Agriculture Reference
In-Depth Information
Methods of vegetative propagation
All vegetative propagation is a form of division and
has been exploited by mankind for a very long time.
In many cases little more than breaking up the plant
or taking the natural propagules is involved e.g. bulbs,
corms, rhizomes and runners. However, there are
several techniques which are less straightforward
including taking cuttings of many kinds.
Divisions
Most gardeners will be familiar with dividing
herbaceous perennials
such as hardy geraniums,
asters,
Achillea
. This usually arises because the shoots
become overcrowded and the thick clumps that
develop often become woody or have bare centres
(Figure 11.8).
Iris
has
rhizomes
(see p. 92) which lend themselves
to dividing. Normally about 10 cm of non-woody
rhizome is cut off with leaves that are reduced to a
third to minimize water loss and wind rocking when
re-planted. Borders are rejuvenated by carefully lifting
the clumps ('crown') of herbaceous plants preferably
with a generous ball of soil. Most of the soil is teased
off carefully to maximize the amount of root for the
new plant. Pieces from the clump are removed each
with a shoot, bud and roots. The unsightly woody
or bare middle section is discarded. Whilst some
specimens can be pulled apart by hand, some are so
tough as to require knives. Splitting some specimens
can be very difficult and, if necessary, they can be
prised apart with back to back forks for good leverage.
Others such as
Phormium
might need an even more
rugged approach. However, care should to be taken
not to cause damage to the propagating material.
The fleshy rooted specimens such as
Hostas
need to
be treated very carefully
.
In general, division can be
undertaken in the autumn as plants die down, but is
usually better done in the spring as the new shoots
appear. The younger sections with strong shoots can
be replanted in prepared ground (normally with many
surplus pieces to give away or sell). This should be
done before the roots have dried and the plants then
watered in. However, there is usually a best time and
method for each type of plant and this is summarized
on the companion website.
Some species such as
Bergenia
flower in the early
spring so it is best to lift the plants in mid-winter and
remove the rhizomes. These should be washed and
the dormant buds found. Sections with a bud can be
taken off and rooted by burying them horizontally to
half their depth in potting compost in trays. As for
propagation generally, it is advantageous to provide
Figure 11.8
Iris with bare centre
'bottom heat' by standing the trays on soil warming
cables. The plants are not ready to go out until
the fibrous roots have emerged from the bottom
of the tray and have been '
hardened off
'. This
acclimatization to the outdoor temperature is usually
achieved by two or three weeks in a cold frame during
which time ventilation is gradually increased until the
covers are left off even through the whole night.
Layering
Layering
is method of propagation that uses the
ability of some plants to produce roots from stems
still attached to the mother plant (see companion
website).
Simple layering
is a cheap means of producing large
new plants quickly, and in the nursery trade, it remains
a good way to produce high-value specimens such
as
Hamamelis
. It is also useful for species that are
otherwise difficult or slow by other methods such as
Cotinus
spp. and
Magnolia
spp. It is usually carried
out in the spring (March to early June). Vigorous
shoots are
wounded
(by cutting half through or a
by giving a sharp twist) to interrupt the sap flow and
induce new root cells, leading to the development
of adventitious roots. These are pegged down
around the mother plant where they can root into
loosened soil with wire pegs. Each stem should then
be covered at the wounded area with fine crumbs
of soil (5 to 10 cm deep) to prevent drying out. The
remaining stem should be tied to a cane to help
produce an upright specimen. All but the slowest (e.g.
Magnolia
) can be lifted with a fork and cut free of the
mother plant when dormant in the autumn or left to
the following spring. Typically these can be bedded
out in a sheltered area. In nurseries, they would be
containerized and grown on in shade tunnels.
