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Chapter 4: vegetative propagation.

KEY TERMS

adventitious root bench grafting bridge grafting cleft grafting clone hardwood cutting herbaceous stem cutting in vitro interstock leaf-bud cutting leaf cutting micropropagules polarity root cutting rootstock scion semi-hardwood cutting side grafting softwood cutting splice grafting whip-and-tongue grafting

Free plants! They are available to anyone who knows the secrets of vegetative, or asexual, propagation. Plants amaze us with their myriad ways of being cloned. Clones are simply replications of an organism that bear the identical genetic makeup of the original. Vegetative propagation is used when seeds are difficult to obtain or when a certain cultivar of a plant will not come true from seed. Plants can be vegetatively propagated by various techniques that will be described in this chapter. If done properly, you can obtain any number of favorite plants from friends and family, and you can pass them on to others, too.

Most of the techniques for cloning plants described here would not be successful without the formation of adventitious roots, or the formation of roots where they would not normally develop. This is particularly true of the techniques in which cutting into a stem is required. Such techniques include cuttings, air-layering, budding, and grafting.

CUTTINGS

Cuttings are widely used and perhaps the easiest vegetative propagation technique to master. They may be taken from leaves, stems, and even roots. The best results are achieved when appropriate media are used, in conjunction with bottom heat (65 to 75[degrees]F) and adequate moisture. In some cases, rooting success is enhanced by the use of rooting hormones or rooting compounds that contain auxin and fungicide. These are not always necessary, but they are available in different concentrations, with higher concentrations recommended for more difficult-to-root species. Appropriate rooting media will provide adequate drainage while maintaining water-holding capacity. Some good combinations include sand and peat moss or perlite and vermiculite. Pure vermiculite, pure perlite, and pure sand have all been used alone with good success. Water alone is often enough to get the rooting process started.

STEM CUTTINGS

This is one of the most popular ways of cloning houseplants and woody plants. In this method, a stem is removed from a plant and placed into soil or water and allowed to root. Whether or not there are leaves depends on what type of stem cutting you make. There are four main categories described here: herbaceous, softwood, semi-hardwood, and hardwood.

Herbaceous stem cuttings are taken from plants that never get woody. Geraniums, poinsettias, chrysanthemums, and many other plants are propagated this way for both private use and for commercial production. Cuttings are usually 4 to 6 inches long and include a shoot tip and three to six leaves. The lower leaves are removed, leaving one fully expanded leaf and one or two partially expanded leaves (Fig. 4-1). The lower nodes are placed into the medium.

Herbaceous cuttings generally develop roots within a few weeks. Alternatively, they may be started in water and then transferred to soilless media once root formation has begun. Any flowers or fruit should be removed before or during the rooting process, as these will rob the developing roots of nutrients needed for their formation. Because herbaceous cuttings have succulent stems and leaves through which water will transpire, they must never be allowed to dry out. It may be advantageous to use a rooting hormone combined with fungicide for cuttings that are placed directly into soilless media.

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Softwood cuttings are taken from woody plants in the spring, after the first flush of growth. Maples and others may be successfully propagated this way. Cuttings are usually 4 to 6 inches long, from succulent new growth. They should be placed into a well-draining medium and kept moist and humid, as the leaves will transpire water during the rooting process. Bottom heat will increase success with rooting and decrease the time for roots to develop. They will usually root within 3 to 4 weeks.

Semi-hardwood cuttings are made in late summer and are taken from partially woody growth of the current season. Rooting is slower than with herbaceous or softwood cuttings, but more rapid than with hardwood cuttings. Remove approximately 4 to 6 inches from new growth and wrap stems with moistened paper towels. Place cuttings into a plastic bag until they are stuck into moistened media to prevent them from drying out. Remove only the lower leaves, leaving one or two mature leaves attached (Fig. 4-2). If there are any remaining leaves that are larger than a few inches in diameter, they may be cut in half to reduce the surface area that would lose moisture during the rooting period.

Hardwood cuttings are taken from matured wood of the past season's growth, in late fall through early spring, from deciduous or evergreen plants (Fig. 4-3). They may be rooted immediately or allowed to heel-in before rooting. In the latter case, cuttings are stored in moist newspaper or perlite or another medium under cool conditions. They must not be allowed to dry out and require monitoring for fungal growth. Callus formation occurs during the heeling-in process. In either case, cuttings may be 6 to 12 inches long or longer, depending on the species. Because these cuttings do not have leaves through which to lose water, they do not require a mist system to maintain high humidity during the rooting process. They should be inserted into the rooting medium about 1 to 2 inches deep and kept moist. Rooting should occur in about 4 weeks.

