The following brief introduction on how to propagate
native plants has been condensed from the Container Tree Nursery Manual, Volume 6 (USDA Forest Service, Agriculture Handbook 674, May 1999).
Introduction - Plant propagation is both a science and an
art. The science of plant propagation requires a knowledge of plant
physiology, nursery cultural practices, and characteristics of the
particular plant that you want to grow. The art of plant propagation cannot
be taught in a book or classroom, however, because it consists of specific
technical skills that must be acquired through innate ability or
experience and often requires a certain "feel." Good plant pagators are
said to have a "green thumb."
Planning - Successful nursery management begins with
planning. Crop planning is one of the most important, yet often neglected,
aspects of seedling culture. The first phase of the planning process is to
determine which propagation method will be most effective and economical
for the crop species. Both the biology of the species and the objectives of
the outplanting project must be considered. If it is possible to propagate
a plant either by seed or vegetatively, then the amount of genetic
variability that is desired in the crop must be considered.
Figure 1. Plants propagated from seed look different
from their parents and each other because they contain a mixture of genetic
characteristics of their parents. Vegetative propagation, on the other
hand, produces exact duplicates of the parent plants.
Sexual reproduction results in a mixture of genetic
characteristics in the offspring, so each plant will appear slightly
different from its parents and each other. Because maintenance of genetic
diversity is so important in ecosystem management and restoration projects,
seed propagation is encouraged whenever possible. It is easier to capture
and preserve biodiversity with seeds than with vegetative propagation
(Figure 1).
Availability of propagation material, time constraints,
and economics must also be considered. Many native plants do not produce
good seed crops each year, so it may be impossible to obtain enough seed.
This is especially true for emergency projects, such as fire restoration,
when crops must be grown in a very short time. As for economics, seed
propagation is almost always less expensive than vegetative propagation
which involve more hand labor and often require special equipment and
structures.
Type of nursery - Native plants can be produced in either
container nurseries or bareroot nurseries. Although some natives are
produced bareroot, most are grown in containers because they offer more
flexibility in scheduling, and can be grown in a shorter period. In
particular, the size and shape of native plant seeds limits how they can be
grown. Most commercial conifers have relatively small, smooth seeds that
can be easily sown in bareroot seedbeds. Most other natives, however, have
seeds that are very small, irregularly-shaped or have appendages that make
mechanized sowing difficult if not impossible. All these characteristics
make container propagation much more attractive.
Seed propagation - There are several ways of growing
native plants from seeds. Direct seeding is the traditional method and
consists of placing seeds directly into the growth container or seedbed and
allowing them to germinate in place (Figure 2). Seedlings either can be
allowed to grow to shippable size or transplanted into larger containers or
into beds in a bareroot nursery. A second option is to sow seeds into
shallow trays, keep them moist, and then hand-sow the germinating seeds
into containers. A third technique consists of sowing seeds into shallow
trays and allowing them to germinate and the seedlings to emerge. The young
"emergents" are then transplanted into containers to finish their
development. Sowing germinants or transplanting emergents must be done
carefully to reduce the possibility of root deformation.
Figure 2. The variable sizes, shapes, and appendages of
most native plants seeds make hand sowing in containers the most practical
propagation method.
Vegetative propagation - Although they vary considerably
in technique, all vegetative propagation methods are a form of asexual
reproduction. The objective is to make multiple "copies" of an individual
plant or select group of plants with similar genetic composition (Figure
1). Species that root easily can be propagated with rooted cuttings (Figure
3). This process involves collecting stem sections, treating the lower part
with rooting hormones, and then either sticking them in trays filled with
growing medium until they form roots ("pre-rooting") or "direct sticking"
them into the growth container. Layering consists of inserting a section of
stem or root that is still attached to the plant into a favorable rooting
environment until roots develop. The rooted section is then cut from the
parent plant and transplanted into the growth container.
Grafting is a very specialized propagation technique in
which shoots or buds from one plant are surgically implanted into another.
The newest and most rapidly developing vegetative propagation technique is
micropropagation. This involves a series of sterile laboratory techniques
in which small sections of plant tissue are chemically stimulated to form
multiple shoots and are then rooted. The resultant "explants" are
transplanted to growth containers and raised under normal culture.
Figure 3. Rooted cuttings are the most common vegetative
propagation method for native plants.
Some species can be propagated either by seeds or
vegetatively, and the decision depends on the objective of the outplanting
project. Quaking aspen is a good example. Aspen seeds are very small and
difficult to handle because they are enclosed in a ball of cottony
material. Seeds can be cleaned relatively easily, but must be sown manually
into growth containers because of their small size. If the objective is to
retain the physical characteristics of a specific ecotype or clone,
however, aspen can be propagated vegetatively from root sprouts, which are
rooted and then transplanted to growth containers.