Basic plant propagation
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Why bother to propagate plants when virtually anything you
may need can be bought at your local garden centre? There are several reasons
but the most appealing is the sense of satisfaction gained from producing a
plant from scratch. Other reasons worth considering include,
The ability to produce
large numbers cheaply.
The production of plants not normally
available in garden centres, i.e. special growth forms or plants that
commercial nurseries deem uneconomical to produce.
The opportunity to produce
your own hybrids or undertake a plant breeding programme.
For profit, even small scale propagation can provide a supplementary income.
Because it’s an enjoyable
and relaxing thing to do.
There are two forms of plant propagation: sexual and
asexual. Sexual propagation involves the production of seed by the pollination
of flowers. It is the normal method used by plants in the wild. Asexual, or
vegetative, propagation is where a new plant is produced from a part of an
existing plant, such as when cuttings or grafts are used.
The main difference between the two forms of propagation is
that by using asexual propagation you will always create an exact replica, or
clone, of the parent plant whereas sexual propagation can never produce clones.
Just as every human being is the sum of all its forebears
so is every seedling, the more diverse the parents the more variable the
offspring.
If you want to copy plants that already exist then choose
asexual propagation. To create new plants choose sexual propagation. The only exceptions to the rule is the seed of true species, which
should be true to type, and certain highly reliable strains that produce very
consistent results from seed.
Growing from seed
The plants that you are most likely to grow from seed are
the annual bedding plants, vegetables and annual fruits such as peppers and
tomatoes. In many cases these plants have been carefully bred over many years
so that they produce reliable and consistent strains. Sometimes you will be
able to save the seeds of these plants for sowing next year but very often this
is not practical because the plants are F1 hybrids.
What is an F1 hybrid?
Early in the study of plant genetics the concept of filial
hybrids was established. When two parent species are crossed the resultant
plants are said to be filial, meaning son or daughter. The first generation is
the first filial or F1. Plants from the F1 generation crossed with one another
produces the second generation, second filial, or F2 and so on.
As a technical term F1 is used quite loosely. Although it
should be restricted to the crossing of two distinct species it is generally
used to describe superior plants that can only be produced by crossing two
established parent strains. When crossed these parents will
always produce consistent offspring, not necessarily greatly resembling the
parents, but plants that show little variation from one another.
However, saving the seed of F1 hybrids is pointless because when they are
crossed with themselves or other strains this consistency is lost. The F2
generation will be highly variable plants, there may be superior individuals
among them, but the generation as a whole will lack the consistency and
reliability that is necessary for mass produced plants to be a commercial
success when grown from seed.
Pre-germination treatment and dormancy
Germinating most seeds is simply a matter of providing the
right moisture levels, light and temperatures but some are a little more
demanding. Not all plants come from areas with regular rainfall and a temperate
climate. The need to survive inhospitable conditions has led to the development
of survival strategies, some of which affect seed germination.
One of the most common is a delaying mechanism that ensures
the seed germinates when the conditions are at their best. Plants from regions
with very cold winters often have seed that requires a period of chilling
before it can germinate, in the wild this delays germination until spring
rather than have it occurring in autumn, which would lead to the tender young
plants being killed over winter. Plants from arid regions would suffer a
similar fate if they germinated with every ephemeral shower so their seeds
often have a hard coating, that will not soften until thoroughly soaked, or may
contain germination inhibiting chemicals that need to be washed away.
Knowledge of a plant’s native environment will often provide
vital clues to its germination requirements. An understanding of the particular
characteristics of a genus also helps. For example most leguminous plants,
regardless of where they come from, have seeds with hard coatings that need to softened by soaking.
Apart from soaking the two most common pre-germination
treatments are stratification
and scarifying.
Stratification is used to provide a dormant period for seeds that require
winter chilling. The usual method is to place the seeds in a plastic bag full
of moist potting mix, which is then stored in a refrigerator (not the freezer)
for several weeks. The exact period of stratification varies but it is usually
from four to twelve weeks.
