Plant Propagation

by Rod Smith

© 2006-2016 Rodney A. Smith
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Plant propagation is the process of increasing the number of plants. The most common method is by planting seeds. Seeds work very well for the plants that “come true” from seed, that is, the seedlings are practically identical to the parents. For other plants, such as trees, shrubs and some perennials, seedlings are highly variable from the parents. These plants are usually propagated by asexual means, such as, cuttings, dividing, layering and grafting. These methods produce plants that are clones, that is, genetically identical to the original plants. Tissue culture is even more versatile but it requires specialized equipment.

Seeds

For many plants, especially annuals, seeds are the quickest and easiest way to propagate plants. Seeds are readily available in seed packets for most annual flowers and vegetables. An increasing number of gardeners maintain “heirloom” varieties by gathering seeds from their own plants and planting them the following year.

Pre-treatment

Seeds of annuals generally do not need any pre-treatment before they will grow, but seed from many trees and shrubs need special treatment before they will grow. Some seeds, such as legumes, have hard, shiny seed coats that are very slow to soak up water. They require scarification to soften the seed coat. The simplest method is to pour hot, not boiling, water over the seeds and let them soak overnight. Another method is to rub seeds between two sheets of sandpaper. Larger seeds can be nicked with a file but care must be taken so the embryo inside is not damaged.

Seed from most plantstrees, shrubs and perennials native to temperate climates require stratification to duplicate winter temperatures before they will grow. Seeds can be planted outside in the fall, or, more reliably, they can be mixed with moist peat moss or vermiculite, put in clear plastic bags, and placed in the refrigerator, not the freezer. Most seeds require eight weeks of 33 to 41 degrees, some as little as four weeks and some as many as twelve weeks. When gathering seeds, harvest them as soon as they are ripe and keep them moist. If seeds become dried out, it is harder to get them to grow.

Sowing

The seeds of hardy annuals and easy to grow perennials can be planted outdoors directly into prepared soil once the soil reaches a minimum temperature which ranges from 40 degrees for peas to 70 degrees for squash. If no planting depth is indicated, plant seeds two to three times as deep as their average diameter. I place the seeds on the surface of the soil and use my finger to push them down to the proper depth, then close the holes.

Seeds that require warmer soil and seeds that are hard to grow can get off to a better start if started indoors about six weeks before being planted outside.

If only a few plants are needed, it is easier to start them in individual pots. For best results place two or three seeds in a Jiffy peat pellet or a peat pot filled with moist potting soil and an inch of seed starting mix on top. Once the seedlings are well sprouted, thin to one seedling per pot. Peat pellets or peat pots can be planted in the garden and the roots will grow right through them, so the young plant roots are not damaged by transplanting out of a plastic pot.

If lots of plants are needed, then plant seeds in a plastic flat with drainage holes filled with moist seed starting mix. Use a yard stick or piece of lath to indent lines in the soil surface about an inch apart for small seeds. Spread the seed in the indented lines and cover with a thin layer of seed starting mix. Keeping an inch between rows of seed will help stop damping off disease from spreading throughout the flat.

Once the seeds are planted, use a spray bottle to thoroughly wet the soil since a watering can will wash the dry seed starting mix away. Then, cover the soil with newspaper or clear plastic to hold in moisture until the seeds start to sprout. Remove the covering as soon as the seeds begin to sprout to prevent disease.

Seedlings under lights

Seeds will sprout best if kept at room temperature but new seedlings need bright light to make them grow short and sturdy. If the light is too dim, they will stretch, become spindly, and easily fall over. Seedlings should be kept in a sunny, but not hot, window or placed directly under a fluorescent grow light. Look for fluorescent lights with the brightest light output. I found tubes with a light output of 3400 lumens. The picture above shows seed flats growing in my sunny south window under fluorescent lights. I tip the fixture up during the day to let more sunlight reach the seedlings. At night, I remove the prop so the lights are directly above the seedlings.

Transplanting

Seeds started in flats will need to be transplanted to individual pots once the true leaves start to appear. I recommend transplanting into peat pots although sometimes I use two inch cell packs or even one inch cell packs for very slow growing plants such as violas. I cut between the lines of seedlings with a knife and carefully separate the seedlings holding only the seed leaves. Then I use a pencil to make a hole in the soil and guide the root into the hole. My mother uses ice cube tongs to lift the seedling and plug of starting mix out of the flat and push it into the soil.

