Primo, TGR and Cutless Influence the Growth of Selected Turf Weeds

David Lowe and Ted Whitwell

Department of Horticulture, Clemson University

Introduction

Plant growth regulators (PGR's) are chemicals which suppress plant growth. Several PGR's, like Primo, TGR and Cutless are becoming popular maintenance tools for intensively managed turfgrass areas such as golf courses. PGR's are used on golf courses to decrease turfgrass height and, consequently, mowing frequency and to enhance turf color. Although many researchers have studied PGR effects on turf growth and color, little information has been published about their influences on turf weeds.

Weeds exist in all horticultural amenities including intensively maintained turfgrass areas. Weeds compete with turfgrass, for available growth factors and weeds can gain a competitive advantage when turf is not actively growing. PGR's significantly decrease turfgrass growth but the effects of these chemical on most turf weeds are unknown. Weeds may be allowed to further exploit the surrounding areas if PGR's inhibit turfgrass growth more than weed growth.

Objective

Greenhouse studies at Clemson University were used to determine the influences of plant growth regulators (Primo, Cutless and TGR) on four common turf weeds.

Procedures

The weeds used were common bahiagrass, purple nutsedge, goosegrass and large crabgrass. Tifway II bermudagrass was included for comparison. Bahiagrass and bermudagrass plugs were taken from turf areas in Florence, SC and then placed in 1 qt pots. Norfolk sandy loam soil was used as growing media for all species. Purple nutsedge plants and goosegrass plugs were taken from areas around Clemson, SC. Plant roots were washed before being potted. Crabgrass was seeded and each pot contained 20 seedlings. All plants were fertilized and irrigated as needed and were allowed 5 weeks of growth before treatments were applied.

Treatments used in the study were Primo @ 0.375 lb ai/ A, Cutless @ 1.0 lb ai/ A, and TGR @ 1.0 lb ai/ A, which represents the highest label rate for turf application. There were 4 replications of each treatment and the experimental design of the study was a randomized complete block.

The measured parameters of the study were plant height and clipping dry weight. Plant height was measured 1 week after treatment (WAT), 2 WAT and 4 WAT. Plants were clipped at 1 inch above the soil surface at 1 WAT, 3 WAT, and 5 WAT. Clippings were dried in a sample drying oven for two days and then weighed.

Results and Discussion

Each PGR reduced the height of each weed. Primo (Figure 1) reduced the height of bermudagrass by 37%, bahiagrass by 21%, goosegrass by 6%, purple nutsedge by 7% and crabgrass by 27% throughout the 5 weeks of the study. TGR (Figure 2) reduced the height of bermudagrass by 35%, bahiagrass by 5%, goosegrass by 1%, purple nutsedge by 16% and crabgrass by 24%. Cutless (Figure 3) reduced the height of bermudagrass by 35%, bahiagrass by 12%, goosegrass by 6%, purple nutsedge by 10% and crabgrass by 35%.

Each PGR reduced the clipping dry weight of bermudagrass, bahiagrass, goosegrass and purple nutsedge. Primo (Figure 4) reduced the clipping dry weight of bermudagrass by 88%, bahiagrass by 57%, goosegrass by 40%, and purple nutsedge by 16% throughout the 5 weeks of the study. TGR (Figure 5) reduced the clipping dry weight of bermudagrass by 86%, bahiagrass by 43%, goosegrass by 40% and purple nutsedge by 41%. Cutless (Figure 6) reduced the clipping dry weight of bermudagrass by 82%, bahiagrass by 49%, goosegrass by 40%, and purple nutsedge by 43%. Crabgrass reacted differently to the treatments than the other plants. Primo and TGR increased crabgrass clipping dry weight by 3% and 30% respectively; whereas, Cutless reduced crabgrass clipping weights by 75%.

Each of these common PGR's hindered bermudagrass growth more than any of the weeds studied. Both plant height and, especially, clipping dry weights of bermudagrass were reduce more than any of the weeds. Bermudagrass clipping dry weights were reduced to a greater extent than heights as compared the weeds. This result demonstrates that PGR's reduce bermudagrass growth more than the growth of each weed; therefore, PGR's should not be sprayed in areas where weeds exist because the weeds may gain greater competitive advantages.

An important observation in this study was an increase in crabgrass density with the use of Primo and TGR. Crabgrass density was measured visually as percent ground cover. Crabgrass density was 79% and 83% with the use of Primo and TGR respectively, whereas, crabgrass density was 54% with the use of Cutless and untreated was 48%. Other studies indicate that Primo and TGR increased the tillering of crabgrass.