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Propagating Native Woody Trees and Shrubs

A number of native woody trees and shrubs can be propagated by the hobbyist using vegetative cuttings. But, from the outset it must be realized that not all natives lend themselves to propagation by this means. In fact, natives that are seldom seen in the nursery trade often go unused because there is no easy means of propagating them.

Cutting propagation relies on the plant’s innate ability to form new roots from stem tissue. To accomplish this seemingly simple task, cells embedded in the plant stem must be switched from a stem cell to a root cell. This ability, or sometimes lack of ability, to differentiate from one type of tissue and change into another is what determines which species can be propagated by cuttings and which can not. Generally speaking, younger plant and more vigorously growing plants are more likely to make this transformation than old, mature trees and shrubs that are just eking out an existence in their native habitat. Frequently plants that are propagated from the wild will display poorer rooting when initially collected but will root easer in subsequent generations when the plant is grown under cultivation.

Cuttings may be classified as either herbaceous or woody. Herbaceous plants, including plants such as Phlox, Amsonia and Baptisia usually root easily provided the herbaceous cuttings are taken while plants are in the vegetative state of growth and proper care is given the more succulent stems. Woody plants, including both trees and shrubs, go through a transition in growth from the softwood stage to the hardwood stage. Softwood occurs when a woody plant begins to grow but before the wood lignifies. Intermediate between these two stages is a stage of development called the semihardwood stage in which the wood has fully expanded in size but only the cellulose of the cell wall has formed, not the lignin which makes the wood truly rigid and woody. The transition from softwood to hardwood takes from two to six weeks, depending on the species and the environmental conditions at the time.

Easy to root species such as Euonymus and Sambucus will root about as easy in one stage as the next. Some species, such as native azaleas and redbuds will only root in the softwood stage and loose the ability to root once the wood makes the transition into the semiharwood stage. Some plants such as Aesculus and Rhododendron only make one flush of growth in the spring and then set a resting bud for next season. With these kinds of plants there is only a short period of time when cuttings can be taken for successful rooting. Other plants, such as pines and oaks, make multiple flushes of growth and have several times during the year when softwood cuttings would be available (note - neither pines or oaks are easy to root from cuttings). A third group of plants, such as sweetgum, red maple and redbud make more or less continuous growth through the summer, provided the environment is suitable for such growth.

Most broadleaf evergreens and deciduous plants root best during the softwood or semihardwood stage of growth. Cuttings of these plants may be made from mid May until mid July and usually root successfully. Cuttings 4 to 6 inches long are best. Conifers such as Juniper only root from hardwood cuttings taken after the first hard freeze.

Rooting hormones speed the rate of rooting, increase root number on the cuttings and, in many cases spell the difference between success and failure in rooting. The softer the tissue (softwood to hardwood), the lower the concentration that should be used. If liquid quick dip materials are being used, a concentration of 500 to 1000 ppm IBA is usually adequate. For difficult to root species a concentration of 3000 to 5000 ppm is often used. On some really difficult species hormones as high as 15,000 ppm are sometimes used.

Hormones may be applied as either a liquid quick dip using pure product obtainable from chemical supply specialists such as Sigma Chemical or as a specially formulated liquid or dry product. The most common rooting hormone available from most garden centers is a dry talc formulation containing 1000 ppm IBA (Indole Butyric Acid). It is suitable for softwood cuttings and most easy to root species. If the pure product is used, the potassium salt of IBA, K-IBA is preferred because of its water solubility. Nurserymen often use a liquid hormone formulation that contains both IBA and NAA. It is diluted with water as needed to obtain the desired concentration.

To achieve successful rooting, the cutting must be kept alive and well while the roots form. For centuries the only time cuttings were rooted was during the winter when plants were dormant. Deciduous hardwood cuttings will root if kept moist but roots do not begin to form until the buds begin to grow in the spring. Moisture conditions during this period are critical.

A second advancement in propagation occurred when cuttings were stuck in sealed cold frames and watered as needed to maintain the desired moisture level. This technique has been updated with plastic film and many hobby propagators use only this passive high humidity system to induce rooting. Small flower pots covered with a plastic bag or larger frames covered with sheet plastic work equally well. These passive systems must either be kept out of direct sun or ventilated carefully to prevent overheating from occurring.

Commercial propagators use some type of mist system to aid in rooting. The mist system is easily adapted using controllers and equipment available for lawn irrigation. Mist permits cuttings to be made during any season of the year and allows one to work with softwood tissue which is difficult to handle with other systems.