Hardening off

Before seedlings can be planted outdoors, they will need a week of hardening off to help them adjust to harsher growing conditions outside. Ideally, plants should be moved to a cold frame and the glass cover lifted a little higher and a little longer each day unless there are strong winds or frost. An alternative is to move plants outside to a sheltered location during the day and into a cool room or garage at night.

Asexual Reproduction

The rest of the propagation methods are asexual. Every resulting plant is genetically identical. This means that plants with superior flowers, fruit or growth characteristics can be multiplied.

Cuttings

Cuttings are made from the current season’s growth of woody plants. Many conifers, houseplants and some deciduous plants are started from cuttings. Cuttings should be collected from healthy plants from strongly growing branches. Softwood cuttings are the tips of new growth collected in late spring and summer. Softwood cuttings usually root better but they need more care. Hardwood cuttings are collected in the fall and winter. They will root better if they are pulled off so a heel of two year old wood adheres to the base of the cutting. Otherwise, cut the bottom off just below a node. Usually, cuttings are trimmed so they are three to six inches long.

All leaves are stripped off the lower half of the cutting and the cuttings are dipped in liquid or powdered rooting hormone. Then cuttings are stuck into a rooting medium such as sand, perlite or seed starting mix. They will root best if the rooting medium is heated by heat cables and the air is not heated.

Cuttings need to be kept moist until they have rooted. For large numbers of plants, automatic mist nozzles can be installed over the cuttings and set to mist the cuttings every hour during the day, more often in hot weather. Cuttings can also be misted by hand several times a day. For a few cuttings, a plastic bag can be put over a pot of rooting medium with sticks to keep the bag clear of the cuttings. Do not let sun shine on the bag. If condensation forms inside the bag, open it a little until the condensation disappears.

Cuttings should be checked regularly and any cuttings showing signs of disease should be removed. Softwood cuttings will root in about six weeks. Hardwood cuttings will take two or three months to root. To check for roots, gently tug on the cutting.

Dividing

Dividing is simply separating plant parts that already have roots and leaves. Many perennials which grow in clumps are easy to divide. Shrubs with multiple stems can also be divided. A simple method is to plunge two shovels or spading forks back to back into the center of the clump. Then the handles are pulled together to pry the clump apart.

Iris are easy to divide simply by cutting each branch off of the main stem and planting it separately. Agapanthus roots can be watered thoroughly then rolled back and forth on their side until the clumps separate.

Stooling

Stooling is a method of encouraging roots to grow on woody stems so they can be divided. Stooling is commonly used to propagate dwarf rootstocks for grafting fruit trees.

In mid-summer, soil is mounded up about a foot high around a young tree or a shrub. A plastic pot with the bottom cut out can be placed over the plant or a wooden frame can be built around the plant to keep the soil neatly in place. Roots will grow out of the stems where they are covered by soil. When the plant is dormant, the soil is removed and the rooted stems are cut off a few inches above the original ground level. In the spring, new shoots will grow from the cut off stems. In mid-summer, the new shoots are covered with soil and the process repeats.

Layering

Layering is similar to stooling except that a branch is bent down, held in place and covered with soil. Roots will form quicker if a short section of bark is removed just below the part of the branch that is buried.

Air layering is useful for plants that cannot be bent below ground level. It works very well for house plants such as rubber plant and dracaena when they get too tall and have a bare trunk. First, a ring of bark about an inch long is removed from the trunk. Scrape the wood to make sure the cambium is also removed. This forces the trunk to develop roots. Then take a piece of clear plastic about fifteen inches square and fold it around the trunk. Tape the edges of the plastic together or fold the ends together and staple them to form a tube around the trunk. Use twine to tie the bottom edge of the plastic to the trunk just below where the ring of bark was removed. Then soak sphagnum moss in water, wring it out like a sponge and stuff it into the tube. Finally, use twine to tie the top of the tube closed. Normally the moss will stay moist without any additional watering.

In a couple of months, roots will grow from the stem just above where the bark was removed. They should be visible inside the plastic. The trunk can be cut off below the tube and planted into a new pot. Usually, the original plant will send up new sprouts and they can also be air layered.

Grafting

Grafting is the process of cutting a twig, called the scion, from one plant and connecting it onto the roots or top of another plant called the rootstock. The scion will grow into a new plant as it receives water and nourishment from the rootstock.