Scarification is the abrading of a hard seed coat, this is
often sufficient in itself or it can be used in combination with soaking. The
most common scarification method is to line a jar with sandpaper, grit inwards,
put the seeds in the jar and shake it vigorously (with the lid on). Large, very
hard, seeds may be dealt with individually by nicking the seed coat with a
knife or by rubbing on sandpaper.
Requirements for growing from seed
Suitable containers for sowing
Shallow pans are better than deep pots, perfect
drainage is essential. Commercial propagating trays are ideal.
A good quality seed raising soil mix
Commercial mixes are satisfactory but regular potting
mix passed through a sieve often produces superior results. Whatever you use should be fresh and, if possible, sterilised.
A soil sieve with approximately 6mm mesh
This is also a useful tool for making up cutting mixes.
Soil mix additives
Especially sphagnum moss and
perlite. These lighten the mix and allow better oxygenation. Sphagnum
moss also contains natural fungicides that may inhibit damping off and other
fungus diseases.
A fungicide
Use Thiram or a similar fungicide that can be used as
a soil drench. Fungus diseases are always likely to cause problems with young
seedlings.
Labels and marker pens
It is useful in all propagating tasks, to know what
you have and when it was sown.
A watering can.
One with a fine rose or a hose
attachment that can provide a fine spray. You will need to moisten the
mix without washing the seed away.
Transparent or translucent seed tray covers
Panes of horticultural glass are ideal but plastic
cling film or other plastic sheeting will do. Condensation tends to form into
large drops on plastic sheeting, this may damage young
seedlings if it falls on them.
Start with a soil mix of about 50% bark based potting mix,
25% fine sphagnum moss and 25% perlite, thoroughly mixed and put through a 6mm
sieve. Blending the mix with a trowel in a wheelbarrow is the easiest way to
make up small quantities.
Having sieved enough mix, fill your tray to about two thirds
depth. If using a shallow tray this need be no more the 50mm for all but the
most vigorous seedlings. Many very fine seeds such as begonia require no more
than 25mm soil depth for initial germination. It is important that you leave
enough room for the seed leaves (cotyledons) to expand without touching the
cover. If they touch they will almost certainly rot due to excess condensation
forming on them leading to fungus diseases.
Having levelled the soil use your fine spray
to moisten it. Take care not to saturate the soil or compact it
unnecessarily, it needs to be moist not sodden.
Once the soil surface is ready the seeds can be sown. Large
seeds may be individually placed in position but most seeds are simply sprinkled
evenly over the surface. Minute seeds may be hard to see. It helps to mix very
fine seed with sand or icing sugar, which will make a far more visible layer.
Having sown the seeds you must decide if you should cover
them with soil or not. Many seeds require light to germinate, these are usually
very fine seeds that do not have large food reserves, others
must have darkness for successful germination. Knowing the needs of particular
seeds usually requires experience and good references but sometimes the information
may be on the seed packet.
When covering use a very finely sieved mix
that will not impede to development of the cotyledons. A good rule is to
cover seeds only to about the equivalent of their own depth.
Before covering the tray with your pane of glass or plastic
cover, gently spray to moisten the covering soil, if any, and to firm the seed
into place. Cover the tray then place a sheet of newspaper over the cover. This
will cut down the light a little and stop the seeds from cooking if the tray is
inadvertently exposed to sun, it will also help prevent algae growing on the
soil surface.
If you know that the seeds need warm temperatures to
germinate or if you are attempting to germinate out of season a heating pad or
hot water cupboard can be an invaluable aid. But heating pads rapidly dry the
soil in the bottom of the trays and hot water cupboards are dark. When
germinating temperate climate seeds in spring or summer all that is necessary
is to place the seed tray in a warm sheltered area out of direct sunlight.
The exact time required for germination varies enormously
with the species, some may only take a day or so, others several months.
Commercial seeds often have information about germination percentage and time
on the packet, otherwise it’s a case of searching out
the information or being very patient.
Once the seeds have germinated remove the newspaper and
slowly, over several days, raise, then remove the glass or plastic cover to
gradually harden the seedlings. At this stage it is advisable to gently spray
then young seedlings with a fungicide and diluted liquid fertiliser. Don’t use
strong chemicals or fertilisers that may damage the tender young plants.
