Guide to Plant Propagation
There are two ways of obtaining new plants. You can raise them yourself or obtain them from someone else. Buying plants from garden centres or nurseries offers you two big advantages. First, with annual bedding plants and vegetables especially, you will have been spared the time, trouble and expense involved in the sowing, germinating, pricking-on and hardening-off of a large number of plants. The downside to this is that you will pay proportionately very much more to offset the nursery’s own costs and provide them with a profit. But, on balance, unless you are very enthusiastic and have a great deal of time to spare, buying the common bedding plants that are required in relatively large numbers makes a good deal of sense for many gardeners.
For those plants required in small numbers and for uncommon types that are less likely to be stocked at garden centres, raising them from seeds represents a better if not the only option. The situation with vegetables is rather different, for most are sown directly outdoors and, in general, those few types that are better raised in pots for transplanting (runner beans, for instance) are required in small numbers that should be within most gardeners production capabilities.
The second big advantage in buying plants is of special importance with perennials. Here, of course, the value of the ‘head start’ is much greater, but the choice in varieties is of even more significance. Many of the best types of herbaceous perennials and shrubs don’t produce seeds; having double(and hence no stamens), they have lost the ability to do so. Even those with single or semi-double flowers are generally complex hybrids and the offspring don’t ‘come true’ from seed. Thus, all of these types must be propagated by vegetative means, generally by cuttings, and so buying plants initially is your only way of obtaining them.
But once you have bought a plant or even raised one from seed to maturity, vegetative methods can be used to multiply it further. All of these techniques make use of plants’ natural abilities to regenerate roots or shoots. There are, however, significant biological and practical distinctions between the two reproductive methods. While a seed-raised plant can be very different in genetic make-up from its parents and so introduce variation, plants propagated by vegetative means, either naturally or using artificial techniques like division, cuttings andare genetically identical to the parent.
To introduce another term, they are clones. Both methods of propagation are important to commercial horticulture and to home gardeners.
Raising plants from seed
Naturally, of course, all plants shed their seeds outdoors and it is there that they germinate, often following a period of dormancy or rest. In gardens, however, we can advance the season, give hardy plants a head start and extend their flowering or cropping by sowing their seeds indoors in warmth. Many garden plants, more-over, are native to climates warmer than ours and if they were to be sown directly outdoors, they wouldn’t experience a long enough period of adequate warmth to enable them to germinate, mature, flower and/or fruit before low temperatures at the end of the season slowed them down or frost killed them. They must be sown under protection.
Plants that originate in warmer climates but can thrive perfectly well outdoors in cool areas in summer, although not in winter, are called half-hardy. Some of them are genuine annuals but most are really perennials. When grown in cool gardens for one season only, however, they all lend to be described as half-hardy annuals. The technique of sowing seeds in indoor warmth before moving the plants outdoors, once the danger of frost has passed, is called the half-hardy-annual technique.
Sowing seed indoors
To raise seeds successfully indoors, some basic equipment is essential. Fresh compost, either a ‘seed-and-cutting’ or a ‘multi-purpose’ formulation, should be purchased each year. A propagator will be needed and there is a range of options from a simple plant-pot covered with a plastic bag to a purpose-made, electric thermostatically-controlled unit. To begin, I recommend you buy several standard plastic seed trays and one or more rigid plastic covers with adjustable manual vents. These are economical and versatile, as the seed trays can be used to hold smaller containers like half-trays or pots and the plastic covers can be easily moved from one seed tray to another.
Seeds require warmth to germinate rapidly and evenly. The ideal germination temperature varies for each species but, in practice, you should aim to provide three broad temperature bands: low, less than 18.5°C (60°F), medium, 15.5-21.5°C (60-70°F) and high, above 21°C (70°F). A window ledge will suffice for seeds that require a medium temperature provided you move the propagator on to a nearby table before drawing the blinds or curtains at night, as the air temperature between the window and drawn curtains can drop dramatically. Seeds that need a high temperature can be placed in an airing cupboard for a brief period but remember to cheek daily and move the propagator into the light as soon as most of the seedlings have emerged.
