enjoy gardening all year with the growing season extended at both ends. Polytunnel sheets are now much improved, although not as transparent as glass their ability to retain infrared is almost as good. They no longer drip and they last much longer. The first sheet I had lasted 4 years whilst my current one is in its seventh year.

Walk into a polytunnel and feel the atmosphere change, experience the different scents, the riot of growth and the freedom from wind and rain. Rake beds, sow seeds, prick out and pot-on whilst the rain lashes down.

I enjoy early potatoes in April, peas, cabbage and cauliflowers in May; lots of peppers, aubergines, cucumbers, melons and tomatoes in summer. A polytunnel is not only about food; I cut chrysanthemums in November, antirrhinums in December, daffodils in February, stocks and sweetpeas in May. In addition my borders are a mass of colour all summer and there are tubs and baskets everywhere, thanks to plants raised under plastic. It rains and blows but whatever the weather I can garden in comfort – what more can anyone ask?

Sadly many gardens are too small to accommodate a polytunnel but this does not prevent the use of cloches and other plastic materials to increase the number and variety of plants. A little ingenuity with a cold frame, a few cloches or a small lean-to structure can produce lots of plants and give hours of enjoyment.

Gardening methods are not written on tablets of stone and there is more than one way of achieving success. The methods I have described here are the ones that have worked for my students and myself. I hope they will also work for you.

Bernard Salt

Chapter One

Protected Cultivation

Inside a domestic polytunnel in July

A polytunnel is very versatile and is used by different gardeners in different ways. The author’s polytunnel gives him the following advantages:

•  Spring is 6 weeks early.

•  Winter is 4 weeks late.

•  Delicious food is harvested when it is most expensive in the shops.

•  Bedding plants are grown very cheaply.

•  Hanging baskets are planted up a month early and in full bloom when hung outside.

•  Two crops are raised each year instead of one.

•  Tender crops are easily grown.

•  Half hardy perennials survive the winter.

•  Vegetable yields are higher and the quality is better.

•  Flowers are not damaged by wind and storms.

•  Fleece placed over plants does not blow away.

•  A no-go zone for pigeons and rabbits.

•  Carrot flies dislike tunnels.

•  Ideal place to raise plants for flowering or cropping outside.

•  Conservatory plants are stored during non-flowering periods.

•  No heating bills.

•  Rain never stops play.

There are of course disadvantages too, these are:

•  Daily watering is necessary during the spring and summer months.

•  Greenhouse pests such as white fly and red spider mites can be troublesome.

•  More skill is required to grow crops in a tunnel than outside.

•  The outside appearance is not very aesthetic.

The growing season on these islands is very short, in many areas it is less than 6 months. A 6 week extension of the growing season represents an increase of 25% and this has a dramatic influence on crops and cropping. Plastic sheeting has been developed which gives almost the same ‘greenhouse effect’ as glass. The ‘greenhouse effect’ is the term used to explain how a greenhouse acts as a heat trap by letting in the sun’s warming rays and preventing the earth’s cooling rays from escaping.

Protected cultivation gives freedom from wind chill, hail, snow and rain. The trapped air is not blown away but remains as a warm blanket around the plants. Incidentally the gardener also enjoys these benefits!

The cheapest method of covering an area for plant production is a polytunnel.

A flower border alongside a polytunnel improves its appearance


Every gardener, who has the space, should consider a polytunnel as they give a whole new dimension to gardening. Whatever the weather, polytunnel gardeners can enjoy their hobby to the full. A polytunnel is the cheapest method of protecting the garden and the gardener! A polytunnel will cover four times the area of a greenhouse for a quarter of the price. Contrary to popular belief the covers are tough and good ones will last for 6 years and more.

A polytunnel is not a ‘plastic greenhouse’. The management and use of an amateur’s greenhouse differs considerably from a polytunnel. A greenhouse is easier to insulate and keep frost free in winter; the light in a greenhouse is a little better than in a polytunnel and a greenhouse is very useful for propagation, especially in early spring when light is at a premium. Ideally a gardener should have both as they are complementary. The author has both; if he had to choose just one, there would be no hesitation – it would be a polytunnel!

A polytunnel consists of a framework of metal hoops, fixed along the top to a metal ridgepole. In some polytunnels the hoops are shaped to give vertical sides. This shape is recommended where tunnel widths are less than 5m (16ft).