To prick out the seedlings gently loosen them with a small
dibbling tool (plastic labels are good), pick them up by their seed leaves, not
the tender stems, and pot them on. Very small seedlings may have to be
transplanted in clumps and thinned further when they are easier to handle.
This procedure is the common method for ornamental seedlings
but many vegetable seeds are sown directly in the open ground.
Producing hybrids
One of the most rewarding aspects of propagation is the
production of new hybrids. For most plants the hybridising is easy, it’s the
waiting afterwards that’s the hard part. Among the more likely candidates for a
basic hybridisation programme are fuchsias, lilies, rhododendrons and any other
plants where the flower parts are readily accessible.
The process of hybridisation is nothing more than
transferring pollen from one flower to another. In nature this is done by
insects, the wind or in the case of primitive non-flowering plants the genetic
material is transferred in water.
When crossing you will need a pollen
parent and a seed parent. These may be separate flowers on the same
plant but usually two plants are involved. For simple crosses you will need
both pollen and seed parents to be in flower at the same time. Freeze drying
pollen or buying pollen already freeze dried can get round this problem.
When choosing a flower to act as a seed parent look for one
that is still closed but on the verge of opening, this will ensure that the
flower is mature but unpollinated. Remove the unnecessary flower parts and
pollinate using an anther from your previously selected pollen parent. It is
not generally necessary to cover the freshly pollinated flower, once pollinated
most flowers will not accept further pollen but if you want to be sure a thin
muslin bag tied over the flower will protect it from contamination. If the
cross has been a success the ovary of the seed parent flower should begin to
swell noticeably within a few days to a few weeks depending on the type of
plant.
Determining when the seed is mature can be difficult, you
don’t want the seed pod to burst and scatter your precious progeny to the wind
but you also don’t want to harvest unripe seed. The only answer to this is
experience, often there will be colour changes or drying as the seed pod nears
maturity, but sometimes you may have to harvest slightly unripe seed pods to
avoid total disaster. Tying a paper bag over ripening seed pods to catch
falling seed is sometimes possible too.
The time it takes to see your final results will vary
enormously depending on the plant, magnolias may take seven to fifteen years to
flower but annuals can be up and blooming in a few weeks. Regardless of the
time it takes there’s nothing more satisfying in propagating than raising a
genuinely new and useful hybrid.
Vegetative propagation
The main reasons for using vegetative propagation are that
it produces exact replicas of the parent plant and as the material used is
generally from mature specimens the plants flower at a younger age than
seedlings, often in the first year. Vegetative propagation is also the only
method for replicating sterile plants or self-sterile plants without a
pollinator.
There are several methods of vegetative propagation.
Cuttings, layering, division, budding and grafting are the most common. Root
cuttings, leaf cuttings and aerial layering are less widely used. A more
technical method, tissue culture, is usually restricted to propagation
laboratories due to its strict hygiene requirements and the need for
specialised equipment.
The main thing that enables vegetative propagation of plants
is their cell structure. All parts of plants, roots, flowers, leaves, stems
develop from minute clusters of cells known as meristems. These meristematic
cells have no predetermined growth habit, any cell may
become part of a leaf or flower or any other part of the plant. Which it
becomes is determined by the composition and concentration of hormones called auxins.
This is very important because it means that a complete
plant may be grown from any part containing meristematic
cells. These cells are usually most highly concentrated at the main growth
points such as leaf nodes or root tips. In practice it requires some experience
to know which techniques are likely to be most successful with particular plants, nevertheless, meristems are the basis of all
vegetative propagation.
Simple vegetative propagation
The most basic methods do not involve removing material from
the parent plant until after it has struck roots. The simplest of these methods
are division and layering. Aerial layering is a similar but slightly more
complicated method.
Division is nothing more than the name implies. It is the
method commonly used with clump forming herbaceous perennials. The right time
for dividing varies with the genus but it’s usually either when the plant is
dormant or when it’s just emerging from dormancy. There are no special tricks
to dividing; about the only question is knowing just
how small the divisions can be yet still survive. Obviously each division must
have some roots and a leaf bud but knowing how far you can go is largely a matter
of experience. If in doubt err on the large side.
