Propagation of Ornamental Shrubs
Propagation of Shrubs
Like all plants, shrubs may be propagated in two ways: by means of seed and by vegetative processes — cuttings, division,, grafting and budding.
Propagation from Seed
Propagated by this means are those shrubs that produce a large number of viable seeds which under favourable conditions yield a large number of seedlings. Young plants raised from seed are hardier than those acquired by vegetative means and also have better root anchorage. A disadvantage, however, is that seed-raised plants are not always true to type, that is, they do not always possess the same characteristics as the parent plants. Some ornamental varieties or cultivars cannot be propagated from seed at all if their special characteristics are to be preserved in ensuing generations.
Gathering and preparing seeds for sowing
The seeds of most species of ornamental shrubs are gathered when ripe. In some, however (particularly members of the family Rosaceae), substances that delay germination are developed in the ovary wall during the ripening process and the fruits must be gathered before the seeds are fully ripe. After they have been harvested, the fruits are spread out in a thin layer in a dry and well-ventilated place (not in the sun) where they are then left to dry. In some shrubs the fruits are in the form of pods or capsules which split open and allow the seeds to fall out. Sometimes the outer covering must be removed by hand. In the case of fleshy fruits the task is more complex and laborious. The seeds may be separated from the fleshy covering by soaking the fruits in warm water, crushing them and washing the seed out, or cutting them open and removing the seeds by hand.
Some seeds are sown immediately after they have been cleaned, washed and dried, usually into special pans. Such is the case with the seeds of actinidia, akebia, berberis, buddleia, caragana, chaenomeles, colutea, cotinus, cytisus, erica,, forsythia, , , , hydrangea, , , , lonicera, rubus, sambucus, syringa and .
In the case of other shrubs it is better to store the seeds for the winter and sow them the following spring. The seeds are stored either spread out in thin layers or in sealed packets (polythene is not suitable for this purpose) in a cool place with temperature between 6 and 10° C (43 to 50° F) and uniform humidity. They are usually sown in early spring, sometimes even at the beginning of the year. This applies to species of berberis, calluna, calycanthus, caragana, chaenomeles, clethra, colutea, deutzia,, hippophae, and vaccinium. As a rule these are species whose seeds are usually sown immediately after harvesting, although it is better to sow them the following spring.
The seeds of another group of shrubs are stratified during the winter and then sown the following spring. This is done by alternating in a pot layers of seed with layers of sharp sand, starting and finishing with a layer of sand. The stratified seeds should be stored in a spot with a temperature between 2 and 6 °C (36 to 43°F); it must not drop below zero. During the winter they should be watered with care (it is recommended to water slightly and more frequently) and checked every now and then for signs of mould and premature or non-uniform germination, in which case water should be applied accordingly. Seeds are stratified in the autumn and sown in spring in the case of the genera actinidia, chionanthus, cornus, cotinus, cytisus, daphne, lonicera, magnolia,, prunus and ptelea.
In the case of some plants the seeds are first stored for the winter, stratified early in spring and then sown in spring, but this is a less common procedure (daphne, cornus). More often the seeds are stored for the winter, stratified in spring and sown in the autumn of the same year. Such is the procedure for actinidia, clematis, cornus, colutea, cotinus, cotoneaster, crataegus,, exochorda, , halesia, , ilex, lonicera, , and viburnum.
Besides, chemical means (hydrochloric acid) and mechanical means (scarification) are used to break the seed coat and hasten germination. However, these are methods that are rarely used.
From what has been said, it is evident that various methods may be used for the pre-sowing preparation of seeds of shrubs of a single genus and the choice of the best method should thus be governed not by the genus but by the species.
Sowing and pricking out
Thewhere seeds are to be sown both in beds and in frames should be made ready well beforehand — in summer for seeds to be sown in the autumn, in autumn for seeds to be sown the following spring. Soil in garden beds should be forked over and left to rest. Then leafmould, a mixture of peat and sand or sand and leafmould, should be worked in as required and the top layer raked to a fine crumb structure.
When sowing in a frame the soil is prepared in the same way. Depending on the species, seed can be sown either in a partly heated or cold frame. In a partly heated frame germination is more rapid but the first seedlings may emerge while there is still danger of spring frosts and they must be provided with a protective cover.
Seed is sown either broadcast or in drills, the latter method being the better because it makes for easier hoeing and weeding. After sowing the seed is covered with a layer of soil no more than twice the thickness of the seed. Stratified seeds are pricked out into beds or frames after they have germinated, that is, as young seedlings. Seeds and seedlings should be watered, shaded, hoed and weeded; those in frames should be ventilated, and those out in the open should be covered with mesh as a protection against birds.
Seedlings are left in the frame or seed bed for one or two years, according to the species. Taller, more vigorous species are transplanted the following spring, less vigorous ones the following autumn or the spring after that. They are planted out in rows spaced 20 to 25cm (8 to 10in) apart, with a distance between each seedling of 5 to 10 cm (2 to 4 in), and are generally kept in the nursery from two to four years.
The container method of growing seedlings has recently gained widespread popularity in large nurseries. Instead of being put in beds in the nursery the seedlings are planted in whalehide, plastic or tin containers. These are filled with a well-mixed growing medium such as John Inns potting mixture or a soilless compost, the components of which include artificial fertilizers and are varied according to the species to be grown in the container. The containers with the seedlings are placed side by side in the nursery and kept well watered, since they cannot absorb moisture from the soil. The plants produce a wealth of roots when grown in this way. When planted out in their permanent site the plants should be carefully removed from their containers so that the soil ball round the roots of each plant remains intact and the roots are disturbed as little as possible while being placed in the planting hole. In this way the plants receive no setback, and seedlings raised in containers may be planted out at any time during the growing period. This is a great advantage, particularly in the case of mass plantings.
Seedlings are pruned for the first time either shortly before being moved to their permanent site or immediately after. Pruning consists of removing undesirable shoots, ones that will cause an untidy appearance, or else cutting them back so that the shrub will be nice and dense at the base.