Plant propagation simply means "making more plants." Reproducing plants from seeds is called sexual propagation. If plant parts other than seeds are used to reproduce a plant, the method is known as asexual propagation. Many ornamental trees, flowering shrubs, foliage plants, and turf grasses are propagated by asexual means. Asexual propagation of plants is generally accomplished by one of three methods: cuttings, grafting, and tissue culture or micropropagation.
Asexual propagation is easy to accomplish, inexpensive, and often requires no special equipment. Asexual techniques are used because larger plants can be produced in a shorter period of time. If a plant does not form viable seeds, or if the seeds are difficult to germinate, asexual methods may be the only way to reproduce the species. Asexual propagation produces clones, and, consequently, all new plants will resemble the parent plant, a benefit for growers who want to multiply and sell a unique plant.
Since the newly propagated clones are genetically identical, they respond to the environment in similar ways. This uniformity makes culture and production easy. However, it can be a problem if a pest or disease attacks the crop. Asexually propagated clones may all be vulnerable to the attackers, which could wipe out the crop.
Asexual plant propagation using cuttings involves removing certain plant parts and allowing each cut part to become a new plant. Common plant parts used as cuttings include stems, leaves, leaf buds, and roots. When exposed to proper environmental conditions and appropriate cultural practices, cuttings form a root system and new foliage. New roots or shoots are termed adventitious growth.
Stem cuttings are one of the easiest and least-expensive methods of plant propagation. Most species will make roots in several weeks. Rooted cuttings are then easily grown to marketable size. Stem cuttings can be classified into groups according to the nature of the plant wood used: hardwood and softwood.
Hardwood stem cuttings are taken from mature dormant branches in late fall or early winter. If the species is deciduous, cuttings should be leafless. A 15-to 20-centimeter section of stem with at least three buds is used. The base of the cutting is cut at a slant to expose a larger area for rooting and to create a distinction between the cutting top and base. For evergreen species, leaves on the lower half of the cutting are stripped off. Then the base is dipped in a root-promoting hormone, either a powder or liquid. The cutting is inserted (about one-half its length) into a moist rooting medium composed of peat mixed with vermiculite or sand. Cuttings should be kept moist at a temperature favorable for optimum growth and development, depending on the plant species. Adequate light is necessary for root formation, but cuttings should never be placed in direct sunlight. Sometimes protected trays of hardwood cuttings are kept outside through mild winters, and they root as temperatures rise in the spring.
Stem cuttings of softwood generally root more easily than hardwood but require more attention and equipment. Softwood cuttings are usually 7.5 to 13 centimeters long with two or more nodes. They are usually made in late spring, using the succulent mature spring growth of deciduous or evergreen plants. Mature shoots, but not old woody stems, are desirable. A slanted base cut is made just below a node , and leaves on the lower half of the cutting are removed. Bases are treated with rooting stimulant before being placed into loose rooting mix. Softwood cuttings require high humidity, which can be achieved through the use of plastic tents, misting systems, and/or fogging. Temperature should be closely observed during rooting. Most species root best between temperatures of 25°C and 30°C at the base, with cooler temperatures for foliage. Having the base warmer than the tip promotes root growth before new top growth begins. Roots form in two to five weeks.
Leaf cuttings are another method of asexual propagation. In this process, an entire leaf (leaf blade plus petiole ), the leaf blade only, or just a portion of the blade can be used to produce another plant. Cut surfaces may be treated with rooting hormones. High relative humidity and warm soil temperature will speed adventitious root formation. The soil mix should be loose but damp. Using the leaves allows the reproduction of more new plants than is possible with stem cuttings: sometimes more than one new plant forms from each leaf. Many leaves can fill a shallow tray. On one greenhouse table hundreds of plants can be reproduced.
Leaf-bud cuttings are basically the same as leaf cuttings, except the leaf is left attached to a short piece of stem with its healthy axillary bud. The cut stem surface can be treated with root-promoting compound. The cuttings are inserted in a rooting medium with the bud about 1.4 centimeters below the surface. High humidity and optimum temperature will speed rooting.