You may prefer to have a heated propagator, but do choose one where the individual seed tray or pots can be removed from the warmth once the seedlings have germinated. Where there is no mains electricity supply nearby, use low voltage heating mats as a heat source under the seed trays. A transformer plugged into a mains socket in the house and a low-voltage wire run to the greenhouse is all that is required. However, my ideal system for the greenhouse that does have mains electricity, is a heated sand bench: a large box, about 15cm (6in) deep and filled with sand in which a thermostatically controlled healing cable is buried.
You can either make your own or purchase a kit. Take care to follow the instructions for calculating the area of bench needed for the lengths of cable. The cable should be laid in a regular pattern for uniform heating and no cable should cross over itself. A heated sand bench is very heavy, so make sure your staging is sturdy enough to cope with the extra weight. The commonest mistakes made when sowing seeds are to have the compost too loose, too wet or too dry, or to sow the seeds at the incorrect depth. Fill the propagator tray with loose compost level with the top and then firm this down evenly (a small block of wood about 5 x 10cm/2 x 4in makes this very easy) so that its surface is approximately 1cm (1/2in) below the lip. Water the compost gently and leave the tray for about one hour to drain before sowing the seeds. There are several methods of sowing, dictated principally by seed size. Very small seeds sold loose in packets (lobelia or fibrous-rooted begonia, for instance) may be mixed with a small amount of inert, coloured ‘filler’ such as line brick dust and scattered over the surface of the compost. Because of the difficulties of sowing these tiny seeds evenly, seed companies have tried various devices, such as supplying them in small, pencil-like tubes, in order to make the task easier, while proprietary sowing aids, including a useful suction device, are also available.
Most tiny seeds such as these should not be covered with compost because their food reserves are inadequate for the emerged seedling to reach the surface, but they can usefully be covered with finely sieved vermiculite to prevent drying-out. A few medium-sized seeds, such as those of primulas, must also be sown on the surface because darkness inhibits their germination. Almost invariably, you will be advised to sow seeds ‘thinly’ and, in general, this means that the space between individual seeds should be between 0.5 and 1cm (1/4 and 1/2in), although you may well need to thin out the seedlings slightly after emergence. The distance between rows should be 2-3cm (1 – 1-1/4in). After sowing, carefully push the compost back over the row and firm it with the wooden block as before.
Seeds, such as those of sweet peas, that are large enough to be picked up with forceps are best sown individually by pushing them into the compost to appropriate depth. With most very large seeds, the resulting seedlings are also large and the seeds are better sown directly into small pots of compost. This technique is also valuable with plants such as cauliflowers orthat resent the disturbance brought about by pricking-on. Sow two seeds in each pot and, if both emerge, pull out the weaker.
Increasingly, gardeners are turning to individual modular inserts comprising several small chambers to be filled with compost and placed within a seed tray, partly for the plants that resent upheaval but also to cut down on the work involved in pricking-on generally.
Once the majority of the seedlings are showing their seed-leaves above the compost, some ventilation is required. Open the propagator vents half-way to increase ventilation as the seedlings elongate. By the time the first true leaves have expanded, the cover should be removed. At this stage the seedlings need regular attention: they need light but not direct hot sun and they need watering little and often.
Most bedding plants should be pricked out into a second tray where they should be spaced 3cm (1-1/4in) apart each way. A small dibber, pencil or plastic label is useful for making planting holes. Larger seedlings can be pricked out into small pots of potting or multi-purpose compost. When pricking-out, take care not to damage the stems and roots; always provide support under the roots and compost.
Perhaps the most critical stage of all is hardening-off, the process through which the plants are encouraged to produce more resistant tissue that will tolerate outdoor temperatures. This is most easily done by using a cold-frame. For the first week, leave the frame cover half open in the day time but closed at night and in the second week, keep the cover fully open during the day and half open at night. If you don’t have a cold-frame, put the trays and pots outside during the day and bring them under cover at night. You need to take your local conditions into account, however, and it is also worth having some garden fleece and newspaper to provide protection if late frosts are forecast.