The frame is covered with a single sheet of polythene, secured to wooden door frames at each end. The doors double as ventilators. One advantage a tunnel has over a greenhouse is that the design of the end is easily varied. The author recommends fitting double doors at each end to provide both good access and good ventilation.


In most gardens the chosen site will be a compromise between what is ideal and what is practical. The following must all be considered:

1. Light – avoid deep shade, this is of extreme importance especially in the winter and early spring. A tunnel with the ridge running east-west will receive more sun than one with the ridge running north-south.

2. Access – easy access from the house to the tunnel and a good surface on the path will probably lead to better care for the plants.

3. Aesthetic – ‘beauty is in the eye of the beholder’ but in most peoples’ eyes a polytunnel is not very good to look at.

4. Trees – these should not shade nor overhang the polytunnel but, when far enough away, make good wind breaks.

5. Distance from the boundary – a clear area of at least 1.2m (4ft) around the polytunnel is almost essential when fixing the cover. This space need not be wasted as it may provide a useful area for standing boxes of plants.

6. Wind – shelter from cold prevailing winds helps to keep the temperature up. A gap between two buildings will sometimes create a wind tunnel. Such situations should be avoided.

7. Foundations are not a problem with polytunnels, a level site has many advantages and every effort should be made to achieve this.

8. Soil – good and free draining is ideal, poor soil can be overcome by creating raised beds inside the polytunnel and filling them with good topsoil.

9. Frost – frost pockets or hollows should be avoided where possible.


1. Water supply – a tap inside a polytunnel is virtually essential for tunnels of 50m2 (538ft2) and above.

2. Electricity supply – is not essential but very useful for lighting, heating a propagator and operating other equipment such as a shredder. This must be installed by a certified electrician and all equipment protected with a residual current device.

Erecting A Polytunnel

Polytunnels are usually supplied in kit form and the erection is easy for the average handy person, but (unless the tunnel is very small) four people are needed to attach the cover. After the tunnel has arrived the first job is to lay out and identify all the parts.

Marking Out The Base

1. Using two pegs and a length of string mark the position of the tunnel side. Drive in the pegs 1m (3ft) or so beyond each end of the tunnel and tie the string taut between them.

Strings pegged out to mark the exact position of the foundation tubes

2. Mark both ends in the same way, squaring the corners with a rectangle of wood or a 3-4-5 triangle.

3. Fix the line that marks the other side. You now have eight pegs and four pieces of string. The points where the strings cross mark the corners of the tunnel.

4. Measure both diagonals, it the measurements die not exactly the same, the base is not square. If necessary, adjust the strings until the diagonals are exactly the same length, this is most important.

5. Mark the positions of the ends of intermediate hoops with pegs and string. The distances between the hoops should be equal.

6. Leave the pegs where they are and remove the strings.

7. Dig the holes.

Fixing hoops

8. Fill the holes with concrete and replace the strings. Push the tubes, which are to hold the hoops into the wet concrete. The tubes must be vertical and their centres exactly underneath the point where the strings cross.

9. Using a long straight plank and a spirit level check that the tops of the tubes are level; drive in any which are proud. (Note: protect the top of the tube with a board before hitting it with a hammer).

After the concrete has set, the sequence of erection is as follows:

1. Slide the hoops into position.

2. Fix the ridgepole with the brackets provided but do not tighten any nuts.

3. Adjust the hoops, by raising and lowering them into the sockets, until they are vertical and exactly in line with each other.

4. Tighten all nuts.

5. Fix the door frames by concreting the bases into the ground and fixing the tops to the end hoops with the clips that are supplied.

6. Cover all nuts and sharp edges with adhesive tape.

7. Stick anti-hotspot tape over the outside of the hoops. Anti-hotspot tape is very smooth which helps the positioning of the sheet; it also extends the life of a polythene cover by at least one year. This tape is usually supplied as an optional extra. In the author’s opinion it is essential.

A solar tunnel frame ready for sheeting up

Covering The Tunnel

Different manufacturers have different methods of fixing the sheet. Points to remember are:

1. Wait for a calm day as the slightest breeze makes a cover unmanageable. The air is most still at dawn and this may be a suitable time to cover the frame.

2. If the plastic is treated on one side with an anti-fogging agent make sure you put it on the right side up. Always follow the manufacturers’ instructions.

An anti-fogging treatment causes droplets of condensation to spread into a thin film. This prevents drips, allows more light in and improves insulation.

