Propagation
Numerous books have been written on the propagation of trees and shrubs. For the
amateur, who only wants a few plants of each type, the following methods are the most
suitable.
Seed
Where seed has been collected in the wild, a species will normally come true, but
will show something of the variation of that wild population. The gardener may be
able to select an especially good form from his seedlings. Where seed has been
collected in a garden, the seedlings are likely to be hybrids, unless the flowers
were hand-pollinated. Seeds of hardy shrubs are best sown in autumn, as many require
a cold period to induce germination. Many, notably some Rosaceae or Viburnum,
do not all germinate in the first or even the second year after planting. More
tender shrubs, from climates in which there is little frost, or growth is mainly
in winter, germinate immediately they are sown, and so are best planted under cover
in early spring. Most very small seeds come into this group. Large nut-like seeds
generally need sowing as soon as possible, as most of them, e.g. Buckeyes or oaks,
germinate as soon as they fall, and quickly die if they dry out. They should be
stored cool and moist. Seedlings are best potted as soon as possible, so they have
made good growth before their first winter: the larger they are, the more likely
they are to survive.
Seedlings
Seedlings are a good way of bringing back trees and shrubs from the wild, if seed
is not available. The smaller the seedlings, the better they will travel; they can
be kept for several weeks potted in a polythene bag held with a rubber band, either
in soil or in moss. Be careful not to let them dry out, keep them cool, and give
them as much light as possible. Self-sown seedlings may also be collected from the
garden and potted up or put into a nursery.
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Cuttings and Layering
These have the advantage that the new shrub is genetically identical to the parent
bush, so a good form or named clone may be propagated. Cuttings are taken either in
summer, 'softwood cuttings', or in autumn or winter after leaf fall, 'hardwood cuttings'.
Hardwood cuttings should be about 30 cm long, and be wounded at the base or have a
heel of the old wood. They are put in sandy soil or compost, outdoors or in a shady
frame, in autumn and begin to grow both shoots and roots in spring.
Softwood cuttings are taken as soon as the wood has begun to harden; too soon and
the shoot will collapse; too late and the shoot is woody and will not form roots
easily. Mist propagation has proved the greatest help in rooting softwood cuttings,
and mist combined with bottom heat and the right degree of shading will induce
many of the most difficult subjects, such as deciduous azaleas, to root. They should
be then kept growing as long as possible into autumn, so that they survive the winter.
An ideal compost for rooting most cuttings consists of perlite and sphagnum peat
in about equal proportions; the size of the perlite used depends on the thickness
of the roots, the thicker-rooted plants needing a coarser compost.
Layering was formerly used commercially for propagation of difficult subjects
such as Japanese maples, deciduous azaleas etc., but has now been replaced by
mist propagation or grafting. Layering can still however be very useful for
amateurs, especially for rejuvenating old plants of Rhododendrons or for difficult
plants such as Magnolias. The layer can remain attached for several years after it
has rooted and so be larger by the time it has to rely on its own roots.
The production of new species by Colchicine treatment Several new plants, e.g.
Forsythia, 'Beatrice Farrand', have been produced by treating a hybrid between
two wild species with Colchicine. Colchicine, an alkaloid derived from the bulb
Colchicum, has the effect of preventing the formation of cell walls between
dividing cells, so that polyploid cells are formed. A sterile hybrid between
two species may become fertile, with double its original number of chromosomes,
if a growing point is treated with lanoline containing colchicine, or if a seedling
is treated. The new fertile plant is a new species, able to breed true from seed.
Several wild plants have evolved in this way, but of course, without Colchicine.
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