Root cuttings when buried and kept warm will produce adventitious shoots. Root cuttings consist of roots less than 1 centimeter in diameter cut into short lengths, 2.5 to 8 centimeters long. These are placed horizontally on the rooting media and covered with a 1.3 centimeter layer of fine soil or sand. The rooting medium is kept moist; it is important not to let it dry out. After the cuttings have developed adventitious roots and shoots, they can be transplanted for further growth. Plants that naturally form suckers at the base are good candidates for cloning through root cuttings.
The process of joining plants together in such a way that they will unite and grow as one plant is commonly referred to as grafting. The part that becomes the upper portion or top of the graft is termed the scion, and the lower portion is called the rootstock or stock. The general process of joining plants is referred to as grafting. When the scion part is just a bud, the operation is called budding. Grafting is a method of plant propagation that is thousands of years old, and there are many methods. All of them involve fastening fresh woody stems together, excluding light, preventing the stems from drying out, and training the new growth.
The advantages of grafting and budding are numerous. Many common fruits such as apples, pears, peaches, and walnuts cannot be satisfactorily propagated by cuttings. In order to meet market demands, the grower is forced to use grafting and budding as a means of reproduction. In other instances, certain rootstocks have desirable characteristics such as resistance to soil diseases, dwarfism (reduced plant size), cold hardiness, and vigor. Through grafting it is possible to combine desirable qualities that are found in these rootstocks with desirable flowering and fruit qualities that are found on different scion varieties. Some stocks, by imparting a dwarf quality to the scion, reduce tree size. This allows smaller trees to be planted closer together in orchards, increasing crop yield per unit of space.
Grafting also enables the agriculturalist to change the variety of established plants. This technique is often beneficial in apple orchards. An older variety may be in ill health or no longer in demand. As long as the new scion wood is compatible, the established variety can serve as a rootstock and a new variety can be grafted onto old trees without digging up the entire orchard.
The fruits of grafted plants are always of the scion variety: they are not combinations or hybrids of the scion and stock varieties. When closely related plants are grafted, most will continue their growth as a single plant. When the scion and rootstock unite and continue to grow as one plant, producing for many years, the graft is said to be compatible. When unrelated plants are grafted, the usual result is failure; the graft is incompatible. Certain combinations of scion and rootstock might grow and continue in a normal manner for several weeks only to have the scion die. Trees that are advertised as being "fruit salad" trees have five or six different scion varieties grafted onto one stock. Typically several of these scions are incompatible and will die off.
Tissue Culture or Micropropagation.
If they are kept under a proper environment, plants can reproduce themselves even from very small parts. When the removed part is not much more than a few cells, the process is called tissue culture or micropropagation. The small section that is removed is termed the explant. It may be taken from the roots, leaves, stem, or growing tips. In tissue culture, the explant is put into nutrient solution in a test tube, petri dish, or other container and kept in a clean, bright environment. The artificial medium contains all the nutrients, food, and hormones to help the explant create new cells. Roots or shoots may emerge within a few weeks. Each rooted explant can then be divided, and each smaller portion cultured again, rapidly multiplying the number of plants.
The advantages of tissue culture or micropropagation include the following:
- All the plants produced through tissue culture are clones of the parent plant.
- Because the explants and lab containers are so small, micropropagation can yield many thousands of plants in one laboratory or greenhouse.
- Many clones can be multiplied relatively quickly.
- The smallest part of the growing tip (not much more than a few cells) is often free from viruses that may be present throughout the rest of the plant. By cloning this very small meristematic portion, a worker can reproduce the plant virus-free.
- Shipping many plants in small sterile containers is easier and safer than shipping larger plants with soil and watering problems.
- Many countries have laws that do not allow soil from other countries to cross their borders. Because these little plants in plastic containers do not have any soil, trade between these countries is not restricted.
Disadvantages or problems associated with tissue culture include the high cost of laboratory setup, the skill needed by workers, and the cost of environmental controls. Problems can also result from the lack of one of the important requirements for performing tissue culture or micropropagation successfully: a super-clean work area, correct medium, and proper environment. Laboratories or companies specializing in tissue culture often have very sophisticated equipment to ensure clean conditions. It is most important to keep bacteria and fungal spores away from the explant and nutrient medium. The explant surface is washed with a weak bleach solution to remove pathogens (bacteria or fungal spores that can spread in the container). Washing hands, cleaning tools, and surfaces with alcohol, and working quickly will prevent the contamination of containers and nutrient medium. Special nutrient mixes can be purchased from a commercial source or created from ingredients in a lab. The mix contains sugar, which supplies carbon to the tiny explant while it is unable to use the CO2 in the air for photosynthesis. Also required are vitamins and nutrients that a healthy plant absorbs through its roots. Hormones stimulate cell division and the formation of roots and shoots. Light is necessary for the plant to develop roots and shoots. Temperatures must be in the range of 20 to 27°C.
