Using plant growth regulators in the landscape

Linda Benedict, Chen, Yan, Bracy, Regina P., Owings, Allen D.  |  2/28/2012 1:28:02 AM

Yan Chen, Regina P. Bracy and Allen D. Owings

Many shrubs and groundcovers used in the southern landscape require routine pruning or shearing to keep their shape neat and compact. Pruning is a significant expenditure of time and a major labor cost for the landscape service industry. Reducing pruning needs has been the key motivator for the development and use of plant growth regulators (PGRs) in the landscape. In addition, more than half of the states have passed laws prohibiting disposing of yard wastes in landfills, which has increased the interest in using PGRs to reduce pruning or the amount of clippings.

A plant growth regulator is an organic compound, either natural or synthetic, that acts like a hormone and modifies or controls one or more physiological changes within a plant.

For the past 20 years, using PGRs to change plant growth patterns has become a common practice for producing quality horticulture crops, including vegetables, flowers and woody ornamental plants. More recently, the turf and sports industry began using PGRs to enhance turfgrass quality and its ability to tolerate environmental stress. Controlling growth of perennial ground covers and shrubs in the landscape is relatively new, and only a few PGRs are currently registered for this purpose.

Cutless .33G is a new formulation of an old active ingredient, flurprimidol, which acts as an enzyme inhibitor. Before Cutless was available, Atrimmec was the only PGR registered for woody landscape plants. The active ingredient in Atrimmec is dikegulac-sodium, which is absorbed through the leaf. It inhibits cell division and differentiation in the meristem (growing point). Although quick in action, the control effects last for only four to eight weeks.

Cutless is root-absorbed and has a different mode of action from Atrimmec. A 50 percent wettable powder formulation is registered on turfgrass to reduce the need for mowing and to suppress  annual bluegrass. The granular formulation at a low concentration is the first slow-release PGR in its chemical class, and growth control usually lasts for four to six months. In addition, it is relatively safer than a PGR that works on the growing point. Cutless is applied by broadcasting onto the soil surface around the plant, which is easier than spraying leaves or drenching roots as with other PGRs.

The LSU AgCenter Hammond Research Station has been testing this product for its appropriate application rate, timing and duration of control effects on common woody species requiring frequent pruning. Some examples are Asiatic jasmine, azalea, boxwood, Indian hawthorn, Knock Out rose and loropetalum.

Three studies were conducted in landscape research plots from 2008 to 2011. Plants were planted in the fall to allow the establishment of roots during winter and then treated with Cutless in the following spring. Low, medium and high rates of Cutless were applied immediately after the plants received a light pruning in early spring. In two studies, the plants were treated again in August, without pruning, to control fall growth. Overhead irrigation helped dissolve the active ingredient and facilitate uptake. After spring application, plant size was measured and visual quality was assessed every month for a period of eight to 12 months.

Results indicate that Cutless is effective in controlling growth of ornamental plants that have significant growth in spring or spring and fall. The degree of height reduction varies among species. However, growth control effects were not significant for slow-growing ornamentals such as azalea Sunglow. Control effects on some plants are summarized in Table 1.

Loropetalum:
Control effects were significant and long lasting in Merlot Lace. Plants treated with 14 pounds per 1,000 square feet in spring 2007 and again in 2008 remained compact with out the need for pruning from 2008 to 2010. This experiment also showed no difference between applying Cutless above or under pine straw mulch. More data are being collected with another popular variety, Zhu Zhou.

Knock Out rose:
Height reduction of 35 percent was observed at four weeks after two applications during the first year after establishment at 10 pounds per 1,000 square feet. Two applications of 20 pounds per 1,000 square feet resulted in similar height reduction and visual quality. A single application of either 10 or 20 pounds per 1,000 square feet provided 10 percent to 15 percent height reduction, which was not satisfactory because Knock Out rose is a vigorous plant.

Overall improvement in plant quality was also observed, including increased lateral branching and darker leaf color in treated plants. For example, darker purple leaves on loropetalum and darker green leaves on Knock Out rose were noted and confirmed by a chlorophyll content meter. The control effect has lasted for three months, and more data are being collected for estimating control duration on the popular landscape rose.

Boxwood and Indian hawthorn:
Height reductions of 25 percent to 30 percent were observed approximately four weeks after spring application at 10 pounds per 1,000 square feet on plants that were established through winter. Additional applications in the fall did not further affect plant growth.

Asiatic jasmine:
Height reduction by 20 percent was observed approximately four weeks after the spring application at 10 pounds per 1,000 square feet. Higher rates at 15 pounds per 1,000 square feet caused some leaf twisting in this plant, although overall visual quality was not affected.

In one study, fertilization was evaluated at various rates with Cutless. Results indicate that using controlled-release fertilizer such as Osmocote 14-14-14 at two pounds of nitrogen per 1,000 square feet will enhance plant quality compared with plants not fertilized. However, fertilizing at higher rates, for example four pounds of nitrogen per 1,000 square feet, increases plant height by 30 percent, counteracting Cutless and giving unwanted growth.

Effective control on ligustrum, sweet olive, cleyera, eleagnus and hollies has been reported from other trials in the southern region of the country. Improved flowering was also reported for star jasmine, hibiscus and ixora after Cutless application.

The cost of Cutless .33G is around $5 per pound, which is slightly higher than cost of Atrimmec. Applying Cutless to maintain a 3- to 4-foot-tall loropetalum without pruning for at least one year is around 70 cents.

To get the most from this new PGR, growers and homeowners must be sure to read and follow the label instructions carefully. Application needs to be timed at the active growing stage, which is usually when leaves are sprouting in early spring or when new growth is visible in late summer. It is recommended to prune and treat with Cutless at the same time, and irrigation or rainfall is needed to get the material into root zone after an application.

Yan Chen, Associate Professor, Regina P. Bracy, Professor, and Allen D. Owings, Professor, Hammond Research Station, Hammond, La.

(This article was published in the winter 2012 issue of Louisiana Agriculture magazine.)

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