Untreated with growth regulators, the single pot of crabgrass at right boasts a relatively thin weed stand compared with the six treated pots.

Crabgrass density may increase over time with the use of certain growth regulators in weakened, crabgrass-infested bermudagrass. Growth regulators have many uses on golf course turf. Grassy weed control -- particularly of warm-season species -- may be a necessary precondition before using growth regulators in a turf stand.

Turfgrass plant-growth regulators create small, dense plants by shortening internodes and leaves (5). They also reduce mowing needs, produce uniform turfgrass stands and help provide a smooth putting surface.

Growth regulators are used in overseeding to prompt an established turf to yield to the newly seeded grass (1,12). They can reduce mowing and turfgrass encroachment at the edges of bunkers, paths and flower beds. Because some growth regulators cause turf to become denser and then "rebound" with vigorous growth after the chemical wears off, they can apparently precondition turf for stressful conditions.

Caution may be advised, however, before using some growth regulators on turf infested with grassy weeds. For example, some growth regulators suppress bermudagrass (Cynodon species) more effectively than they do weeds, thereby giving an advantage to pre-existing weeds.

Growth reduction
Applications of growth regulators, which have so many different uses, have increased dramatically in the 1990s.

In turfgrass, the Type II growth regulators in common use are flurprimidol (Cutless), paclobutrazol (TGR) and tri-nexapac-ethyl (Primo). They inhibit gibberellic acid synthesis (a plant hormone needed for cell elongation). Mefluidide (Embark), a Type I growth regulator, inhibits cell division and differentiation.

According to various studies, growth regulators can reduce plant height and clippings (4,8,10):

Three trinexapac-ethyl applications reduced mowing of Tifway bermudagrass by 70 percent within a 12-week period (10).

Kentucky bluegrass (Poa pratensis) was reduced by four to five mowings within an eight-week period following growth-regulator applications (7).

Tall fescue (Festuca arundinacea) mowing was also reduced by three to four mowings within a five-week period following mefluidide and trinexapac-ethyl treatments (9).

Growth regulators may also influence weeds. One researcher noticed increased spring broadleaf weeds in bermudagrass treated with flurprimidol and mefluidide (2). Decreased turf density led to increased crabgrass (Digitaria species) infestations in Kentucky bluegrass-red fescue (Festuca rubra) turf following applications of an experimental growth regulator (4).

In contrast, annual bluegrass populations declined in creeping bentgrass greens after experimental paclobutrazol and flurprimidol treatments (3,11), and growth regulators have become commonly used to control this tenacious weed on golf courses.

Experiment
Weeds generally invade weakened or open areas in turf and compete with turfgrass for resources. Turfgrass growth is inhibited when growth regulators are applied, so existing weeds may gain an advantage.

To better understand this effect, our research compared the responses of weeds and bermudagrass turf treated with growth regulators.

We evaluated trinexapac-ethyl, paclobutrazol and flurprimidol on Tifway bermudagrass, crabgrass, purple nutsedge, goosegrass and bahiagrass. Application rates represented the lowest to the highest labeled rates of each product for bermudagrass. Applications were made with a carbon dioxide-powered sprayer calibrated to 30 gallons per acre.

Crabgrass was seeded into pots, and all other plants were transplanted from local populations and grown in a greenhouse one month before treatments. Pots remained in the greenhouse following treatments to facilitate plant measurements.

Plant heights were measured one, two and four weeks after treatment, and clipping dry weights were recorded at one, three and five weeks. Height and weight values were averaged for the three measurement dates and presented as percentages of untreated plants.

The increase in crabgrass visual density prompted a subsequent study of tillering in which 20 crabgrass seedlings per pot were treated with trinexapac-ethyl (at the equivalent of 0.375 pound active ingredient per acre) or paclobutrazol (at 1 pound active ingredient per acre). After four weeks, crabgrass tillers were counted and weighed.

Growth reduction
Bermudagrass was the most sensitive species to growth-regulator treatments for height reduction, whereas crabgrass was the most sensitive among weeds.

Trinexapac-ethyl, across all rates, stunted bermudagrass height by 28-37 percent, but goosegrass (Eleusine indica) and purple nutsedge (Cyperus rotundus) heights were reduced less than 13 percent. Higher rates (the equivalent of 0.2 and 0.375 pound active ingredient per acre) suppressed crabgrass and bahiagrass height from 20 to 30 percent.

Paclobutrazol reduced bermudagrass height by 25 to 35 percent, whereas goosegrass and bahiagrass heights were less affected (by less than 8 percent). Flurprimidol suppressed bermudagrass and crabgrass height by 25 to 35 percent but only suppressed bahiagrass, goose-grass and purple nutsedge height less than 13 percent.

