Types of Propagation
Grafting is a technique that vegetatively joins two plants into one. Instead of cross-pollinating two plants and producing a hybrid seed, grafted plants use the roots and the bottom portion of one plant (rootstock) and attach it to a tender shoot (scion) from the top portion of another plant. This is often done with trees and shrubs, to combine the best characteristics of the two plants.
Asexual plant propagation methods produce new plants from vegetative parts of the original plant, such as the leaves, stems and roots. These methods are generally referred to as vegetative propagation. Many plants can reproduce this way naturally, but vegetative propagation can also be artificially induced.
The main advantage of vegetative propagation methods is that the new plants contain the genetic material of only one parent, so they are essentially clones of the parent plant. This means that, once you have a plant with desirable traits, you can reproduce the same traits indefinitely, as long as the growing conditions remain similar.
Root cuttings are pieces of roots that form new shoots. These are often made in winter or early spring before plants begin growing. Roots should be the thickness of a thick wire. Cut into one to two inch lengths, place on a moist rooting medium, cover, and keep moist but not wet. Some plants have underground corms, such as rhubarb, which can be split or quartered and rooted.
The development of roots on a stem while the stem is still attached to the parent plant is called layering. This method of vegetative propagation is generally successful, because water stress is minimized and carbohydrate and mineral nutrient levels are high.
Some plants propagate naturally by layering, but sometimes plant propagators assist the process. Layering is enhanced by wounding the stem where the roots are to form. The rooting medium should always provide aeration and a constant supply of moisture.
Seed propagation, or sexual propagation, involves the union of the pollen (male) with the egg (female) to produce a seed. The seed is made up of three parts: the outer seed coat, which protects the seed; the endosperm, which is a food reserve; and the embryo, which is the young plant itself. When a seed is mature and put in a favorable environment, it will germinate (begin active growth). In the following section, seed germination and transplanting of seeds will be discussed.
The advantages of sexual propagation are that it may be cheaper and quicker than other methods; it may be the only way to obtain new varieties and hybrid vigor; in certain species, it is the only viable method for propagation; and it is a way to avoid transmission of certain diseases.
How to Graft Fruit and Nut Trees
For a complete guide to grafting fruit and nut trees refer to our growing guide on grafting: Raintree Guide to Grafting
How to Propagate Vegetative Cuttings
Propagation by stem cuttings is the most commonly used method to propagate many woody ornamental plants. Stem cuttings of many favorite shrubs are quite easy to root. Typically, stem cuttings of tree species are more difficult to root. However, cuttings from trees such as crape myrtles, some elms, and birches can be rooted.
A greenhouse is not necessary for successful propagation by stem cuttings; however, maintaining high humidity around the cutting is critical. If rooting only a few cuttings, you can use a flower pot. Maintain high humidity by covering the pot with a bottomless milk jug or by placing the pot into a clear plastic bag. Cuttings can also be placed in plastic trays covered with clear plastic stretched over a wire frame. Trays must have holes in the bottoms for drainage. The plastic will help keep the humidity high and reduce water loss from the cuttings.
Cuttings should generally consist of the current or past season’s growth. Avoid material with flower buds if possible. Remove any flowers and flower buds when preparing cuttings so the cutting’s energy can be used in producing new roots rather than flowers. Take cuttings from healthy, disease-free plants, preferably from the upper part of the plant.
The fertility status of the stock (parent) plant can influence rooting. Avoid taking cuttings from plants that show symptoms of mineral nutrient deficiency. Conversely, plants that have been fertilized heavily, particularly with nitrogen, may not root well. The stock plant should not be under moisture stress. In general, cuttings taken from young plants root in higher percentages than cuttings taken from older, more mature plants. Cuttings from lateral shoots often root better than cuttings from terminal shoots.
Early morning is the best time to take cuttings, because the plant is fully turgid. It is important to keep the cuttings cool and moist until they are stuck. An ice chest or dark plastic bag with wet paper towels may be used to store cuttings. If there will be a delay in sticking cuttings, store them in a plastic bag in a refrigerator.
While terminal parts of the stem are best, a long shoot can be divided into several cuttings. Cuttings are generally 4 to 6 inches long. Use a sharp, thin-bladed pocket knife or sharp pruning shears. If necessary, dip the cutting tool in rubbing alcohol or a mixture of 1 part bleach to 9 parts water to prevent transmitting diseases from infected plant parts to healthy ones.
Remove the leaves from the lower one-third to one-half of the cutting. On large-leafed plants, the remaining leaves may be cut in half to reduce water loss and conserve space. Species difficult to root should be wounded.
Treating cuttings with root-promoting compounds can be a valuable tool in stimulating rooting of some plants that might otherwise be difficult to root. Prevent possible contamination of the entire supply of rooting hormone by putting some in a separate container before treating cuttings. Any material that remains after treatment should be discarded and not returned to the original container. Be sure to tap the cuttings to remove excess hormone when using a powder formulation.
