Gardener's Guide to Propagation Techniques E-Book

Nikki Barker

• Herausgeber: The Crowood Press
• Kategorie: Lebensstil
• Serie: A Gardener's Guide to
• Sprache: Englisch

Seitenzahl: 216

Veröffentlichungsjahr: 2022

Beschreibung

Plant propagation is an economical, rewarding and sustainable way to increase your plant stock and develop your horticultural skills. A Gardener's Guide to Propagation Techniques is a comprehensive, practical guide to the various methods of propagation suitable for a wide range of trees, shrubs, herbaceous perennials, bulbs and tender plants. Chapters cover seeds, cuttings, layering, division, grafting, bulbs and even micropropagation, as well as providing an extensive list of plants that are suitable for each method, rating them from easy to difficult. In addition, the book includes: step-by-step photographic guides to the different propagation techniques; advice on selecting the right plant material, at the right time; help with deciding on the best method for different plants; guidance on choosing the appropriate propagation environment and aftercare of the material, and finally an overview of pest and disease challenges.

Leseprobe

First published in 2022 byThe Crowood Press LtdRamsbury, MarlboroughWiltshire SN8 2HR

[email protected]

This e-book first published in 2022

© Nikki Barker 2022

All rights reserved. This e-book is copyright material and must not be copied, reproduced, transferred, distributed, leased, licensed or publicly performed or used in any way except as specifically permitted in writing by the publishers, as allowed under the terms and conditions under which it was purchased or as strictly permitted by applicable copyright law. Any unauthorised distribution or use of this text may be a direct infringement of the author’s and publisher’s rights, and those responsible may be liable in law accordingly.

British Library Cataloguing-in-Publication Data

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

ISBN 978 0 7198 4080 7

Cover: Blue Sunflower Creative

The author and publisher are grateful to the following for the provision of photographs: Serge Barker, Rob Jackson, Palmstead Nurseries, Andy Vernon and C.J. Waters.

CONTENTS

1 Introduction to Plant Propagation

2 Propagation Systems, Environments and Materials

3 Seed Sowing

4 Softwood Cuttings

5 Semi-Ripe Cuttings

6 Hardwood Cuttings

7 Root Cuttings

8 Leaf, Leaf-Section and Petiole Cuttings

9 Layering

10 Division

11 Grafting and Budding

12 Bulb Propagation

13 Micropropagation

14 Pests, Diseases and Disorders

Index

CHAPTER 1

INTRODUCTION TO PLANT PROPAGATION

The Art of Propagation

Propagation is a skill that takes time and practice to master, and there is always something new to learn. Within each method there are plants that propagate quickly and easily, and plants that take longer and need more resources and equipment. Practising each method with the easier plants will give you confidence and improve your technique before moving on to more difficult subjects. This book aims to show you the techniques involved in each method and lists different plants that can be propagated by those methods, showing the level of difficulty for each plant. This will help you to select plants that you wish to propagate by the equipment and materials available to you, as well as your own skills.

What is Propagation?

Plant propagation is the method by which new plants are produced from seed or other parts of a plant. People have been propagating plants for centuries, and some of the techniques we use now have been refined from the way plants reproduce in their natural environment. Roman and Greek texts from over 2,500 years ago indicate that grafting, one of the more difficult methods of propagation, by which the growing plant is grafted on to another plant’s rootstock, was being practised widely in orchards and fruit production. Weeds proliferating in garden borders are plants sexually reproducing and multiplying naturally, and when we sow seeds from packets, we are trying to provide the conditions that will promote maximum germination for the plants we want to grow. Other plants, such as Lamium maculatum and Ajuga reptans naturally use more than one method of propagation, and layer themselves by producing adventitious roots along their stems, which is why we consider them to be excellent climbing or ground-cover plants, as well as producing seed. Looking at how a plant reproduces in its natural environment can often give us clues about the best way to propagate from it.

Example of natural layering in Ajuga, Lamium and thyme.

So, people have been propagating plants, selecting desirable characteristics, for thousands of years, both for food production and to provide ornamental plants to make a pleasing environment. Propagation in the twenty-first century can have a high degree of mechanization, using mist systems, computer-controlled temperatures, ventilation and irrigation, but most propagation techniques can still be applied using less expensive equipment and resources, particularly if you wish to propagate smaller numbers of plants. One thing that hasn’t changed much over the centuries is that most of the actual propagation still needs to be done by a human being, so whilst the propagation environments become more computer controlled, artificial intelligence has yet to replace the skilled propagator.

