Vegetative Reproduction

Vegetative reproduction is the 'foolproof' propagation method of producing new plants with exactly the same genetic makeup and physical appearance as the original 'parent.' This method can be used to produce a whole team of Huskers that looks identical to ’Husker Red’ or to overcome the sterility or germination inconsistency seen in polyploids and hybrids. Methods of vegetative reproduction used for herbaceous landscape plants include cuttings, tissue culture, division of the original plant, or layering.

Cuttings

While cuttings result in identical plants, the process may be too time-consuming, labor-intensive, or require too much space to be used by most gardeners on most plants. However, it is used extensively in commercial production. As with seed production, the success of cuttings relies to a large extent on the physical characteristics of the plant from which they will be taken. Stems and roots are the most commonly chosen tissues of herbaceous landscape plants used for cuttings.

Stem cuttings taken from the terminal or tip of a stalk or branch are usually most successful. (Fig. 9) Some penstemons can be propagated in this manner. With some plant species, if cuttings are taken from a prostrate side branch, the growth habit of the resulting offspring may be low and spreading rather than assuming the form of the original plant. The quantity of offspring material available will depend on the growth rate and vigor of the plant supplying the cuttings. Unless the parental plants are being kept solely for vegetative reproduction purposes, most people want to avoid removing so much growth that the bloom, form, or overall character and usefulness of the plant in the landscape is damaged or destroyed. Most herbaceous plants produce successful stem cuttings from soft or new growth, rather than older growth. Often the quantity and spacing of nodes from which new roots will emerge from the stem cutting plays a part in the success. Applying hormones may aid in more rapid development of roots on the cuttings. Some plants refuse to root from stem cuttings regardless of the care that is taken in collecting and handling the material. In cases such as this, it may be easier to create new plants from root cuttings.

Root cuttings should not be taken from plants that are immature or have been recently moved, because the very process of taking the cuttings involves severe disturbance of the plant. Look at the physical characteristics of the root structure to help decide whether root cuttings will be successful. For example, many penstemon species grow from a woody caudex, which may form a taproot-type of structure underground. This type of root structure is often not very conducive to producing root cuttings. On the other hand, fibrous root systems, particularly those in which the individual roots are large or fleshy, provide more material for cuttings, usually with less damage to the original plant.

Many commonly grown herbaceous plants should have a 'Do Not Disturb' sign hanging from their stems. These plants often have root systems that are brittle, composed of very few roots, or have difficulty regenerating roots once they are disturbed. Balloon flower (Platycodon grandiflorus) and gas plant (Dictamnus albus) are examples of plants that respond poorly to disturbance.

Root cuttings are made by carefully digging whole plants or sections of plants and cutting sections of actively growing root tips. (Fig. 10) This is either done in the fall when plants are preparing to go dormant for the winter, or in the spring when plants are being divided. Fine roots one-fourth of an inch in diameter or less, like those of butterfly milkweed (Asclepias tuberosa) are sectioned into pieces one to two inches long. The root pieces are laid on their sides in a light soil or coarse sand and covered with half an inch of sand, which is then moistened and placed in a cold frame or other container to overwinter in cold conditions. Thicker roots, like those of common bleeding heart (Dicentra spectabilis) are cut into pieces three to four inches long. To keep track of which end is up, make a flat cut on the root closest to the crown of the plant and an angled cut toward the root end. Insert the cuttings vertically with the flat (top) end up into clean potting soil, cover, water, and over winter in a cold frame.

Division

Division might be considered a rather abrupt cutting or propagation method. However, home gardeners often use division more as a management technique to maintain the health and vigor of a herbaceous plant than to produce more plants. Again, an elementary understanding of the root structure of the plant will give you a clue about its ability to be successfully divided. Plants that produce small plantlets from a central crown, like Penstemon digitalis or Penstemon grandiflorus, can be divided by simply removing the smaller plants (each with a generous portion of root) and replanting them. Many herbaceous plants not only can be divided, but should be, since they develop a dead center section surrounded by healthy growth. 

The commonly grown chrysanthemum is an example of this type of herbaceous plant, and many grasses also exhibit this characteristic. As with other propagation methods, the timing for successful division depends on the species. Some plants, like daylilies (Hemerocallisspecies) can be divided nearly any time during the growing season. Plants can be successfully divided by either lifting the entire clump and breaking or cutting it into smaller pieces, each with a good portion of root and crown attached, or by using a sharp spade to cut an outer piece off a larger plant. The divisions should be inspected for insects and disease, and Division (Fig. 12) is done by:

A slightly different form of division is used for plants that grow from bulbs, corms, or rhizomes. Bulbs typically produce smaller bulbils around the original bulb; these can be dug, separated, and replanted. Tulips are a good example, and daffodils can also be propagated from bulbs. Corms develop offsets either on top of the original corm, or around it. Crocus produce corms. The method of division is the same as that used for bulbs. Plants that spread by rhizomes can be divided by cutting cleanly between the original plant and a rooted section of the rhizome. Daylilies are a common example of a plant that is rhizomatous. 

Layering

Plants with a prostrate or spreading habit often have long trailing stems that can provide the material for vegetative reproduction through layering. Penstemon pinifolius, pineleaf penstemon, is one such plant. The flowers of this plant are bright orange or yellow, but smaller than those of many other penstemons, and held within the mound of fine foliage. The trailing stems can be layered simply by mounding soil over a node and allowing roots to form. The resulting plants can then be separated from the original plant and moved to a new location. Many groundcovers typically have characteristic trailing stems.

Tissue Culture

Tissue culture seen in (Fig. 13) is a method of vegetative reproduction intended to produces exact replicas, or clones, of the original plant. However, in 2-10% of cases, variation occurs, so this is not a perfect method of creating exact replicas. The term ’micropropagation’ is descriptive of the most widely used tissue culture technique. When stock plants are propagated by cloning, all the time-consuming processes of cross-breeding or sexual reproduction are eliminated but so is the introduction of new traits. Large-scale use of this technique by producers demands strict attention to environmental conditions and a commitment to following through with human and fiscal resources. It is widely used for production of many foliage house plants and orchids, as well as rapid development of woody species. Tissue culture techniques also allow the production of virus-indexed plants, plants with greater resistance to stress or pests, or pathogen-free plants, as we discussed previously.