The rooting medium should be sterile, low in fertility, and well-drained to provide sufficient aeration. It should also retain enough moisture so that watering does not have to be done too frequently. Materials commonly used are coarse sand, a mixture of one part peat and one part perlite (by volume), or one part peat and one part sand (by volume). Vermiculite by itself is not recommended, because it compacts and tends to hold too much moisture. Media should be watered while being used.
Insert the cuttings one-third to one-half their length into the medium. Maintain the vertical orientation of the stem (do not insert the cuttings upside down). Make sure the buds are pointed up. Space cuttings just far enough apart to allow all leaves to receive sunlight. Water again after inserting the cuttings if the containers or frames are 3 or more inches in depth. Cover the cuttings with plastic and place in indirect light. Avoid direct sun. Keep the medium moist until the cuttings have rooted. Rooting will be improved if the cuttings are misted on a regular basis.
Rooting time varies with the type of cutting, the species being rooted, and environmental conditions. Conifers require more time than broadleaf plants. Late fall or early winter is a good time to root conifers. Once rooted, they may be left in the rooting structure until spring.
Newly rooted cuttings should not be transplanted directly into the landscape. Instead, transplant them into containers or into a bed. Growing them to a larger size before transplanting to a permanent location will increase the chances for survival.
How to Propagate from Root Cuttings
Some plants can be propagated from a section of a root. Root cuttings of woody plants are usually taken from plants during the dormant season, when carbohydrate levels are high. Root cuttings of some species produce new shoots, which then form their own root system, whereas root cuttings of other plants develop root systems before producing new shoots. Examples of plants that can be propagated from root cuttings include raspberry, blackberry, rose, crabapple, fig, lilac, and sumac.
Plants with large roots are normally propagated outdoors. The root cuttings should be 2 to 6 inches long. Make a straight cut on the proximal end (nearest the crown of the parent plant) and a slanted cut on the distal end (furthest from the crown) of each root cutting. Tie the cuttings in bundles with all the same type ends together. It is important to maintain the correct polarity of the cuttings. Store for 3 weeks in moist sawdust, peat moss, or sand at 40°F. Remove from storage. Space the cuttings about 2 to 3 inches apart in well-prepared garden soil. The tops of the cuttings (proximal ends) should be 2 to 3 inches below the soil surface.
For plants with small roots, cut the roots into 1 to 2 inch sections. Lay the cuttings horizontally on the medium surface in a flat and cover with about 1⁄2 inch of soil or sand. Place the flat inside a plastic bag or cover with a pane of glass. Place the flat in the shade and remove the protective cover after new shoots appear.
How to Propagate from Layering
Simple layering can be accomplished by bending a low growing, flexible stem to the ground. Cover part of it with soil, leaving the remaining 6 to 12 inches above the soil. Bend the tip into a vertical position and stake in place. The sharp bend will often induce rooting, but wounding the lower side of the bent branch may help also. Simple layering can be done on most plants with low-growing branches.
Simple layering can be done in early spring using a dormant branch, or in late summer using a mature branch. Periodically check for adequate moisture and for the formation of roots. It may take one or more seasons before the layer is ready to be removed for transplanting.
Tip layering is quite similar to simple layering. Dig a hole 3 to 4 inches deep. Insert the tip of a current season’s shoot and cover it with soil. The tip grows downward first, then bends sharply and grows upward. Roots form at the bend. The re-curved tip becomes a new plant. Remove the tip layer and plant it in late fall or early spring. Examples of plants propagated by tip layering include purple and black raspberries, and trailing blackberries.
Compound (serpentine) layering is similar to simple layering, but several layers can result from a single stem. Bend the stem to the rooting medium as for simple layering, but alternately cover and expose sections of the stem. Each section should have at least one bud exposed and one bud covered with soil. Wound the lower side of each stem section to be covered.
How to Propagate from Seed
To obtain quality plants, start with good quality seed from a reliable dealer. Select varieties to provide the size, color, and habit of growth desired. Choose varieties adapted to your area which will reach maturity before an early frost.
Although some seeds will keep for several years if stored properly, it is advisable to purchase only enough seed for the current year’s use. Quality seed will not contain seed of any other crop, weeds, seeds, or other debris. Printing on the seed packet usually indicates essential information about the variety, the year for which the seeds were packaged, and germination percentage you may typically expect, and notes about any chemical seed treatment. If seeds are obtained well in advance of the actual sowing date or are stored surplus seeds, keep them in a cool, dry place. Laminated foil packets help ensure dry storage. Paper packets are best kept in tightly closed containers and maintained around 40◦F. in a low humidity. The door shelves in a refrigerator work well.
Some gardeners save seed from their own gardens; however, such seed is the result of random pollination by insects or other natural agents, and may not produce plants typical of the parents.
Germination will begin when certain internal requirements have been met. A seed must have a mature embryo, contain a large enough endosperm to sustain the embryo during germination, and contain sufficient hormones to initiate the process. In general, do not expect more than 65% to 80% of new seeds to germinate. From those germinating, expect about 60% to 75% to produce satisfactory, vigorous, sturdy seedlings. There are four environmental factors which affect germination: water, oxygen, light, and heat.
