Gary J. Keever and John W. Olive

Height control of bedding plants and development of good form in woody landscape plants during production are essential to obtaining quality products. In this study, Primo (cimectacarb), a growth retardant labeled for warm- and cool-season turfgrasses, provided acceptable growth suppression in most species tested; however, phytotoxic symptoms which developed on all bedding plant species and two of four woody landscape species, resulted in unacceptable quality. Based on these results, the use of Primo as an alternative to other chemical growth retardants in the production of herbaceous and woody landscape plants is not recommended. Additionally, concentrations of Primo applied to species in this study are similar to those recommended for turfgrasses; this raises the concern of potential injury to herbaceous and woody plants in the landscape from drift or overspray when Primo is applied to turfgrasses.

METHODS
In the first experiment, plants of ‘Celebrity Lilac’ petunia, ‘Accent Deep Pink’ coleus, ‘Goldcrest’ cosmos, ‘Accent Deep Pink’ impatiens, ‘Pinkie’ periwinkle, and ‘Bronze Yellow’ French marigold in 32-cell flats were treated with single foliar sprays of Primo at 0, 1, 10, 100, 500, or 1,000 parts per million active ingredient (ppm ai). A second experiment evaluated the response of four woody landscape plants to Primo. Single foliar sprays of Primo at 0, 500, 1,000, 1,500, 2,000, 2,500, and 3,000 ppm were applied to ‘Nellie R. Stevens’ holly, ‘G. G. Gerbing’ azalea, Japanese privet, and ‘Royal Red’ butterfly-bush.

RESULTS
EXPERIMENT 1. Within 3 days of treatment, foliage of all bedding plant species sprayed with 100, 500, or 1,000 ppm Primo appeared bleached. Symptoms were relatively minor on plants treated with 100 ppm but severe on those treated with the two highest concentrations. Bleaching occurred primarily on younger foliage and was concentrated near leaf tips and margins. At the termination of the experiment, 6 weeks after treatment (WAT), bleached foliage was still evident on petunia, coleus, impatiens, and marigold plants treated with the two highest concentrations of Primo. Flowers were present on petunia, impatiens, and periwinkle when treated; these flowers were bleached by 100, 500, and 1,000 ppm Primo and subsequent flowers that formed and opened during the study were bleached. Flowers that formed on marigold and cosmos opened without any abnormal symptoms while coleus did not flower.

Heights of impatiens, petunia, cosmos, and coleus were affected by Primo application (Table 1), while heights of marigold and periwinkle were not. Impatiens treated with the highest rate were 22% shorter than controls. Concentrations 100 ppm had minimal effect (5% or less), on the height of petunia or cosmos, whereas at 1,000 ppm heights of petunia and cosmos were 64 and 31%, respectively, below that of the control. At 10 ppm height of coleus was similar to that of the control, whereas at 100 and 1,000 ppm heights were 22% greater and 19% less, respectively, than that of the control. The study was terminated 6 WAT because treatments had either made plants unmarketable or were ineffective in controlling shoot growth.

EXPERIMENT 2. At 2 and 4 WAT, new growth of azalea treated with Primo was distorted and chlorotic to bronze in color. Symptoms occured on all plants treated with Primo but intensified with increasing rate. At 10 WAT, new growth was healthy and vigorous in appearance; however, bronzed older foliage was still present at 2 and 4 WAT.

At 1 WAT new leaves of butterfly-bush were smaller compared to controls and chlorotic to bleached in appearance. Symptoms were present on all plants treated with Primo rates but were progressively more severe at higher rates. At 3 WAT, foliage of all treated plants was lighter green than that of control plants. Bleached leaf tips were present on plants treated with 1,500 ppm Primo. Flowers on control plants were a normal deep purple color, while those of plants treated with 500 ppm Primo were a pale lavender, and those of plants treated with 1,000 ppm were bleached white. No discoloration of the foliage was present on holly or privet at any of the observational dates.

Growth index of azalea, butterfly-bush, and holly decreased with increasing Primo rate at four and 10 WAT. (Table 2). At 10 WAT, growth index of privet was not affected by Primo treatments, indicating that the growth rate of treated plants was faster than that of the control between 4 and 10 WAT. At 4 WAT, treated plants of privet were compact, but by 10 WAT shoots of treated plants had elongated excessively.

Findings of these two experiments indicate that Primo was effective at suppressing shoot growth of several herbaceous and woody landscape plants. However, at rates necessary for shoot control, phytotoxicity symptoms were common on foliage, flowers, or both of most tested species. Results also suggest that herbaceous or woody landscape plants may be injured by Primo application to nearby turf due to overspray or drift. The recommended rate of Primo for turf application is 1.5 ounces of product per 1,000 square feet in a volume of 0.5-2.5 gallons per 1,000 square feet. At these recommended rates, Primo would be applied at rates up to 2,810 ppm. These rates are much higher than rates that injured both herbaceous and woody landscape species in these experiments.

Table 2. Growth Index1 of Four Woody Landscape Plants Sprayed with Primo, Experiment 2
Primo rate (ppm)	‘G. G. Gerbing’ azalea		‘Royal Red’ butterfly-bush		‘Nellie R. Stevens’ holly		Japanese privet
	4 WAT2	10 WAT	3 WAT	10 WAT	4 WAT	10 WAT	4 WAT
0	27.7	44.9	60.3	78.6	33.7	46.7	33.7
500	22.0	41.3	62.0	94.3	29.3	44.6	33.0
1,000	25.5	42.1	59.1	81.8	28.1	41.0	30.2
1,500	23.8	38.5	55.5	71.6	24.7	34.2	27.8
2,000	18.9	33.2	55.0	84.1	25.5	36.7	29.8
2,500	21.3	37.0	51.0	71.5	24.7	34.7	30.0
3,000	21.3	33.9	42.2	61.3	23.8	31.8	30.4
1Growth index = (height + width at the widest point + width 90o to the widest point) ÷ 3, in centimeters. 2WAT = weeks after treatment.