Trends and Uses of Plant Growth Regulators on Herbaceous Perennials

Production of herbaceous perennials has been one of the most rapidly growing areas of floriculture for the last 8 to 10 years. However, greenhouse and nursery production of herbaceous perennials requires a significant effort to control the growth of some of these very vigorous plants. Until very recently the primary methods of control were scheduling and pruning. If a grower missed the target date, he cut the crop back. However, in the effort to keep these plants in balance with their containers and shipping carts, growers have the same arsenal of chemical plant growth regulators (PGRs) that has been used on annual bedding plants and other potted floricultural crops since the early 1960s. Herbaceous perennial growers understand that plant height affects the perceived quality of the plant, i.e., a plant in balance with its container size is of higher quality, which means that these plants are the most saleable. Growers using PGRs to accomplish this height control have fewer production losses due to the more saleable plants as well as the physiological enhancement of stress tolerance, which results in longer shelf-life in the greenhouse or in the retail setting. Another major advantage to growers using PGRs is that more compact plants can be more easily and economically shipped and displayed on typical retail racks. After many years of conducting research on crop responses to a variety of PGRs, we are finally seeing growers testing these methods on herbaceous perennials in their own nurseries. The primary plant processes that we seek to affect with PGRs in perennial production are growth regulation of plant height and branching. Other chemicals are used in other crops primarily to affect flowering or in propagation of cuttings. Controlling Plant Height Plant Growth Retardants. The primary chemicals used to control plant height are gibberellin (GA) inhibitors. The gibberellins are the natural plant hormones that cause the elongation of cells in the plant. They are present in the greatest quantities in those tissues that are elongating the most rapidly, usually stems, petioles, and, in some crops, the flower inflorescences. The PGRs that are used to reduce height are commonly called plant growth retardants. They consist of several older compounds that were introduced to floriculture in 1960s and 70s like B-Nine (daminozide, Crompton/ Uniroyal Chemical Co.) and Cycocel (chlormequat chloride, Olympic Horticultural Products) and A-Rest (ancymidol, SePRO Chemical). These products are still commonly used in the industry. In the 1980s, a new class of compounds was introduced, the triazoles, which include the paclobutrazols, Bonzi (Syngenta Professional Products) and Piccolo (Fine Americas) and uniconazole which is sold as Sumagic (Valent USA). These products are much more potent than the older chemistry. Around that same time, flurprimidol was also being tested in floricultural crops but was not released in the United States for this market. Soon to be released as Topflor (SePRO Chemical), flurprimidol is also a very potent GA inhibitor. With respect to growth retardant use on herbaceous perennials, all of these PGRs are used in this market. B-Nine or a tank mix of B-Nine and Cycocel is the common starting place for many producers new to using PGRs. These are safer PGRs with more short-term effects on 1 Department of Horticulture, 301 Saunders Hall, Virginia Tech, Blacksburg, VA 24061-0327