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Root cuttings are taken from plants such as pink coreopsis or phlox. Cuttings are most successful if they are first allowed to heel-in for a few weeks. When taking the cuttings, cut straight across at the distal (far) end and make a slanted cut on the proximal (crown) end. This will ensure that you are planting with the correct polarity--top end up. If cuttings are not to be planted immediately, they should be bundled together and placed into a moist medium such as sawdust or perlite and stored at 40[degrees]F. When planting, place the cuttings about 2 to 3 inches into the rooting medium and provide bottom heat. The soil should be warmed to about 65 to 75[degrees]F. Within a few weeks, new roots and shoot growth should be visible.

Leaf section cuttings are taken from plants with long, fleshy leaves or from plants that form a basal rosette. Mother-in-law's-tongue, or Sansevieria, is commonly propagated this way (Fig. 4-4). Remove the leaf from the plant and then cut it into 3- to 4-inch sections from tip to base. Each section may be placed into rooting media, bottom side down. New roots and shoots will form at the base of the leaf section, and the original leaf piece will disintegrate. Be mindful that variegated leaves will not result in variegated clones, which are best obtained by division of the entire plant. Begonia rex, fibrous-rooted begonias, and African violets may also be propagated by leaf cutting (Fig. 4-5). However, the entire leaf is used, with or without the petiole attached, and it is not cut into sections.

Leaf-bud cuttings are similar to leaf cuttings, except that, in addition to a leaf, a piece of stem and a bud are also included. Most herbaceous houseplants may be propagated in this way, including pothos and philodendron.

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BULB PROPAGATION

Storage organs such as true bulbs and related structures may be propagated in special ways that will enhance the number of propagules obtained. Some bulbs naturally produce bulblets and bulbils, but the numbers of these produced each year may be small or they may require several years to develop. By using the special techniques of scooping, scoring, coring, sectioning, and scaling, you can increase the number of bulblets produced. Review the parts of a bulb (chapter 1) and familiarize yourself with bulb structure before proceeding. (Fig. 4-6a and b) Corms produce cormels around the new corm every year. The old corm is used up during leaf growth and blooming.

Scooping

In scooping, remove the entire basal plate, including the center flower bud and main shoot at the center (Fig. 4-7). New bulblets will form at the base of the fleshy leaf scales. Treat the bulb with fungicide and store at 70[degrees]F in a dark place. The scales will begin to swell in about 3 weeks. Raise the temperature to 85[degrees]F. About 25 to 50 small bulblets will have formed. When roots begin to form you may transplant the new bulblets. Plant the bulb and attached bulblets about 1 inch deep. Four to five years of growth are required for plants to flower.

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Scoring

Make three V-shaped cuts into the basal plate of the bulb, so there are six equal wedge shapes. Do not cut all the way through the bulb but only enough to kill the main shoot. The cut should stop below the midsection of the bulb (Fig. 4-8). Place the cut bulbs upside-down 2 inches deep in clean, dry sand. Treat the same as for scooped bulbs. Scoring produces about half as many bulblets as scooping, but they should be larger and will bloom in only 3 to 4 years.

Coring

Remove the center portion of the bulb. You may use a boring instrument to accomplish this (Fig. 4-9). Treat the same as for scooping and scoring. You will get larger but fewer bulblets from this procedure, and they should bloom in 2 or 3 years.

Sectioning

Cut the bulb longitudinally from base to tip, in 5 to 10 sections. Be sure to include a portion of the basal plate with each section (Fig. 4-10). Treat the same as for the other bulb-propagation techniques discussed here. Bulblets will form on each section and should bloom in 2 to 5 years, depending on the size of the section.

Scaling

Scaling is a modified version of sectioning, in which two to three scales are left together, including a portion of basal plate (Fig. 4-11). Scaling can also be done on scaly bulbs such as lilies. Scales are planted at 1-inch spacing. In the fall, they will be ready to transplant to their final destination.

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LAYERING

Layering is a method of propagation in which adventitious roots develop before removal of the shoot from the mother plant. Layering works well for some large plants for which stem cuttings are not successful. In layering, the upright shoots of a bramble, such as blackberries and raspberries, are bent toward the ground so that the tip and about 6 inches below it touch the ground. They may be laid in a trench or hole that has been prepared ahead of time. The tip may be buried or allowed to remain uncovered, while several inches of the shoot below the tip are covered with 3 to 4 inches of soil. The shoot may require pinning to keep it in place. Adventitious roots will develop at leaf nodes after 4 to 6 weeks. To remove, simply cut the shoot below the covered area and transplant it with roots intact into a new location (Fig. 4-12). Other plants that may be treated this way are red twig dogwood, bearberry cotoneaster, English ivy, coleus, Swedish ivy, philodendron, and other multistemmed shrubs, groundcovers, and climbing plants.