For many garden plants, sowing seed directly outdoors into the garden , in positions where they will grow (the hardy annual technique), makes more sense than using the half-hardy annual method. Many of those annuals that originated in cool climates will grow quickly enough to flower or crop satisfactorily within the season of sowing. A few types of plant are sown outdoors, but closely together into a seed bed from which they will be transplanted later: this is because they grow fairly slowly and would thus occupy valuable growing space for a long, unproductive period.
Whether seeds are to be sown directly into their final growing positions or into a seed bed, the soil preparation is the same. The area will usually be roughly dug and organic matter incorporated in the autumn and then left with fairly large clods overwinter. By the spring, the winter rains and host will have broken these down but the soil will still be in a lumpy and uneven state, almost certainly with some weed growth. It should then be dug again as soon as the land begins to dry out in the spring. About one week before sowing, the area should be raked to remove any remaining large clods and at the same time, an appropriate fertilizer should be scattered over the soil and incorporated into the upper few centimetres. Rake alternately in directions at 90 degrees to each other to obtain a level surface.
Spring sowings are invariably more successfully if the soil is warmed gently beforehand. Cloches will do the job admirably, and can be replaced over the seeds after sowing. Their only drawback is they are easier to use with seeds sown in rows rather than broadcast and they need to be well secured against wind. An alternative is to use plastic sheeting over the surface of the soil before sowing, secured in place by digging it into a shallow trench. Use black or white, but not clear plastic as this will encourage weed growth.
Most vegetables are sown in rows, the spacings and sowing depths varying slightly with each vegetable. Use a garden line to mark out a straight line, then cane out the seed drill with the end or a bamboo cane or the edge of a draw hoe. Sow thinly, either from the packet or from your hand, then cover the seed with soil and press down gently using the back of a rake. Summer-flowering annuals are usually sown in drifts or patches rather than in straight rows.
If you need to transplant seedlings from a seed bed to their final flowering positions, use a hand fork to remove the plants and a trowel for digging the planting hole. Plant at, or slightly greater than, the depth at which the plant was growing previously. Firm in with your hands and apply liquid fertilizer immediately afterwards.
Special seed techniques
Most seed will germinate readily if sown correctly, but a few species are tricky as the seed has an in-built dormancy mechanism to help it survive through periods of adversity. I’ve listed below the commonest of the techniques that may help to overcome seed dormancy.
Stratifying: Often used for tree, shrub and alpine seeds. Place the ripe seeds (or fruits) in shallow pans of coarse sand, bury them about 2cm (3/4in) below the surface and cover the pans with fine-mesh netting to exclude pests. Leave the pans outdoors in a sheltered place over winter. In spring, place them in the slightly warmer conditions of a cold-frame and germination should then begin slowly and erratically.
Nicking: Some seeds, like certain sweet pea varieties, have an exceptionally thick outer coat making it very difficult for seedlings to emerge. By nicking or cutting the seed coat with the tip of a sharp knife on the side opposite the ‘eye’ the seed coat can be encouraged to degrade quicker. Where the seed is too small for the seed coat to be cut by a knife, place the seeds in a screw-top jar with a cylinder of sandpaper and shake hard.
Soaking: Seed coats can often be softened by soaking. Try using warm water in a vacuum flask overnight. Seed that has sunk after soaking is more likely to be viable than any still floating on the surface.
Firing: For some plants from the tropics or sub-tropics, a high temperature is often needed to break dormancy. When seeds originate in areas where bush fires are common, germination may be induced by covering the seeds with dry straw and setting it aflame. If you want to try this do make sure you use a terracotta pot, not a plastic one.
Collecting and storing seeds
There are advantages, apart from satisfaction, in collecting seed from plants in your own garden. With rare or unusual species, it may be impossible to obtain seed commercially and, if the plant happens to be an annual, saving the seed from existing plants provides you with the only way of perpetuating your stock. Many of the gardeners most successful at horticultural shows routinely save seed from strains of the particular vegetable specialities that they have kept season by season.