There are four stages of the tissue culture or micropropagation procedure:
- establishment and stabilization, when the small explant is inserted into the container (onto the medium) and survives the initial transplant;
- shoot multiplication, when each explant expands into a cluster of microshoots and each cluster is divided and re-cultured to increase numbers;
- pretransplant or in-vitro rooting, when the explant is transferred to another media mix that stimulates roots to form inside the container (in vitro means "in glass"); and
- transplanting, acclimation, and more rooting, when the explant is transplanted into a clean potting mix and moved into a warm bright greenhouse where it will begin independent photosynthesis and establishment.
see also Horticulture; Reproduction, Asexual; Reproduction, Sexual; Tissue Culture.
Elizabeth L. Davison
Dirr, Michael A., and Charles W. Heuser, Jr. The Reference Manual of Woody Plant Propagation. Athens, GA: Varsity Press, 1987.
Hartmann, T. Hudson, Dale E. Kester, Fred T. Davies, Jr., and Robert L. Geneve. Plant Propagation: Principles and Practices, 6th ed. Upper Saddle River, NJ: Prentice-Hall, Inc., 1997.
Thompson, Peter. Creative Propagation: A Grower's Guide. Portland, OR: Timber Press, 1992.
Plant propagation is the art and science of increasing numbers of plants utilizing both sexual and asexual methods. It is not an exaggeration to say that the continued existence of modern civilization depends upon plant propagation.
Sexual plant propagation is accomplished using seeds or spores. Many crops grown this way are essential for environmental quality, food, fiber, fuel, medicines, shelter, and myriad other plant-derived substances essential for quality of human life.
Seeds may be harvested from wild plants or from those subject to carefully controlled cross-pollination, which produces plants known as hybrids . These hybrid plants may have characteristics superior to their parents such as increased protein , better flavor, and pest resistance. Sexual plant propagation begins with seed harvesting and is separate from the creation of the cross-pollination process.
Seeds of most grains and vegetables require specific environmental conditions to germinate and grow. For these plants, proper seed harvest and storage to maintain viability and vigor are essential. Once a seed is sown, it can be expected to germinate in a period of time ranging from a few days to a few weeks.
Many seeds require special events or processes to occur before they can germinate. These may include cycles of warm and/or cool, moist treatments (stratification), cracking or wearing away of seed coats (scarification), smoke, intense heat from fire, or even passing through the digestive tract of an animal. Seeds of many perennial flowers as well as most trees and shrubs originating in temperate climates require physical and/or chemical treatment to overcome dormancy.
Some natural and human-made plant hybrids will not retain their desirable traits if allowed to reproduce sexually, so they must be propagated by asexual means to produce clones. A common technique in asexual plant propagation is stimulating root growth on plant parts such as stems that have been cut off. This is known as cutting propagation and is the most common form of propagation used in ornamental nursery production.
An ancient yet common asexual propagation technique involves joining the top of one plant (the "scion") with the root system of another. This is called grafting . Grafting allows combinations of desirable root characteristics of a plant (such as pest resistance) with desirable shoot characteristics of another (such as flavorful fruit). Often grafting is the only economical means to produce plants with those desirable characteristics. Grafting is a skill commonly employed in the production of fruit and nut-producing plants.
Another asexual plant propagation method is micropropagation, or tissue culture. In micropropagation, a very small piece of plant tissue is placed on an artificial growth medium under conditions similar to a hospital laboratory. Once sufficient tissue increase has occurred, plants are hormonally stimulated into differentiating to create a plant that can be grown outside the laboratory.
see also Clone; Hormones, Plant; Horticulturist
Richard E. Bir
Bir, Richard E. Growing and Propagating Showy Native Woody Plants. Chapel Hill, NC: University of North Carolina Press, 1992.
Hartman H., et al. Plant Propagation Principles and Practices, 6th ed. Englewood Cliffs, NJ: Prentice Hall, 1997.