3. When pulling the sheet taut, take care not to stretch it.

4. Lay the sheet out along one side of the tunnel with an equal amount at each end.

5. Drag the sheet over the tunnel and adjust to make the overlap at the sides equal.

6. Make two vertical cuts at one end, to allow for the door, 30cm (1ft) less than the width of the door.

7. Allow 60cm (2ft) for fixing and cut off surplus plastic that would otherwise cover the door.

8. Take a wooden batten, the width of the door frame, and roll the plastic that is hanging below the door frame around it.

9. Use galvanised nails to fix the wood (with plastic wrapped around) to the outside face of the top of the frame.

10. Repeat the operations at the other end. Before nailing in position pull the sheet as tight as possible along the length of the ridge, the final appearance of the tunnel depends upon this.

11. Beginning in the middle and working towards the ends, fix the sheet along each side, pulling out any creases and getting it as taut as possible. The sides of the sheet are either buried in the ground or fixed to special horizontal rails.

If using rails fix them to the hoops about 20cm (8in) above their final position; when the sheet is attached to the rails push down hard and tighten the nuts. Bury the surplus sheet to form a draught proof seal.

12. Pull the sheet around the end hoop as tightly and crease-free as possible. Fix the sheet to the sides of the door frame by nailing on wooden battens.

13. Hang the doors.

14. Cover the doors with surplus plastic. A staple gun is the best tool for this purpose, double the plastic over to form ‘washers’.

A 14ft wide polytunnel


Ventilation is provided by opening doors or vents at both ends of a polytunnel. Apart from cold spells in the depth of winter, one end (or sometimes both) should be opened every day. This is most important in autumn when conditions become favourable for grey mould and other fungus diseases. On sunny days which are followed by frosty nights the tunnel should be ventilated by day but closed up an hour or so before the sun goes down. A layer of condensation then forms on the inside of the sheet, this freezes and gives additional insulation.

A polytunnel should always be given maximum ventilation by day during the summer. It is best to close up in the evening to keep the night time temperature as high as possible. A large difference between day and night temperatures can cause problems – especially with tomatoes.

In winter, during very cold spells, keep the polytunnel closed both day and night.

A frame made of polystyrene sheets and a polycarbonate top. This is used inside a polytunnel to protect the most tender plants during severe weather

A propagator, with a thermostatically controlled heated base with twin wall polycarbonate top, is a cost effective method of raising plants early in the growing season


A polytunnel warms by the greenhouse effect, much of this warmth remains as there is little chill factor from the wind. It is unheated and the temperature inside will fall below freezing. Closing up well before sundown in spring and autumn is an essential part of tunnel management. Tender crops such as early potatoes are best covered with fleece. This will not need anchoring as there is no wind to blow it away. In very cold spells a body blanket (aluminium foil) can be used as a second cover during night time only.

In summer a polytunnel can become very hot even when doors and vents are fully open. Plants withstand these high temperatures providing the air is humid and they are well watered.

The polytunnel bench in May

Inside Organisation

Once the polytunnel is complete the inside will need to be organised. It is better to have the path down the length of the polytunnel off centre to allow the ridgepole to be used for plant support and/or hanging baskets. The polytunnel in the photographs is organised into a number of 1m (4ft) wide raised beds with wooden surrounds. There is 1m (3ft 6in) benching along one side with a path running the full length between the beds and the bench.

The bench consists of old tables, purchased cheaply from a scrap merchant. The tables are covered with white plastic sheeting; this is clean and hygienic, it also reflects light.


Staging helps to organise a polytunnel into a convenient production unit, with places for pot and tray grown plants. A slatted top allows air to flow around the plants. This helps to control fungus diseases in autumn but in summer it makes plants dry out more quickly. A solid top gives a good working area for sowing, potting, taking cuttings, etc. The height of staging should be adjusted to a comfortable working height by placing bricks under the legs. The area underneath is a good place to grow chrysanthemum stools, dahlia tubers for cuttings, ferns and shade tolerant plants. It is not a good place to store used boxes and pots.

An average handy person could produce home made benches from a variety of materials. Care should be taken however not to provide nooks and crannies that may encourage pests and diseases. Wooden benches are better when covered with a sheet of white plastic. This is easy to clean, reflects light and looks good. All benches and staging should be level. This is very important for a capillary bed or it will not function properly.