Propagation – Sexual Plant Reproduction

Propagation from seed is the method by which plants sexually reproduce, through either being self-fertile or cross-fertilization. Dioecious plants, such as species in the genus Ilex (holly), have either male or female flowers on a single plant, so they require both a male and a female plant to produce seeds, which are inside the berries. Therefore, it is quite common to see a holly with no berries on next to one covered in berries – the plant with no berries is male, and the plant with berries is female. Monoecious plants, ranging from Quercus (oak) to cucurbits such as squashes and pumpkins, have both male and female flowers on a single plant, so can produce fruit or seed without having another plant nearby, although cross-pollination by another plant is still desirable for genetic diversity. However, all these plants may still require fertilization assistance, whether from pollinating insects, wind, mammals or, sometimes, a paintbrush to move pollen from a male flower to a female flower.

Male squash flower – no swelling below flower.

Female squash flower with swelling below flower.

Plants have evolved a wide variety of methods to ensure that pollination takes place, and we see many of these examples in our gardens and the wider environment, often without noticing what is happening. Plants produce fragrance, nectar and brightly coloured flowers to attract insects, and many plants, including apples and pears, rely on insect pollination, for which bees are particularly important. Others such as Salix (willow) and Betula (birch) produce catkins, which release pollen on the wind. Flowering at the right time, to attract the right pollinator, is also important, as flowering too early or too late may result in low levels of fertilization. Some orchids have developed flowers that mimic both the appearance and scent of female wasps to attract male wasps, which try to mate with the flower – unsuccessfully – but transfer pollen very effectively!

Flower Parts for Sexual Reproduction in Flowering Plants (Angiosperms)

Pollination and fertilization are two different processes, but fertilization cannot happen without pollination taking place. Pollen is produced in the male organ of the plant, which is known as the stamen, and transferred to the female organ, known as the carpel. Fertilization then takes place, and part of the carpel becomes whatever type of fruit the plant produces, which contains the seed.

Agapanthus seedlings, monocot seed leaves.

Hydrangea paniculate flower head.

Seedling showing seed leaves and first true leaves on dicotyledon.

Cross-section showing flower parts for sexual reproduction.

Some flowering plants are monocotyledons and can be identified by the parallel veining in the leaf and having no distinction between sepals and petals. They also tend to be non-woody plants, such as grasses, tulips and lilies. When the seed of a monocotyledon germinates, it only has one seed leaf.

Dicotyledons generally have distinct sepals and petals, and the leaves have a network of veins. They can be woody plants, as well as annuals and perennials, ranging from petunias to magnolias. When the seed of a dicotyledon germinates it has two seed leaves.

Terms for flower parts

Carpel

The female reproductive part of the flower, composed of the stigma, style and ovary.

Stigma

This is the top of the style and is usually sticky to catch the pollen grains. This is where the pollen germinates.

Style

This holds the stigma in position to be most effective at catching pollen. When pollination has taken place, the pollen tubes grow down the style to the ovary.

Ovary

Surrounds the ovule, which contains the female gametes. The ovary has a thick wall that becomes part of the fruit after fertilization.

Stamen

The male reproductive part of the flower, composed of the anther, filament and pollen.

Anther

This is where pollen is produced.

Filament

The stalk that holds the anther in place.

Pollen

Pollen contains the male gamete(s) in angiosperms. (In gymnosperms these are produced in male cones.)

Receptacle

Holds the carpel and the stamen at the top of the flower stem.

Nectary

Where nectar is produced. Not all flowers produce nectar.

Corolla

Collective name for petals.

Calyx

Collective name for sepals.

Sexual Reproduction in Non-Flowering Plants (Gymnosperms)

Gymnosperm means ‘naked seed’ and this sub-kingdom of plants produce seeds but from cones rather than flowers, and the seeds are only partially enclosed. The most common type of plants in this group for propagation purposes is conifers. Whilst they are rarely propagated from seed commercially, conifers such as Chamaecyparis lawsoniana can often be found selfseeded in gardens. Conifers have male and female cones and are usually wind pollinated.

Cone of pine.

Propagation – Asexual Plant Reproduction

Vegetative reproduction uses different parts of the plant, stems, leaves, roots, offsets, to produce new plants and this is asexual reproduction – commonly known as vegetative propagation. Vegetative propagation is widely used commercially to grow large quantities of plants for the wider horticultural industry, from landscapers to garden centres. Vegetative propagation gives uniformity of production, with the cuttings being genetically identical to the parent plant (although mutations do occasionally occur) and can be large- or small-scale.

Micropropagated orchid in flower.

