Gary J. Keever and John W. Olive

Primo is a growth retardant labeled for use on warm- and cool-season turfgrasses. Though effective in retarding shoot growth, foliage of some treated plant species in an earlier study became chlorotic to bleached when treated with Primo at 100 to 1,000 parts per million active ingredient (ppm ai), and flowers of plants treated with Primo at 100 to 3,000 ppm experienced a loss of pigmentation that ranged from a slight fading to bleached white.

Recommended rates of Primo for turfgrasses range from 0.25 to 1.0 ounce per 1,000 square feet for approximately 50% growth suppression over a 4-week period, at one and one half times these rates for excessively vigorous growth, and at two times these rate for up to 8 weeks of suppression where temporary discoloration can be tolerated (Primo label). It is also recommended that Primo be applied in 0.5 to 2.5 gallons per 1,000 square feet. When Primo is mixed at 0.25 to 1.5 ounces of product in 0.5 to 2.5 gallons, solution concentrations range from 94 to 2,810 ppm ai; these concentrations caused severe phytotoxicity to both herbaceous and woody landscape plants in a previous study. However, in the previous study, Primo was applied in a volume of 0.5 gallon per100 square feet, a recommended volume/area for most plant growth regulator application to herbaceous and woody landscape species.

The objective of this study was to determine if application of Primo at rates and in volumes recommended for turfgrasses will adversely affect herbaceous and woody landscape plants that may be exposed due to overspray or drift.

METHODS
Uniform plants of heavenly bamboo, azalea, variegated liriope, coleus, impatiens, and periwinkle were treated on May 5, 1994 with all combinations of the followings: Primo at 0.25, 0.5, 1.0, and 1.5 ounces per 1,000 square feet in volumes of 0.5, 1.5, and 2.5 gallons per1,000 square feet. Solution concentrations of active ingredient ranged from 94 to 2,810 ppm.

Plants were examined daily for the first 2 weeks following treatment for phytotoxicity symptoms and less frequently thereafter. Data were collected periodically. Bedding plants were pruned to a uniform height above the substrate to remove all foliage adversely affected by the first treatment application and treatments reapplied on July 6, 9 weeks after initial treatment and 3 weeks after pruning.

The experiment was repeated in 1995 with privet, ‘G. G. Gerbing’ azalea, ‘Royal Red’ butterfly-bush, ‘Polo Velvet’ petunia, ‘Bronze Jazz’ coleus, and rose periwinkle. Bedding plants were not pruned or retreated as in the first experiment.

RESULTS
EXPERIMENT 1. ‘Bronze Jazz’ coleus normally has dark red foliage with bright yellow margins. Within 2 days of treatment, foliage of coleus in all Primo treatments appeared bleached. Symptoms varied among treatments but was most evident on new foliage and was concentrated near leaf tips and margins. No other species showed any symptoms of phytotoxicity.

Injury rating made on coleus 4 days after treatment (DAT) reflects the effects of Primo application volume and rate (see table). Within volumes 0.5 gallon per 1,000 square feet and 2.5 gallons per 1,000 square feet, the injury rating increased as Primo rate increased. At the highest rate, bleaching was present on 20-40% of the foliage. Within a volume of 1.5 gallons per 1,000 square feet, the highest injury rating was given to plants treated with 1.0 ounce per 1,000 square feet of Primo. Within a given Primo rate, the injury rating increased as volume increased. At the three lower Primo rates, the injury rating was lowest when Primo was applied in the smallest volume. Considering that Primo was most concentrated in the smallest volume, one would expect the most injury in these treatments. However, treatment application in a volume of 0.5 gallon per 1,000 square feet resulted in incomplete leaf coverage which possibly resulted in less injury compared to that from the higher volume treatments.

Two weeks after reapplication, treatment differences were visually apparent among coleus treated with different Primo rates but not volumes. Plants treated with the lowest rate were paler than control plants which exhibited dark red leaves with bright yellow margins. Symptoms increased in severity as Primo rate increased. Thirteen weeks after treatment (WAT) and 4 weeks after reapplication,the foliar color rating reflected a similar trend to that reported in the foliar color rating at 4 WAT and observed 2 weeks after reapplication of treatments.

EXPERIMENT 2. Symptom development in coleus followed a similar pattern to that in the first experiment. Bleached spots on petunia flowers occurred within 24 hours of treatment with the two highest Primo rates. At 5 DAT, bleaching of flowers was more general and most severe in plants receiving the highest Primo rate. At 30 DAT, there were no obvious treatment effects in petunia, possibly because flowers showing injury at 5 DAT had fallen off by 30 DAT; this suggests injury to petunia from Primo was not long-term. Butterfly-bush was not in flower when treated and showed no visible adverse effects until flowering began. At 30 DAT, flowers on plants treated with either of the two highest Primo rates were slightly faded. At 60 DAT, flowers on plants treated with the three lowest Primo rates were slightly faded, while flowers on plants receiving the highest rate were almost white. At 30 DAT, immature leaves of azalea treated with the two highest rates of Primo were bronze in appearance.

In these experiments, several species of herbaceous or woody landscape plants were injured by Primo application. Foliage of coleus and ‘G. G. Gerbing’ azalea and flowers of petunia and butterfly-bush were particularly sensitive. Bleaching of the flowers in these cultivars represented a loss of red-purple pigmentation. Injury in most cases was worsened by increasing Primo rate, while application volume had less effect. In general, injury was less severe than previously reported on some of the same species. This is probably due to Primo being applied in a volume of 0.5 gallon per 100 square feet in the previous study, a recommended volume/area for most plant growth regulators, and in volumes recommended for turfgrass application, 0.5 to 2.5 gallons per 1,000 square feet, in the current study. While injury was less severe than previously reported, it was severe enough to adversely affect the landscape quality of sensitive species that may be exposed to Primo from drift or overspray during turfgrass application.