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AIR-LAYERING

This technique is widely used on many tropical and subtropical plants. Roots are stimulated before removal of the shoot from the plant. A cut is made into the stem of the plant, through xylem, phloem, and cambium tissue, but leaving approximately one-half of the stem intact and attached to the plant. The cut may be a wedge that is removed or a nick into the center of the stem that is then held open with a matchstick-sized prop. The inner stem tissue is dusted with rooting hormone and then the entire cut area is wrapped in a clump of moistened sphagnum peat moss, about the size of a fist. Hold the peat moss in place with plastic and tie the upper and lower ends of the plastic to the stem using twist ties or string (Fig. 4-13). You may wrap the entire thing with aluminum foil if desired. This technique requires a long period of time, from 1 to several months. Remove the foil to view the peat moss wrapped in plastic. When roots are visible through the plastic, the shoot may be removed entirely from the plant. Cut just below the area where the moss and plastic are and carefully remove the plastic but not the moss. Place the entire root system and moss into an appropriate container for your new plant. The mother plant will stimulate new growth below the cut. This is a good way to prune a leggy dracaena, croton, or rubber tree and gain a new plant along the way.

CROWN DIVISION

This technique works well with plants that do not spread by stolons or rhizomes and lack vertical stems with axillary buds. These plants, which include ornamental grasses, hostas, and many herbaceous perennials, have crowns, from which shoots and roots emerge and develop.

When a plant is divided by this technique, variable-sized plants are possible, depending on the size of the original plant and the desired size of the daughter plants. Regardless of how large or small a division is taken from the original plant, each must include some crown tissue, at least on the shoot, and some root tissue (Fig. 14-14a-f). However, a larger division may be taken. The larger it is, the sooner it will fill in he new space when it is transplanted.

BUDDING AND GRAFTING

Budding and grafting techniques have been used for centuries. Theophrastus described it in 323 BC but the Chinese diplomat, Feng Li, gave up his profession for a business venture of budding and grafting fruit trees in 5000 BC. These techniques have been used extensively on woody fruit trees such as pears, apples, peaches, and plums. However, they are also applied to watermelon in Korea to bud onto disease-resistant rootstocks.

Budding and grafting are techniques that can be used to join two different cultivars or even species together into a single plant as a means of combining the best qualities of the two plants. The bottom part of a grafted or budded plant is the rootstock, and the top part is the scion. An example of combining characteristics of two plants is disease and pest resistance in the rootstock combined with large, sweet fruit in the scion. There is sometimes a third part, an interstock, that is grafted between the rootstock and the scion. Tree roses, or "standards" are grown with an interstock that provides the straight stem.

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Budded and grafted plants are able to grow together because contact is made at the vascular cambium, one of the meristems in plants. Because cell division occurs there, the dividing cells from the two different plants are able to grow together and become fused into one single plant (Fig. 4-15).

Scions are selected for their improved fruits or flowers, such as on fruit trees or ornamental trees or shrubs. Rootstocks are selected for their improved resistance to diseases or insect pests or vigorous growth. Rootstocks may also impart a special characteristic, such as dwarfing, on the scion. Apple rootstocks developed in England serve this purpose. The resultant trees require less pruning, can be grown at closer spacing, and require smaller ladders for harvesting fruit.

The major types of budding techniques are June budding, or T-budding, and chip budding (Fig. 4-16a and b). The former type of budding is done when the bark is slipping or generally in June. It is also called T-budding because of a T-shaped cut that is made in the rootstock for insertion of the bud. Chip budding does not require that the bark is slipping.

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Major types of grafting techniques include splice grafting, whip-and-tongue grafting, cleft grafting, bench grafting, side grafting, and bridge grafting (Fig. 14-17a-f).

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MICROPROPAGATION

Micropropagation is a general term that really applies to propagation of tiny pieces of plant tissue called micropropagules grown in test tubes, or in vitro. Other in vitro techniques include ovule, embryo, and seed culture. Other methods that are used on a lesser scale are creation of somatic seeds and anther culture. Somatic seeds are seeds produced from vegetative rather than reproductive tissue. These could provide a relatively inexpensive method of seed production in plants that do not normally produce seeds or produce too few for commercial purposes. Offspring are clones of the parent plant. In anther culture, haploid plants result. These can then be treated with colchicine, a chemical that causes doubling of chromosomes. The resultant plant is completely homozygous--genes are in identical matching pairs. These "inbred" plants can be used in developing hybrids or for other plant-breeding purposes and for genetic studies (Fig. 4-18).

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Advantages of micropropagation include the ability to rapidly reproduce large numbers of plants in a short period of time, the ability to propagate new plants from a very small amount of tissue, leaving the original plant intact, and the ability to maintain micropropagated tissue in vitro indefinitely, providing an infinite source of propagation tissue. Micropropagation provides a means of generating clones of plants that are otherwise difficult to propagate any other way.