There are several methods of vegetative propagation, including cuttings, grafting, division and micropropagation, and different species have different success rates with each method; so, commercially, the plant is produced using the most viable method. Orchids used to be an expensive plant to reproduce by traditional vegetative methods, but they are now commercially successfully reproduced using micropropagation, and are readily available almost all year round at reasonable prices.

Desirable characteristics that occur on a plant, such as variegation, can only be maintained by vegetative propagation, and it is by these methods that some of our most popular garden plants are produced for sale, ranging from Euonymus fortunei ‘Emerald and Gold’ to Begonia rex. Other methods of vegetative propagation confer other desirable characteristics, so fruit-tree cultivars can be grafted on to rootstocks that have dwarfing characteristics, as well as pest and disease resistance. This offers major advantages both for commercial orchard production, as well as the domestic garden.

The age of the plant material used for vegetative propagation is important, as juvenile material has higher hormone (auxin) levels, which increase the chances of successful rooting. Commercially, parent plants can be pruned to prevent flowering and encourage leaf growth, to provide more propagation material. Plants generally have higher hormone levels in spring, so cuttings taken at this time of year often root quickly and easily, whereas hardwood cuttings taken in winter are slower. However, there are still advantages to both methods, as softwood cuttings may require more environmental protection to help them to produce roots.

Propagation material should always be healthy, pest- and disease-free, and material should be selected from non-flowering stems to achieve high-percentage success rates. Collecting the material early in the day when it is turgid (full of water) and the temperature is cooler will also increase success rates.

Environmental Factors for Successful Propagation

Parts of a plant taken for vegetative propagation are called propagules. In the case of stem and leaf cuttings, these have no roots, so the environment must provide all the needs of the propagule until it has produced its own root system. The propagule must be able to photosynthesize and respire, or it will die before roots start to form, but also water loss through transpiration needs to be reduced, as there is no root system to replace it. The environment should encourage the propagule to produce roots as quickly as possible. Most cuttings absorb water through the cut at the base of the stem but, after a few days, the cut calluses over, and this reduces the amount of water the cutting can absorb. Often cuttings can look fine for a few days, and then drop all their leaves and fail. The humidity levels need to be kept high at the point where the base of the cutting has callused over.

The root environment

Growing medium

High-quality growing medium to allow good drainage but also moisture retention.

Water

Moisture levels should be stable, so that the growing medium does not dry out or become too wet.

Temperature

The temperature at the base of the cutting should be appropriate to the plant and method of propagation.

Air

There must be air in the growing medium to prevent waterlogging, which can lead to disease.

Nutrient

Nutrient levels should generally be low and increased once the root system is developed.

The aerial environment

Light

Adequate light is essential for photosynthesis to take place, but there should not be an excess of light, which can cause scorching. Shading can be used at times of year when excessive light levels may be an issue, and artificial lighting can be used when light levels are low.

Temperature

The temperature should be optimum to the plant and the type of cutting. A stable temperature is better than a fluctuating temperature in most protected environments, such as glasshouses. This can be achieved through ventilation and shading, as well as heating at cooler times of year.

Humidity

Higher humidity can reduce water loss through transpiration, so particularly for leafy propagules, the humidity around the cutting should be kept high. This can be achieved in a variety of ways, from automatic mist systems to putting a plastic bag over a pot of cuttings on the windowsill and hand misting them.

Air quality

The air needs to have carbon dioxide for photosynthesis and oxygen for transpiration. The air normally has the right balance, but sometimes increasing the humidity can reduce the amount of oxygen in the air, so a balance needs to be struck between humidity levels and ventilation for air quality.

Advantages and Disadvantages of Seed and Cuttings

All methods of propagation have advantages and disadvantages, and some plants will grow from almost any method of propagation. Buddleja davidii, for example, will produce seed freely, which is why they are so successful on railway bridges and embankments. They also propagate easily from softwood, semi-ripe and hardwood cuttings. Therefore, the method you would use to propagate Buddleja davidii would be the method that suits the environment you can provide and is the most economically viable. For this reason, Buddleja davidii cultivars are often commercially propagated from hardwood cuttings, as this requires low input costs in the form of heating, water or other environmental protections, whilst still producing a saleable plant quickly. However, if you want to practice your technique for softwood or semi-ripe cuttings, Buddleja davidii is an easy plant to start with, and there is always plenty of material available.

Advantages and disadvantages of propagating from seed (sexual reproduction)

Availability

Some plants produce large quantities of seed, which are easily harvested. Vegetable seeds and many annual flowering plants are grown from seed, both commercially and domestically.