Trinexapac-ethyl reduced clipping weights of bermudagrass and bahiagrass by 53 to 87 percent, and crabgrass and purple nutsedge clipping weights were least affected. Paclobutrazol reduced bermudagrass clipping weights by 72 to 84 percent, but goosegrass, purple nutsedge and bahiagrass were reduced less than 44 percent.

High rates of flurprimidol reduced clipping weights of bermudagrass and crabgrass by 76 to 82 percent compared with goosegrass (45 percent), bahiagrass (50 percent) and purple nutsedge (46 percent).

Denser crabgrass
When paclobutrazol and trinexapac-ethyl were applied to crabgrass, the weed canopies appeared denser. This effect has also occurred with kikuyugrass and bermudagrass after trinexapac-ethyl use (6). Paclobutrazol also increased turf density by enhancing tillering of various grass species (13).

In our follow-up study, crabgrass treated with trinexapac-ethyl and paclobutrazol had a 13-19 percent increase in tillers four weeks after treatment. Crabgrass clipping weights also increased, presumably from increased tillering. In weakened turfgrass stands with pre-existing crabgrass, crabgrass density may increase over time following the use of certain growth regulators.

In previous research, crabgrass infestation in a bluegrass-fescue turf was greater (45 percent) after four years of mefluidide treatments compared with untreated plots (8 percent) (4). In contrast, flurprimidol-treated plots contained significantly less crabgrass infestation (less than 1 percent) compared with untreated plots.

In our study, flurprimidol decreased both crabgrass height and clipping weight. Because flurprimidol was initially screened as a post-emergence grass herbicide, this probably explains its greater activity on crabgrass compared with other growth regulators.

Conclusions
Growth-regulator applications were more effective on bermudagrass than on weeds. Difficult-to-control weeds such as purple nutsedge and goosegrass were not responsive to most growth-regulator treatments.

Continual growth-regulator applications may alter interactions between turfgrass and weeds by suppressing turf- grass growth and allowing weeds to become more competitive. Weed control may be an important prerequisite to using plant-growth regulators on bermudagrass turf.

Literature cited

1. Blalock, A.M., T. Whitwell and G. Goss. 1996. Plant growth regulators as overseeding aids in hybrid bermudagrass fairways. Proceedings Southern Weed Science Society 49:64.
2. Brede, A.D. 1984. Plant growth regulators on bermudagrass turf. Proceedings Southern Weed Science Society 37:271.
3. Breuninger, J. 1993. Poa annua control in bentgrass greens. Golf Course Management 61:68-73.
4. Dernoeden, P.H. 1984. Four-year response of a Kentucky bluegrass-red fescue turf to plant growth retardants. Agronomy Journal 76:807-813.
5. Grossmann, K. 1992. Plant growth retardants: Their mode of action and benefit for physiological research. p. 788-797. In: C.M. Karssen, L.C. van Loon and D. Vreugdenhil (eds.), Progress in plant growth regulation. Kluwer Academic, Amsterdam.
6. Houseworth, D., and J.M. DiPaola. 1996. Primo: For managing turfgrass growth. Technical Bulletin. Ciba-Geigy Corp., Greensboro, N.C.
7. Jagschitz, J.A. 1982. Evaluation of growth retardants in cool-season lawn turf. Proceedings Northeastern Weed Science Society 36:334-339.
8. Johnson, B.J. 1992. Response of centipedegrass (Eremochloa ophiuroides) to plant growth regulators. Weed Technology 6:113-118.
9. Johnson, B.J. 1993. Response of tall fescues to plant-growth regulators and mowing frequencies. Journal of Environmental Horticulture 11:163-167.
10. Johnson, B.J. 1994. Influence of plant growth regulators and mowing on two bermudagrasses. Agronomy Journal 86:805-810.
11. Johnson, B.J., and T.R. Murphy. 1996. Suppression of a perennial subspecies of annual bluegrass (Poa annua spp. reptans) in a creeping bentgrass (Agrostis stolonifera) green with plant growth regulators. Weed Technology 10:705-709.
12. McCarty, L.B., and J.S. Weinbrecht. 1996. Trinexapac-ethyl (Primo) as an overseeding and transition establishment aid. Proceedings Southern Weed Science Society 49:72.
13. Watschke, T.L., M.G. Prinster and J.M. Breuninger. 1992. Plant growth regulators and turfgrass management. p. 557-588. In: D.V. Waddington, R.N. Carrow and R.C. Shearman (eds.), Turfgrass Agronomy Monograph 32. ASA, CSSA and SSSA, Madison, Wis.

Todd Lowe is a turfgrass research assistant at Clemson (S.C.) University. Ted Whitwell, Ph.D., and Bert McCarty, Ph.D., are professors in the Clemson horticulture department.