But against all of this there must be set some disadvantages. Many of today’s garden plants are F1 hybrids and they certainly have many merits: they are vigorous, large, strong growing and ripen uniformly. But they are also expensive because of the labour-intensive method by which they are produced: a cross between parental plants must be made afresh each year. Seed saved from F1 hybrid varieties, therefore, is useless as the resulting offspring will be a complete and unpredictable hotchpotch.
Even with varieties that are not F1 hybrids, there is no guarantee that the seedlings you obtain will be identical to their parents. Plants that naturally are self-pollinating will almost invariably come true but those that naturally cross-pollinate may well have been fertilized by related plants growing nearby, although this in itself can offer the exciting prospect of something new and worthwhile turning up.
It is important to make preparations for collecting seed just before it ripens and spills on to the ground. Cut off the heads of plants such as lupins, poppies, peas and beans that produce dry seeds and hang them upside down in a warm but well ventilated and dry place, with a paper bag tied over them. The seeds should then tall naturally into the bag. Seeds, such as those ofand that are produced within soft and fleshy fruits, should be separated from the surrounding tissues and washed thoroughly to remove any germination-inhibiting chemicals that may be present. Spread them to dry at room temperature before storing them in small paper envelopes.
Both for seeds that you have collected yourself and for commercial packet seed (once the metal-foil packet has been opened), storage conditions are important if the viability is to be retained. The two factors that diminish the life of seeds are high temperature and high humidity. The ideal storage conditions, therefore, are provided by placing your seed packets inside a screw-top glass jar together with a small sachet of silica gel drying agent. The jar itself should then be placed in a refrigerator. Of course, even stored in this way, seeds will not last forever but, in general, small and fairly hard seeds such as those of brassicas.or poppies will maintain their viability for several years. Larger and more fleshy seeds, such as peas and beans, may only last for two years at the most.
The simplest method of multiplication is by division; an old gardening quip but a true one. Large clumps of almost all types of herbaceous perennials (but not woody plants) can physically be pulled apart and the smaller pieces replanted. The age at which they should be divided depends on their rate of growth but in most instances, once the crown reaches 12-15cm (5-6in) in diameter, it can be divided. The best times of year to do this are autumn or early spring, the latter being preferable for plants that are less hardy or that have large, fleshy tubers or rhizomes.
The procedure is straightforward enough. Dig up the mature clump with a fork and pull if first into two, then more pieces; if possible, do this by hand, but if not, by inserting two forks, back to back, and levering them apart. Never use a spade for this will sever and damage the roots. From a clump of about 15cm (6in) diameter, it should be possible to obtain approximately ten new plants, but always tear off and discard those parts that lay in the centre of the original crown; these will be degenerate and never give rise to vigorous new growth.
After division, the most important method of vegetative propagation is taking cuttings and, in one form or another, this technique can be used with almost every type of plant, ranging from tiny alpines to forest trees. Very few species can’t be induced fairly easily to form new root tissues, most of the exceptions being slow-growing plants with very close grained wood or those that produce copious amounts of latex.
Cuttings fall neatly into different categories, depending on the time of the year that they are taken and the part of the plant from which they are cut. A few general principles, however, apply to all.
Cuttings should almost always be removed from the parent plant with a clean cut made close to a bud. The exceptions are heeled cut-lings and inter-nodal cuttings. While rooting powder containing growth-promoting hormones is not essential for all types of cutting, used correctly it never does any harm. And because the powder also generally contains a fungicide to prevent rotting, it is sensible to use it routinely. Moisten the freshly-cut end of the cutting, dip it in the powder (always buy fresh each season) and then knock off the excess.
All cuttings, with the exception or hardwood cuttings, should be rooted (or ‘struck’) in a covered chamber, either a propagator as used for seed sowing or a covered cold-frame. It is very important to maintain a moist atmosphere around the cuttings for they will otherwise lose water through their leaves at a time when, lacking roots, they are unable to replace it from below. Even with a covered propagator, therefore, you should pay careful attention to the moisture content of the rooting medium and use a hand sprayer to mist over, the cuttings regularly. The cold-frame can also be used for hardwood cuttings, although I prefer to root these in a sheltered spot in the open garden, inserting the shoots in a narrow Y-shaped trench in which sand has been layered.