The first step in the germination process is the imbibition or absorption of water. Even though seeds have great absorbing power due to the nature of the seed coat, the amount of available water in the substrate affects the uptake of water. An adequate, continuous supply of water is important to ensure germination. Once the germination process has begun, a dry period can cause the death of the embryo.
Light is known to stimulate or to inhibit germination of some types of seed. The light reaction involved here is a complex process. Some crops which have a requirement for light to assist seed germination are ageratum, begonia, browallia, impatiens, lettuce, and petunia. Conversely, peas, beans, calendula, centaurea, annual phlox, verbena, and vinca will germinate best in the dark. Other plants are not specific at all. Seed catalogs and seed packets often list germination or cultural tips for individual varieties. When sowing light-requiring seed, do as nature does, and leave them on the soil surface. If they are covered at all, cover them lightly with fine peat moss or fine vermiculite. These two materials, if not applied too heavily, will permit some light to reach the seed and will not limit germination. When starting seed in the home, supplemental light can be provided by fluorescent fixtures suspended 6 to 12 inches above the seeds for 16 hours a day. High intensity lights will provide more light over the course of the day and will enhance the quality of seedlings. These lights cost more than the common shop lights, but are often worth the investment if you plan on growing plants indoors.
In all viable seed, respiration takes place. The respiration in dormant seed is low, but some oxygen is required. The respiration rate increases during germination, therefore, the substrate in which the seeds are placed should be loose and well-aerated. If the oxygen supply during germination is limited or reduced, germination can be severely retarded or inhibited.
A favorable temperature is another important requirement of germination. It not only affects the germination percentage but also the rate of germination. Some seeds will germinate over a wide range of temperatures, whereas others require a narrow range. Many seeds have minimum, maximum, and optimum temperatures at which they germinate. For example, tomato seed has a minimum germination temperature of 50 degrees F. and a maximum temperature of 95 degrees, but an optimum germination temperature of about 80 degrees. Where germination temperatures are listed, they are usually the optimum temperatures unless otherwise specified. Generally, 65 to 75 degrees F. is best for most plants. This often means the germination flats may have to be placed in special chambers or on radiators, heating cables, or heating mats to maintain optimum temperature. The importance of maintaining proper substrate temperature to achieve maximum germination percentages cannot be over-emphasized. It’s also very important to note that the recommended temperatures need to be maintained 24 hours a day.
Transplanting and Handling
If the plants have not been seeded in individual containers, they must be transplanted to give them proper growing space. One of the most common mistakes made is leaving the seedlings in the seed flat too long. The ideal time to transplant young seedlings is when they are small and there is little danger from setback. This is usually about the time the first true leaves appear above or between the cotyledon leaves (the cotyledons or seed leaves are the first leaves the seedling produces). Don’t let plants get hard and stunted or tall and leggy.
To transplant, carefully dig up the small plants with a knife or wooden plant label. Let the group of seedlings fall apart and pick out individual plants. Handle small seedlings by their leaves, not their delicate stems. Gently ease them apart in small groups which will make it easier to separate individual plants. Avoid tearing roots in the process. Punch a hole in the medium into which the seedling will be planted (see below for information about media). Make it deep enough so the seedling can be put at the same depth it was growing in the seed flat. Small plants or slow growers should be placed 1 inch apart and rapid-growing, large seedlings about 2 inches apart. After planting, firm the soil and water gently. Keep newly transplanted seedlings in the shade for a few days, or place them under fluorescent lights. Keep them away from direct heat sources. Continue watering and fertilizing as in the seed flats.
Most plants transplant well and can be started indoors, but a few plants are difficult to transplant. These are generally directly seeded outdoors or sown directly into individual containers indoors. Examples include peas, beans, carrots, beets, chard, zinnias and cucurbits, such as melons and squash.
Hardening is the process of altering the quality of plant growth to withstand the change in environmental conditions which occurs when plants are transferred from a greenhouse or home to the garden. A severe check in growth may occur if plants produced in the home are planted outdoors without a transition period. Hardening is most critical with early crops, when adverse climatic conditions can be expected.
Hardening can be accomplished by gradually lowering temperatures and relative humidity and reducing water. This procedure results in an accumulation of carbohydrates and a thickening of cell walls. A change from a soft, succulent type of growth to a firmer, harder type is desired.
This process should be started at least 2 weeks before planting in the garden. If possible, plants should be moved to a 45 to 50 degree F. temperature indoors or outdoors in a shady location. A cold frame is excellent for this purpose. When put outdoors, plants should be shaded, and then gradually moved into sunlight. Each day, gradually increase the length of exposure. Don’t put tender seedlings outdoors on windy days or when temperatures are below 45 degrees F. Reduce the frequency of watering to slow growth, but don’t allow plants to wilt. Even cold-hardy plants will be hurt if exposed to freezing temperatures before they are hardened. After proper hardening, however, they can be planted outdoors and light frosts will not damage them.
The hardening process is intended to slow plant growth. If carried to the extreme of actually stopping plant growth, significant damage can be done to certain crops. For example, cauliflower will make thumb size heads and fail to develop further if hardened too severely. Cucumbers and melons will stop growth if hardened.