Layering is a very simple process and one that often occurs
naturally on low spreading plants such as Parahebe
or ground cover azaleas. These plants have a tendency to form roots at the
points where their branches come in contact with the soil. Most plants are
capable of doing this but they don’t grow in a manner that allows it to happen
naturally, their branches do not contact the soil.
Basic layering is just a matter of bending a branch so that
it touches the soil and then keeping it there long enough to form roots. In
some cases this isn’t possible so aerial layering is used instead. This takes
the approach that if you can’t bend the branch down to meet
the soil then take the soil up to meet the plant. By packing tightly wadded
sphagnum moss around a branch that has been slightly wounded (and the wound
dusted with root forming hormones) the conditions of soil based layering can be
simulated above ground.
Advanced vegetative propagation
Although layering gives satisfactory results by far the most
common vegetative propagation methods are cuttings, budding and grafting.
Cuttings and budding are the easier methods for gardeners.
There are several
types of cutting based on the ripeness of the material used:
Softwood
Softwood or tip cuttings are taken from the soft new
tip growth. These cuttings usually strike quickly but can be difficult for home
gardeners as they are inclined wilt, rot and die before striking unless grown
under controlled conditions. Most nurseries use misting or fogging units to
keep softwood cuttings turgid until they can fend for themselves.
Semi-ripe or semi-hardwood
Firm yet pliable pieces of stem wood, they are usually
taken from autumn tip growth but may also come from further down the branch.
You have probably noticed that the new growth of most plants is soft green wood
that develops the characteristic brown bark as it ages and hardens. Semi-ripe
cuttings are still quite green and soft while semi-hardwood is approaching the
bark forming stage.
Softwood and semi-ripe cuttings are the most common
types of cuttings in both nurseries and home gardens. They will nearly always
perform better under mist but are usually firm enough to strike quite well in
cold frame or even if just kept in a sheltered place.
Hardwood
Normally used for the propagation of deciduous plants.
They are taken from the fully hardened wood of the previous summer’s growth.
While usually inserted in the open ground outdoors they can also be struck
indoors, in boxes of soil, and may develop faster in spring if treated this
way.
Taking cuttings
Taking cuttings is not a complicated procedure but the time
it takes for the cuttings to strike, and the percentage of successful strikes,
varies enormously depending on the species or variety. Experience will teach
you which plants are the easiest but among them would be fuchsias, pelargoniums
and marguerite daisies. Dahlias and delphiniums are also easy to strike from
the fleshy new shoots that form in spring.
A good soil mix is important, I prefer to use a good
quality, bark based potting mix blended at about three
parts potting mix to one of perlite or fine pumice by volume. The mix is then
passed through a 6mm mesh sieve.
Apart from the soil mix other useful aids include
A good quality pair of secateurs.
Propagating trays, purpose built cutting
trays are best but any shallow container with drainage holes will do.
A sharp knife is useful occasionally although most of the time you will use your secateurs.
A selection of labels and a waterproof marker pen.
A small atomiser or hand
mister to keep the cuttings and soil moist while you are working.
A root forming hormone (liquid or powder) of appropriate strength for the type of cutting you are working with.
If you don’t have access to a misting unit you will also
need large transparent or translucent plastic bags for covering the propagating
trays.
Fill a tray with cutting mix, use a board or some other
straight edge to level the soil off with the top of the tray, then moisten the soil with your atomiser. It’s a good idea
to add a fungicide to the atomiser spray but do not use anti-wilting agents,
these are usually oil based and can inhibit the cuttings’ respiration.
Softwood and semi-ripe cuttings are prepared in the same
way. The size of the cutting varies with the type of plant but is usually about
three to four nodes long. Meristematic cells are
present in large numbers at the nodes, the points at which the leaves are
attached, so most cuttings are taken just below a node. Use your secateurs to
get a neat straight, or slightly sloping, cut.
Prepare the cutting by carefully removing the lower leaves
until the three to five top leaves remain. Sometimes the leaves come away
easily but often you need to take care not to strip the bark away with the
foliage. If the remaining leaves are large (over 40mm long) cut them back to
about half length to lessen their transpiration rate.