Cost

Easy availability of seed keeps the cost low, so large quantities of seed are comparatively cheap. Carrot seed packets often contain 2000 seeds, at low prices.

Reliable

Many plants grow readily from seed, whether direct in the ground, such as peas and beans, or started under protection. Seed purchased from reputable suppliers must undergo strict testing.

Genetic diversity

Plants grown from seed can give rise to genetic variations, which may be desirable in plant breeding.

Pest- and disease-free

Generally, seeds do not transmit pests, diseases or viruses in the same way that vegetative material can do. There are exceptions to this, such as tomato brown rugose fruit virus (ToBRFV).

Range

There are a wide range of seeds available, with some plant species having thousands of cultivars available from seed, especially vegetables.

Storage

Seeds can remain dormant for long periods of time; in the case of some seed this can be decades. This means that many seeds can be used as required and then stored into the next season. Other seed does need to be used within a shorter time frame, carrots being an example of seed that reduces in viability significantly after the first year.

Germination

Some seed is difficult to germinate, requiring specific conditions, which may include breaking dormancy mechanisms. For example, in the natural environment, some seeds need to pass through an animal gut to break dormancy, and those conditions need to be replicated for germination on a nursery or at home.

Viability

Some plants produce seed that has low germination rates as a ratio of seed sown. This can make it economically unviable to propagate from seed.

Variation

Most hybrids and plant cultivars will not produce seed that is true to type; therefore, for many popular garden plants, vegetative propagation is vital.

Commercial viability

Many hybrids and cultivars will only be true to type via vegetative propagation. Seed propagation may also be slower to produce saleable plants, and can take much longer before becoming mature enough to flower.

Advantages and disadvantages of vegetative propagation (asexual reproduction)

Clones

Vegetative propagation produces genetically identical material, so for cultivars and hybrids, this is the only reliable method of reproduction to maintain the same desirable characteristics of the parent plant.

Commercial viability

Many plants will be saleable more quickly if produced vegetatively, whether from cuttings or grafting. Grafted plants, such as wisteria, will produce flowers at a much younger age than if seed grown.

Uniformity

Batches of plants can be produced at the same time, which means that they will be ready for sale at the same time.

Normal reproduction method

Vegetative reproduction may be the plant’s most successful natural method of reproduction, so replicating this is the easiest form of propagation. Strawberries are produced from runners both naturally and commercially. Bulbs are another example, where they produce bulblets or offsets naturally.

Preserving cultivars

Some plants, including many rose and fruit-tree cultivars, do not grow well on their own roots, so require grafting or budding on to more suitable rootstocks.

Pest and disease resilience

Cloned plants will all be equally susceptible to new pests, diseases or viruses. This can cause major environmental issues; for example, elm trees naturally reproduced from suckers, so did not have the genetic diversity to withstand Dutch elm disease.

Quantity

Whilst high volumes of vegetatively propagated plants are produced commercially, it requires more space and planning for the material to be available from the parent plants, in comparison to the volume of seeds that some plants can produce.

Labour

Propagation requires a skilled workforce to be able to produce the volume and uniformity of material.

Expense and environment

Generally, the environment required by vegetative propagation material is more expensive than seed sowing. Commercial glasshouses and propagation units are also high maintenance, as well as having ongoing utility costs.

Lack of diversity

Cloning plants does reduce the likelihood of genetic mutation or adaptation, which can result in new cultivars.

As you can see from the last two Tables, the selection of the right method of propagation for a plant depends on a wide variety of environmental and economic factors, which apply both to commercial production and the domestic situation.

CHAPTER 2

PROPAGATION SYSTEMS, ENVIRONMENTS AND MATERIALS

Greenhouses, polytunnels and cold frames are all environments protected from the weather. This protection offers advantages for propagation, as the environments can be controlled, unlike the weather. Polytunnels and greenhouses can be heated to maintain constant air temperatures or controlled so that the heating only comes on when the temperature falls below a pre-set minimum, such as a frost-free setting. Heating large, open and non-insulated spaces like this can be expensive, so using a smaller, protected environment within a glasshouse or polytunnel is commonplace, both commercially and domestically. Greenhouse heaters for domestic use are readily available, with a range of thermostat controls, but the electric ones need a safe source in the greenhouse. Systems using either oil or electric are available for larger commercial glasshouses, as well as solar panel systems.

Setting up a smaller protected environment within a larger one offers versatility should the environment need changing from one crop to another. Some plants may be easily propagated without bottom heat, some root or germinate much more quickly with both bottom heat and high humidity, so the aerial environment around the propagules needs regulating. This could be in the form of a propagator lid in a domestic greenhouse or it could be a closed environment on a larger scale, created with metal or plastic hoops over a greenhouse bench, covered with clear plastic sheeting.