Inducing new roots to form on any type of cutting is always helped by good aeration in the rooting medium. For this reason, a soilless compost alone is generally unsatisfactory and I almost always use a two-layered system, half-filling the propagator tray or pot with firmed soilless potting compost and then topping up with a layer of horticultural sand. For leaf cuttings, the layer of sand may need to be shallower than the compost layer. Each cutting is pushed into the sand until its lower end is at the sand-compost junction. Thus, when the new roots form, they immediately have available a good supply of nutrient-rich growing medium.
Principle types of cuttings
Softwood cuttings: These are cuttings taken early in the season, while the tissues are still soft and sappy. The ideal length is about 10cm (4in); they should always be handled carefully as the tissues, being soft, are readily damaged. Pipings are softwood cuttings 5-7cm (2 – 2-1/2in) long, taken from pinks and carnations where short lengths of stem tip are pulled away from the remainder, telescope fashion.
Semi-ripe cuttings: There’s no strict definition of when a cutting is ‘semi-ripe’ but I think that the answer lies in the degree of springiness. If it bends readily when you twist it between your fingers, and then stays bent or oozes juice, it is still a softwood cutting. If it bends readily, but then springs back, it is semi-ripe. If you can’t readily bend it by simple finger pressure, it is already hardwood. Most plants produce semi-ripe cuttings during the second half of summer and the ideal length is about 10cm (4in). There are two main ways of taking semi-ripe cuttings.
The first is as normal stem cuttings, formed when a length of shoot is cut from the end of the stem and then cut off cleanly at a length of 10cm (4in).
The second is as a basal cutting, where a side-shoot is selected that is already about 10cm (4in) in length and then pulled or torn from its parent stem to leave a small piece of the older stem at the base to form a ‘heel’. When taking cuttings for the first time from a new plant, I take about half of each type, although there are some types of shrub that clearly are trying to tell you something, for it is almost impossible to pull away a basal heel: they simply snap off straight.
Inter-nodal cuttings: These are exceptions to the rule that cuttings should be taken close to a bud. They are used for a very few types of plant (clematis are the most important) which have their greatest concentration of natural root-promoting hormones between, rather than at the nodes from which buds arise. The cuttings are, therefore, severed midway between the nodes although they should still be pushed into the compost up to the level of the bud.
Hardwood cuttings: These are cuttings taken of wood that has already become fairly tough and woody; ideally during the current year. It is a mistake, however, to imagine that tough old wood from ancient branches can form hardwood cuttings. Timing is less critical and they can be taken at any time between autumn and winter. The shoot should be cut to a length of about 25cm (10in) and inserted to about two thirds of its length in either a cold frame or a V-shaped trench lined with sand in a sheltered spot in the open garden.
Leaf-bud cuttings: There is a limited range of shrubs that can successfully be propagated from single buds but they do include some very important ones: camellias,, grapevines, clematis, ivy and mahonias. Early spring and autumn are the best times for leaf-bud cuttings although, of course, cuttings taken in spring make use of last year’s buds; summer and autumn cuttings make use of those of the current season. The bud must be mature and healthy and the cuttings should be trimmed to leave approximately 1cm (1/2in) below each bud and about 25mm (1in) above it. The lower part of the stem and the lower half of the bud itself should be buried in the rooting compost. I find that bud cuttings are best with little or no bottom heat. They should be covered but kept ventilated and placed somewhere sheltered.
Root cuttings: Herbaceous perennials with thick storage roots are especially amenable to being propagated by root cuttings. Much the best way is to dig up the parent plant, wash off the soil and select healthy, white roots from close to the main crown. Cut the roots into pieces about 3cm (1-1/4in) long but make a straight cut across the top (the plant end) and a sloping cut across the bottom to help distinguish which is which. Push them into pots of compost such that the top of each cutting is flush with or just below the surface. Cover them in the usual way and incubate them, ideally with a little bottom heat. They should root within about six weeks and should be grown on until stems and leaves have formed.