The same bench in winter

Plant Support

The ridgepole provides a good place for hanging baskets and for attaching strings or nets for plant support.

A useful method of providing other anchor points is to run a wire, complete with strainer, along the length of the polytunnel and attach it to the wooden uprights which form the door posts. Strings from this wire to the ridgepole provide intermediate anchor points.

A strainer holds the plant supporting wire taut. The wire runs the length of the tunnel and is used as an anchor for plant supports

Choosing A Polytunnel

When selecting a polytunnel there are four golden rules:

1. Make certain that the hoops are strong enough to withstand snow loading. Steel hoops galvanised inside and out are ideal. In addition the structure is stronger if the hoops are no more than 2.4m (8ft) apart.

2. Choose a wide tunnel rather than a narrow one. A short wide polytunnel is better than a long narrow one both for working space and for ventilation.

3. Make sure that there is a clearance of 1m (3ft) on both sides and the far end of the tunnel.

4. Choose the best available sheet and one that is treated with an anti-fogging agent. The extra cost will be more than repaid by longer life and greater energy efficiency. The anti-fogging agent also prevents annoying drips of water down the gardener’s neck and (more important) harmful drips of water onto the plants.

In addition to the ‘standard’ polytunnel there are more sophisticated ones available for amateur gardeners. These are all excellent in their way advantages over the standard ones. They are also more expensive.

John Walker, the well-known garden expert, is a solar tunnel enthusiast

Solar Tunnel

Inside a polytunnel

A solar tunnel is best described as a hybrid between a polytunnel and a greenhouse. The sides are vertical and the roof domed. The cover is PVC reinforced with a green 1 cm sq mesh. The green appearance and vertical sides make a solar tunnel a more attractive addition to a garden than a standard polytunnel. A solar tunnel is secured with large corkscrew anchor bolts. These can be unscrewed from the ground and the whole structure carried to a new site. This operation needs four strong (or six not so strong) people. Unlike the standard polytunnel the sheeting is supplied in pieces. Each piece has its edge folded over and welded to form a continuous loop. ‘Draw strings’ threaded through the loops are used to hold the sheets in position. It is possible (but not desirable) to erect a solar tunnel in a fairly strong wind.

Solar tunnels are available in 3 and 4m (10 and 13ft) widths and any length. The cost is more than a polytunnel and less than a greenhouse. 01903 742615 for a catalogue.

The solar tunnel, with its green appearance, fits well into the garden design

Keder House

A Keder house is covered with a type of plastic very similar to the large bubble plastic that is available as a greenhouse insulator. It is very strong, the bubbles are virtually unbreakable and a roof section will carry the weight of three people. Keder plastic is half the cost of glass with double the insulation properties. Manufactured in 2m (6ft) wide sheets, the edge of the sheet has a continuous cylinder that fits into a female plastic strip. This strip is fixed to galvanised steel supports by means of self tapping coach bolts. Many commercial growers now use Keder plastic and a firm in Malvern is supplying ‘midi houses’ with this type of sheeting. Apart from the additional insulation qualities offered by the Keder plastic, these houses have excellent side ventilation – a real boon in the summer and autumn. The delivered and erected price of a 4×8m (13×26ft) midi house is around £2,000 which is much more than a standard polytunnel but much less than the traditional greenhouse. 01386 49094 for a catalogue.

A Keder midihouse. Smaller Keder houses are also available

Planning Permission?

Regulations are subject to change. At the time of writing, domestic greenhouses and small polytunnels are not subject to planning. It is always worthwhile checking with your local planning department, especially if you are in a conservation area.

What To Grow?

Polytunnels have many different uses including housing animals, growing mushrooms, protecting fishponds, shading houses, staging exhibitions, covering swimming pools and of course growing plants.

The last item begs the question, ‘what plants?’ and the answer is ‘Any type of plant the owner wishes to grow’.

In practice there are plants that thrive in a polytunnel and others that grow as well outside.

Basically most domestic polytunnels are used to:

•  extend the growing season.

•  propagate plants.

•  protect less hardy plants in winter.

•  grow plants from warmer climates in summer.

•  produce food for the family.

•  produce cut flowers for the house.

•  raise plants for the vegetable garden.

•  raise half hardy annuals for the flower borders.

•  propagate hardy perennials for flower borders.

•  store conservatory plants that are not in flower.

•  allow the gardener to enjoy his/her hobby regardless of the weather.