Maintaining humidity around cuttings helps to reduce water loss through transpiration. Softwood cuttings and more difficult-to-root subjects may require high levels of humidity over longer periods of time.

Ventilation is also an important consideration within a protected environment because it may become extremely hot, and air circulation becomes necessary. Opening doors and roof vents is usually sufficient in a domestic setting, but air fans can also be used in commercial environments to circulate air and cool the temperature.

Light, Shade and Temperature

Whilst plants require light to photosynthesize, propagules need protection from very high light and air temperature levels, as they have no root system to support them. Even when they are rooted, the root zone is not going to be able to support high transpiration levels. Shading can be painted on to domestic greenhouses, but shade netting can be used in both domestic and commercial settings. Shade netting can be purchased at differing shade densities, ranging from 10 to 80 per cent, although 20 per cent shading is suitable for most propagators’ requirements. This will also cool down the area in the warmer months. Thermal screens are also used commercially for insulation in cooler months, and to provide shade for crops in the warmer months. These can be manually operated, or computer controlled with temperature and humidity ranges set at a pre-determined level, depending on crop requirements.

Shade netting can provide different percentages of shade for different conditions and is used for protecting soft material from direct sunlight.

Humidity

Humidity can be monitored in commercial systems through the use of sensors, which can be regulated to the specific requirements of a particular crop. On a domestic scale, humidity thermometers are readily available.

Mist Systems

Mist systems use overhead irrigation nozzles that produce fine water droplets – the size of the droplet will vary depending on the nozzle size. The aim of mist units is for the water droplets to increase humidity and for the water to evaporate from the surface of the leaves on cuttings, thereby reducing transpiration.

A mist unit provides optimum humidity to maximize rooting percentages, usually in conjunction with bottom heat.

Mist systems can be used on a wide range of vegetative propagation material, and is beneficial for difficult-to-root species, but will also speed up rooting of other subjects. Uniformity of rooting and increased rooting percentage are also benefits of using mist systems.

Commercial mist systems are usually computer controlled using environmental sensors to activate the mist frequency. The environmental sensors, such as electronic leaf sensors, monitor the humidity at the level of the cuttings. Light, temperature and humidity sensors are also used to control shading and screening.

Bottom heat is usually required with a mist system, as well as a well-drained propagation bed to quickly remove the excess water. Mist systems can be set up over benches or the beds can be on the floor, but there should be a sand bed underneath to allow for sharp drainage and capillary action to the growing media used for the cuttings.

Not all plants require a misting system – they are an expensive outlay but they can significantly speed up rooting times and increase the percentage of cuttings that root successfully.

Closed and Open Mist

Closed mist is when an area within a glasshouse or polytunnel is covered by a plastic film using hoops or similar structures over the nozzles. The advantage of closed mist is that it can enable sections of a propagation unit to be used as needed, which means valuable resources such as water are not wasted.

Different mist nozzles give varying droplet sizes.

Computerized pump systems provide the water pressure necessary for a mist unit.

Open mist systems can be used in polytunnels, glasshouses and outdoor beds, and are generally used on easier-to-root subjects – in the case of outdoor beds, they may not require bottom heat – and on high-volume material where entire areas can be used for one crop.

It is possible to install mist systems in domestic greenhouses, and domestic kits are available, although for small greenhouses it is unlikely to be costeffective.

Fog Systems

Fogging units use much smaller water droplets than mist units – generally less than 15 microns – and are able to keep the humidity in the unit at in excess of 95 per cent. They create a thin layer of moisture over the leaf, which reduces transpiration and cools excessive air temperatures. Another advantage of fogging units is that they do not saturate the growing medium as much as mist units, so heating costs for bottom heat are reduced. Having less water going to the rooting medium also reduces the chances of cuttings rotting or damping off, and fogging uses less water resources than mist. They are expensive systems to install, however, so may not be cost-effective for most species that root well in closed mist systems.

Heated Bench and Polythene

Many species do not require mist or fog for successful rooting, and the use of bottom heat under a low polythene cover with a thickness of 20–25 microns. Hoops and polythene can be used in a closed mist system on heated benches or heated floor space. Polythene can be clear or white, depending on the shading requirements of the crop and the time of year. This has the advantage of being cheaper than a mist unit and uses less water, although it may not be suitable for more difficult-to-root subjects.

Soil-warming cables can be used to provide bottom heat for propagation.