From The Test Tube To The Garden
Dix Hills, New York
Reprinted from the New York Chapter, ARS, newsletter, March 1989
Tissue culture (micropropagation) is a relatively new method of propagating rhododendrons when compared to the traditional methods of cuttings, seed, grafting, layering, and so on. This approach has the chief advantage of producing large quantities of a particular plant relatively quickly and easily. New varieties are brought into the marketplace in a short period of time. Rare, interesting or difficult to propagate varieties are mass produced.
As with any new industry, some errors have occurred. Most people in the nursery trade have shown a willingness to correct such problems as: improperly labeled cultivars, mutations, excessive callus formation, weak stems, incorrect levels of chemicals involved in the multiplication process, and poor cultural practices outside the laboratory. Moreover, many of the problems that did occur were due to human errors and lack of knowledge. These should almost disappear with a more diligent approach by management and a better understanding of the process involved.
Considerable misconceptions exist today regarding growing tissue culture plants. First, there is a lack of information being disseminated by the producers about the aftercare of these plants. Thousands of tissue culture plants are sold at rhododendron conventions, by local rhododendron societies, and by garden clubs to their members. In all probability, some of these plants have died due to improper aftercare. Many home gardeners lack the proper facilities and do not know the correct growing techniques to handle these small plants.
Our nursery grows many different varieties of tissue culture plants. We have obtained plants as micro cuttings, rooted propagules, and liners from many different sources or labs. As they are grown under different conditions, the quality varies widely. Some fellow nursery growers have expressed their displeasure with these plants due to the variation in growth and performance, as well as the many losses incurred when growing these plants in their nursery. We have been consulted many times concerning our growing methods and would like to pass on our experience in successfully growing these plants from the test tube to a suitable size for the garden.
Tissue culture plants behave like seedlings. They have lower branching and if properly maintained, they are more compact and form better shaped plants over a shorter period of time compared to plants produced by conventional methods. They have shorter internodes and will branch readily at lower heights. As a mature plant, they will match exactly the mother plant in hardiness, growth rate, floriferousness and overall performance.
Many tissue culture liners are grown in open mesh 2" pots which root prune the plant and cause the outer roots to be nonviable. It is imperative to establish these plants in a larger pot or an especially prepared bed utilizing an artificial soil mix similar to the one in which the plant is growing. These plants cannot be planted in the open ground or field as they will have great difficulty establishing themselves due to the different soil density. In addition, tissue culture plants are watered almost every day in the greenhouse, but a similar watering schedule in the field would certainly result in the plant's demise.
The roots of tissue culture plants are fragile, and in some cases, their attachments to the stem are not completely secure. Transplanting operations are best done with care. Excessive firming of the soil mixture should be avoided. These plants need gentleness and tender loving care to avoid root damage, but they will take off quickly if handled properly. As with other rhododendrons, these plants will survive and prosper if they are planted with the top of the root ball at the same level as previously. One of the greatest causes of transplant death is planting too deeply. This especially holds true for these young tissue culture plants, and may cause other problems such as stunted growth, diseases, and poor root development.
After transplanting into larger pots, these young plants should be carefully acclimated to their new conditions, and proper watering techniques practiced to wean them from their daily watering schedule. It will usually take 4 to 6 weeks for a plant to become established, as root damage and cultural shock is inevitable during shipping and subsequent transplanting.
Tissue culture plants that are received by us during the spring and summer months are placed under 50% shade for a period of 4 weeks and then gradually exposed to greater light. Their ultimate size at the end of the growing season will determine the winter protection necessary for survival. Large plants are placed in unheated overwinter houses or cold frames, whereas small plants may enter an accelerated growth program or be placed in minimum heat structures. Good judgment and experience are necessary in determining which treatment to implement as these young plants, not having reached their ultimate hardiness, need some winter protection. This may be due to the lack of development of woody tissue which protects the trunk and stems.
Plants obtained in the fall or winter months are placed in an accelerated growth program to reach a saleable size in a much shorter period of time. Dormant plants need a period of 4 to 8 weeks of temperatures below 40°F, depending on variety, before they will commence growth. Failure to allow a plant to complete this chilling requirement delays new growth, often until midsummer.
After the plants have been kept below 40° until mid December, gradually the temperature is increased in 5° increments. This increase in temperature will be accompanied by strong root growth. After top growth begins, a weak soluble fertilizer solution every 10 days may be applied to most plants. Some rhododendron cultivars resent fertilizer as exhibited by leaf tip burn injury, and these plants are not fertilized. Pesticides and fungicides are used on a periodic basis to prevent and/or eliminate any problems, although these plants usually are trouble free. By late March or early April, the new flush of growth has hardened off and the plant is ready to be moved into a larger container and gradually acclimated to its move outside the greenhouse.
Generally speaking, rhododendron liners have been well fertilized by the grower and a continued program of fertilization will insure proper growth. In a accelerated growth program however, nitrogen levels should be reduced to compensate for the reduced light levels and to avoid the danger of ammonium buildup. Slow release fertilizer pellets or tablets can cause problems in greenhouses at this time, dilute liquid feeds are more effective in these growth programs.
Although there have been some difficulties with tissue culture plants in the past, they certainly offer a useful method of plant production. Many new plants will be offered in the future, and in the hands of the professional grower, they will reach the marketplace as a quality horticultural product. Experience is necessary to properly handle these young plants, and more information as well as effective growing methods should be available to growers and other interested parties in the coming years.