There are many grafting methods and some methods work better for particular kinds of plants. In all of the different methods of grafting, it is necessary that the cambium layers of the scion and rootstock are placed next to each other so the graft will heal together. The cambium layer is the boundary between the wood and the bark. All growth occurs at the cambium layer.

grafting

The most common graft is the whip and tongue graft. Both the scion and the rootstock have to be dormant for this method. An angling cut, about an inch long, is made in the rootstock. Then a second cut is made beginning about a quarter of the way down from the tip. The second cut should almost parallel the first cut so the wood does not split. This produces a thin wedge of wood called the tongue. A matching cut is made at the base of the scion. Care should be made when cutting the scion so the scion ends up right side up. The scion is then pushed onto the rootstock so the two cuts interlock. Ideally, the rootstock and scion are about the same diameter so the two cambiums match all of the way around. If not, then cock the scion to one side a little so the cambiums cross each other at several points. Finally, wrap the graft with grafting tape or a rubber strip, partially overlapping the wraps, to hold the graft tightly together and seal in moisture. Also, the top of the scion needs to be sealed with a cap of grafting tape or a drop of latex based tree paint. The graft should heal together about the time new growth begins. If the graft grows rapidly, it may be necessary to cut the grafting tape or rubber strip so it does not choke the new growth.

A simpler graft, which is usually used on conifers, is the side graft. An oval shaped cut is made in the side of stem through the bark and slightly into the wood. A matching cut is made in both the scion and rootstock. The cuts are held tightly together with grafting tape or rubber strips. Once the graft has healed, the rootstock is cut off just above the graft.

The side graft is also used on really difficult to graft plants in approach grafting. The rootstock plant is placed next to the scion wood plant and they are side grafted together. They can stay like this as long as it takes for the graft to heal. Then the scion is cut off below the graft and the rootstock cut off above the graft.

Budding

Budding is a form of grafting where only a single bud is used for the scion. The most common budding method is T-budding, which has the highest success rate. For this method, the scion has to be dormant and the rootstock has to be actively growing so the bark will easily peel from the wood. T-budding is usually done in mid-summer when the buds on the base of the scion wood twigs are mature and the tips of the rootstocks are still growing. As the scion wood is being collected, the leaves should be cut off leaving a short piece of leaf stem as a handle. T-budding can also be done in the spring if scion wood is collected in late winter and refrigerated until the rootstock is actively growing.

T budding

First, a horizontal cut is made in the rootstock about a third of the way around the twig. Then a vertical cut is made downward from the first cut about an inch long. This forms the T. The flaps of bark are peeled loose on either side of the T. Next, on the scion wood, starting about a half inch below a bud, a thin slice of wood is cut to a point a half inch above the bud. Then a horizontal cut is made to create a square top on the bud so it is shaped like a shield. Some grafters like to peel the bud and bark off of the thin slice of wood while others leave the slice of wood attached.

The bud shield is then slipped into the T under the flaps of bark until the square top of the bud shield fits the top of the T. Then it is wrapped with rubber strips or grafting tape so the shield is covered except for the bud. If the piece of leaf stem drops off after a week or two, it signals that the bud is healing properly.

chip budding

Chip budding can be done when both scion wood and rootstock are dormant. A cut is made in the scion wood starting a half inch above a bud and cutting to a half inch below the bud. A second cut is made starting below the bud and meeting the first cut. This produces a wedge of wood with the sharp edge at the bottom. A matching cut is made in the rootstock and the bud is tied in place with grafting tape or rubber strips.

When budding in the field, buds should always be placed on the north side of the rootstock. This protects the bud from hot sun and helps the new growth straighten up quicker. In addition, shields are available to tie over the bud to protect it from the sun and guide the new growth straight up.

Once the bud has healed, the rootstock can be cut off above the bud to force it to grow. Another option is to make a half-moon shaped cut through the bark to the wood just above the bud in late winter to encourage the bud to grow. This cut traps the sap rising through the bark at the bud so it is more likely to grow.

Grafting Supplies

Special grafting and budding knives, grafting tape and rubber strips are sometimes available through nurseries and garden centers. They can also be ordered from A. M. Leonard & Sons, Piqua, Ohio. 1-800-543-8955 www.amleo.com


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