Some African violets are produced commercially by micropropagation. This technique replaces the traditional leaf-cutting method. A leaf-cutting takes about 4 to 6 months to produce one sellable plant, while micropropagation with the same amount of plant material can produce thousands of plantlets in the same period of time. Orchid seeds are grown in test tubes to avoid the need to germinate seeds in soil containing the requisite mycorrhizae. Orchids are notoriously difficult to grow from seed. Test-tube seed culture has helped orchids become one of the largest-selling flowering potted plants in the United States.

In tissue culture, the plant hormones auxin and cytokinin are used in an agarbased medium, along with vitamins and plant nutrients. Auxin and cytokinin levels are manipulated to cause development of either undifferentiated tissue, such as callus, or shoots or roots. In this way, new plants can be obtained from virtually endless amounts of callus tissue, and when the shoots and roots are prompted to grow, the new plants are still identical to the parent plants.

SUMMARY

Plants can be propagated in ways other than by seed. These ways are categorized as asexual reproduction, or vegetative propagation. In a generic sense, they can be considered cloning. The types of vegetative propagation commonly used include various types of cuttings, specialized bulb techniques, layering and air-layering, crown division, and budding and grafting. Micropropagation involves the use of very small pieces of plant tissue that are propagated in test tubes or jars. Some vegetative propagation techniques have been known and used for thousands of years, others are fairly recent, and still others occasionally occur naturally.

HANDS-ON ACTIVITIES

* Research plants that will root in water. Try several different species by taking stem cuttings.

* Propagate a tuber (potato) or a tuberous root (sweet potato). Cut a piece of potato, making sure to have at least one bud, or "eye" in the cutting. Stick toothpicks into the potato piece to suspend it over a glass or jar of water, inserting one end of the potato in water. The bud should be on top. Place in a light, sunny location.

* Take cuttings of woody plants. Try treatments with and without rooting hormone, mist, and bottom heat.

* Use crown division to propagate large, overgrown hostas.

REVIEW QUESTIONS

1. What are adventitious roots, and what role do they play in vegetative propagation?

2. What are clones, and how are they obtained from plants?

3. Compare and contrast the various forms of stem cuttings: herbaceous, softwood, semi-hardwood, and hardwood. Include types of plants used, season of collection, and size of cuttings required.

4. Describe and discuss air layering. What plants can be propagated this way? What plants must be propagated this way?

5. Describe the method of leaf cuttings. What plant is regularly propagated this way?

6. Why is cambium important in budding and grafting?

7. When storing hardwood cuttings, what happens to the cut ends? What conditions must be provided for their storage?

8. What is crown division? What kinds of plants are normally propagated this way?

9. What is the bottom part of a grafted tree called? The top part?

10. What are the advantages of micropropagation? List some plants that are commonly propagated this way.

SELECTED REFERENCES

Dirr, M. A., & Heuser, C. W., Jr. (1987). The reference manual of woody plant propagation: From seed to tissue culture. Athens, GA: Varsity Press.

George, E. F. (1993). Plant propagation by tissue culture. Part 1: The technology. Blacksburg, VA: Exegetics Ltd.

George, E. F. (1996). Plant propagation by tissue culture. Part 2: In practice. Blacksburg, VA: Exegetics Ltd.

Hartmann, H. T., Kester, D. E., Davies, F. T., & Geneve, R. L. (1996). Plant propagation, principles and practices, 6th ed. Upper Saddle River, NJ: Prentice Hall.

Kyte, L., Kleyn, J., & Kleyn, J. G. (1996). Plants from test tubes. Portland, OR: Timber Press.

Dr. Marietta Loehrlein currently teaches horticulture classes at Western Illinois University in Macomb, Illinois. She earned both her bachelor's degree in Agronomy and her master's degree in Plant Genetics at The University of Arizona. Her master's research project was concerned with germination problems associated with triploid seeds, from which seedless watermelons grow. Following that she worked for 5 years in a breeding and research program for Sunworld, International near Bakersfield, California. She worked with peaches, nectarines, plums, apricots, and cherries. Then she returned to school to earn her Ph.D. in Horticultural Genetics at The Pennsylvania State University. Her Ph.D. research focused on flowering processes in regal pelargonium (also called Martha Washington geraniums). While at The Pennsylvania State University, she bred a new cultivar of regal pelargonium, "Camelot." At Western Illinois University, Dr. Loehrlein teaches nine courses: Greenhouse and Nursery Management, Introductory Horticulture, Landscape Design, Landscape Management, Home Horticulture, Plant Propagation, Turf Management, and two courses in Plant Identification.
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Author:Loehrlein, Marietta M.
Publication:Home Horticulture: Principles and Practices
Date:Jan 1, 2008
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