Dip the cutting in the root forming hormone, just the cut
end not the stem. The root forming solution contains an auxin (indole butyric acid) that induces meristematic
cell to develop into roots. Some cuttings strike so easily that they don’t need
it, others are so difficult that it makes little difference, but it most cases
it does have a positive benefit.
After dipping, gently push the cutting into the cutting mix
to about a third of its length. The cuttings can be packed quite tightly, up to
the point where their leaves are touching but not overlapping. As you work use your atomiser to mist the cuttings and moisten the
soil surface.
When the tray of cuttings is completed you will need to have
some way of stopping them wilting before they strike roots, they need to have
high humidity. The best way is to use a misting or fogging unit but failing
that, enclose the tray in a plastic tent. Make sure the cuttings do not touch
the plastic as they will probably begin to rot at any points of contact.
Cuttings strike best when their tops can be kept cool and
humid while the root zone is kept warm and moist. Ideally mist and some form of
bottom heat, such as a heating pad or, even better, heating cables in moist
sand, should be used. If you don’t have access to such things a warm sheltered
position out of direct sun will usually suffice.
Start to check your cuttings after about three weeks, those
that are easy to strike such as marguerite daisies should show some signs in
that time. More difficult plants could take several months longer and may form
callus tissue before striking roots.
Hardwood cuttings are treated less tenderly,
they are usually pieces of stem of about 100-200mm in length depending on the
plant, that are simply inserted into prepared nursery beds. This is normally
done in late autumn, as the last of the foliage falls, and the cuttings are
left through the following growing season to be lifted when dormant in winter.
Budding and grafting
Budding and grafting are more advanced but straightforward
procedures. Budding is the more reliable method for inexperienced propagators.
There are two forms, shield and chip. Shield is the more common, being the
method most widely used on roses.
The best time for budding is early summer, when the leaf
axil buds first become obvious. At this time the bark of the stock plant should
be at its most pliable thus enabling easy insertion of the bud. Also there is
plenty of growing season left for the wound to heal
and the bud to take.
It’s tempting to think that because budding is an early
season job the results will be visible quickly but that’s not so. The bud will
probably show no sign of development until the following spring, about ten
months after it was inserted.
There are many types of grafting but all work on the same
principle. Just below the bark is a layer of cells known as the cambium layer, this is the
plant’s network for distributing water and nutrients. When grafting it is vital
to match the cambium layer of the scion (grafting wood) with that of the stock,
the better the match the greater likelihood of the graft taking.
The stock and scion must be reasonably closely related
plants for a graft to be successful, they should be within the same family but
they don’t necessarily have to be in the same genus. There are many examples of
bi-generic grafts; lilacs on privet (syringa/ligustrum),
citrus on trifoliata rootstock (citrus/ponciris),
but for simplicity keep your first grafts within a genus.
Grafts of deciduous plants are normally made in late winter
or early spring just as dormancy comes to an end. Evergreen grafts can be made
over a wider season but late winter is still a popular time.
Budding and grafting are usually restricted to plants that
are difficult to strike from cuttings or that grow poorly on their own roots.
Due to the time, expense and uncertainty involved they have never been popular
methods for mass production.
Propagating bulbs, corms, tubers and rhizomes
Left alone these plants will often
propagate themselves but there are ways of speeding up the process and making
it more reliable.
Rhizomes
Typical rhizomatous plants, such as asparagus, often strike
roots as they spread so propagation is simply a matter of division. In the
instances where there are no root already formed they will usually develop if a
large enough piece of rhizome is used.
Tubers
Tubers can be lifted when dormant, cut into pieces,
each with a growth point (‘eye’), and replanted. Each cut surface should be
dusted with a fungicide to prevent rotting.
Bulbs
Both types can be propagated by
dividing established clumps but if greater numbers are required scaling and
scooping can be used. Scaly bulbs are propagated by separating the scales and
growing them on as individual plants or by keeping the scales in moist potting
mix until they develop small bulblets. Tunicate bulbs
should be cut into vertical segments, which are treated like scales.
Scooping involves scraping out the
base of the bulb, and storing it in a warm dark place, usually an airing
cupboard. After about eight weeks small bulblets will have formed on the cut
surface. The original bulb is then potted, upside down, and placed in a
greenhouse or a sheltered position outdoors. Within a few weeks the young
bulblets will start to grow.
Corms
Corms may be cut up, much like tubers, but the most
common method of propagation is to lift the corms at the end of the season and
remove the small cormels that have formed. Often there will be dozens of them,
they can be potted, or replanted in the garden.
Less common techniques
Root cuttings
Very few plants are grown from root cuttings. The problem is
that there is often a lack of suitable roots. This means that the plants have
to be lifted and root pruned the season before propagation to ensure the roots
are in the right condition. As you can imagine this added time makes it an
unpopular method with commercial nurseries.
Once you have a plant with suitable roots it’s a
straightforward and reliable procedure. The plant is lifted in the dormant
season and the roots washed clean of soil. Strong healthy roots are chosen and
removed near the crown. The roots are then cut into pieces about 50mm to 150mm
in length depending on the type of plant and the growing conditions. Larger
pieces are used in outdoor beds while smaller pieces are better under
controlled conditions.
The cuttings are then inserted vertically so that their tops
are level with the soil surface. A light layer of fine gravel or sand (about
1cm.) is put over them to prevent excessive drying. With time leaves and roots
develop and the plants may be removed for growing on.
An important aspect of root cuttings is remembering which
way is up. It’s all too easy to insert the cuttings upside down.
Leaf cuttings and offsets
Many plants can be grown from cuttings that utilise just the
foliage or a leaf and a small portion of stem. Others produce young, ready to
grow, plantlets from the mature foliage.
Plantlets
Several plants, such as Asplenium, Kalanchoe and Tolmiea
produce young plantlets at the base of the leaves or around the edges. These
may be carefully removed and grown on.
Offsets
Many succulents produce offsets, small plants with
roots yet still attached to the parent plant. These may be removed and grown
on.
Leaf-bud cuttings
Some plants, particularly climbers, have long
internodes. This makes it difficult to take normal stem cuttings as they are
large and unwieldy. The answer is to use just one node. Where leaf buds are
directly opposite this can even be taken further by splitting the stem.
Leaf cuttings
There are several different types of leaf cuttings.
The most straightforward is the petiole cutting. Remove the whole leaf, including
the petiole, and insert it into moist potting mix, petiole downwards. The
petiole will develop roots and small plantlets or new growth will form at the
base of the leaf.
Leaf sections
A variation used on long thin succulent leaves, most
commonly Sanseveria. Cuts are made across the leaf so that each section is
about 6-8cm long. These are inserted into the mix in an upright position and
the plantlets develop at the base.
Leaf slashing
Only suitable for plants that have broad leaves with
distinct veins, most notably the ornamental foliaged begonias. Remove a healthy
leaf and make several cuts through the prominent veins on the underside. Pin
the leaf flat down on moist potting mix preferably with the cut surface of the
petiole in contact with the mix. After a few week
small plantlets should start to appear at the points where the cuts were made.
Leaf squares
A variation of leaf slashing.
The leaf is cut into pieces about 1.5cm square, these are pinned down and the
plantlets develop at the cut edges.
Tissue culture
Many difficult to propagate plants are now grown by tissue
culture. This is a process whereby meristem cells or small pieces of material
called explants are cultivated on nutrient enriched agar jelly under strictly
controlled laboratory conditions. By varying the auxins
in the nutrient solutions it is possible to induce root and stem development.
Growing from meristems has the advantage that it usually produces virus-free
plants whereas other vegetative means perpetuate viruses as they are transferred
when mature genetic material (cuttings, grafts, etc) is used.
Meristems are usually the last part of a plant to be
affected by viruses. So by excising the meristem and growing from it alone it’s
possible to eliminate the built-in viruses. Explant
tissue culture is not as effective at eliminating viruses nor does meristem
culture make the plants any less likely to contract viruses later but at least
they start off healthy. Tissue culture also allows rapid build up of plant
numbers from very small initial stocks.
Tissue culture requires high levels of hygiene and constant
monitoring of temperature, fungal problems and nutrient solutions. These
complications usually restrict its use to commercial propagation laboratories.
Copyright Geoff Bryant
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