Spoon Feeding

One of the most challenging aspects of golf course management is putting green culture. Successfully maintaining turfgrasses at ⅛ or 5/32 inch mowing height is highly dependent on your commitment to sound agronomic practices. However, superintendents often have to compromise on these fundamental cultural practices in order to meet the demands of greens committees, club owners and tournament play conditions. Providing putting greens that feature fast and consistent ball roll is alluring to those who are not responsible for their management and survival.

There is a lot of pressure to lower mowing heights, double cut and roll greens to meet these standards. Unfortunately, these practices are extremely stressful and, over time, can wear out the turfgrass. Ideally, your job would allow you to promote quality putting greens with consistent and acceptable ball speeds without deleterious effects on plant health. One alternative to consider is the incorporation of plant growth regulators (PGRs) into your putting green maintenance. Reducing diurnal growth fluctuations may further provide consistent putting surfaces and could be a safer approach to producing quality putting green conditions on a long-term basis.

PGR PROFILE

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PGRs were introduced to the turf industry in the 1950s to reduce mowing requirements of utility turf such as that found on roadsides and highways. Reducing turfgrass growth and reducing the need for mowing, can be beneficial in turfgrass management. Studies have assigned actual dollar values in money saved from reducing roadside mowings from five to six per season to one or two as a result of using PGRs. One study determined that a single PGR applications cost less than or equal to one mowing cycle. In addition to economic benefits, growth inhibition is often desirable for areas that are difficult to mow, such as steep inclines and golf course bunker slopes. Research indicates the cost of PGR applications on these areas equal approximately two mowing/trimming operations. PGRs also promote smoother putting surfaces by decreasing friction from persistent and uneven leaf growth.

PGR USE ON BERMUDAGRASS PUTTING GREENS

Currently, there are no herbicides or PGRs that are labeled for use on the new “ultradwarf” bermudagrass putting greens. The dwarfed growth habit and finer leaves of these grasses make them more sensitive to these compounds compared to creeping bentgrass putting greens and other hybrid bermudagrasses. Turf discoloration and root growth restrictions may limit the potential for PGR applications in ultradwarf bermudagrass maintenance.

Ultradwarfs are becoming popular selections for putting green turf because of their abilities to withstand long-term mowing heights of ⅛ inch or closer. Bermudagrass putting green quality has traditionally been considered inferior to finer textured creeping bentgrass due to the inability of these of there cultivars, such as Tifgreen and Tifdwarf, to withstand routine mowing heights lower than 3/16 inch. Ultradwarf bermudagrasses, however, are characterized by finer leaf textures, higher per-area shoot densities and low growth habits suitable for close mowing and producing ball roll distances that were once exclusive to creeping bentgrass greens. PGR safety and effectiveness on these grasses will be a critical issue to help achieve their full putting green potential.

A gibberellic acid (GA) inhibitor, Primo, is the most popular PGR used for putting green turf and according to research, is currently is the safest PGR chemistry for ultradwarf bermudagrass. A foliar-absorbed PGR, Primo represents a newer generation of growth inhibitors that disrupt later stages of GA biosynthesis by blocking the 3b-hydroxylase conversion of GA20 to GA1. Sequential applications of Primo on Tifway bermudagrass provide consistent growth suppression, avoidance of post inhibition growth enhancement and improvements in turf quality. Multiple applications of Primo also delays fall dormancy and may promote spring greenup. Reduced canopy heights and increased shoot density following Primo applications may buffer bermudagrass from mowing injury and positively impact mowing quality. Research has indicated that Primo suppressed Tifway bermudagrass seedheads by more than 70 percent with fewer required mowings 12 weeks after initial applications compared to other PGRs. These results indicate that golf course superintendents who manage bermudagrass may have a valuable tool in Primo for promoting high quality turf.

BERMUDAGRASS INJURY

Bermudagrass that is resuming active growth during spring and early summer may be extremely sensitive to PGR applications. Leaf discoloration of Tifway bermudagrass was observed following initial Primo treatments of 0.2 kg ha-1 in late May, but turf generally recovered to an acceptable level within one or two weeks. In one study, turf discoloration and leaf chlorosis on Tifway bermudagrass was reported three weeks after initial Primo treatments at 0.11 kg ha-1 but recovered the following week.

The concept of “spoon feeding” nitrogen (applying low rates at frequent intervals) is commonly implemented in putting green management to prevent rapid growth fluctuations. For example, applying 0.5 lb./1,000 ft2 of N per month may be achieved through four applications of 0.125 lbs. N/1,000 ft2 per week. In both instances, the 0.5 lb. N is still being applied in the same time frame; however, the turf does not respond to it all at once.

The approach of spoon feeding PGRs has the same principle for reducing fluctuations of turf growth caused initially or as PGR efficacy decreases. Instead of applying greens with a standard PGR rate every three to four weeks, which may be injurious, research indicates incremental weekly or biweekly applications is a safer regimen to reduce bermudagrass injury. Early in the growing season, reducing PGR rates by 25 to 50 percent and gradually increasing them as June and July approach will enable bermudagrass greens to adjust to growth regulation effects of the newly introduced compound.

From May to August 2003, Clemson, S.C., averaged a 42-percent higher monthly rainfall than the yearly summer average with high cloud coverage and low average daily air temperatures. This particular summer had poor growing conditions for ultradwarf bermudagrass, as these grasses require eight hours minimum of daily full sunlight and consistent temperatures from 80 to 90°F. Ideally, turfgrasses should be actively growing to minimize injury and discoloration from PGR applications, and researchers observed extreme sensitivity early in the summer on bermudagrass research greens. For example, applying Primo at 6 oz./acre for three weeks displayed unacceptable injury especially following initial applications. However, sementing the 6 oz./acre into three applications of 2 oz./acre per week drastically minimized deleterious effects and was much safer on ultradwarf bermudagrass. The bermudagrass still received the same PGR amount over the three-week period, but the phytotoxicity was greatly reduced, and the “spoon feeding” approach proved to be much safer.

MINIMIZING THE “REBOUND EFFECT”

PGR safety on bermudagrass greens is an important aspect in promoting green speeds and surface uniformity. Another phenomenon that raises concerns among many superintendents who consider PGRs is the potential “rebound effect.” Rebound effects are a rapid period of growth caused by the overproduction of growth hormones and release of stored root carbohydrates when the applied PGR is no longer effective. For example, Primo inhibits the conversion of GA20 to GA1, the precursor signaling molecule for cellular elongation. Recent studies on Kentucky bluegrass showed Primo reduced GA1 concentration 47 percent but increased GA20 concentration by 146 percent. As time progresses and the PGR efficacy has diminished, the increased GA20 concentration has the potential to overproduce GA1 and the “rebound effect” may cause an overwhelming and undesirable period of shoot growth. For putting green turf, periods of rapid rebound shoot grow reduces ball-roll consistency and green speeds.

One experiment found that PGRs enhanced ball roll on several observations throughout the same day. The researchers concluded long-term ball roll improvements were more consistent when they reduced mowing height, while PGRs may be beneficial over the course of a given day. Furthermore, improved ball roll from Primo, rather than Trimmit, suggests late GA inhibition may produce more desirable bentgrass leaf morphology suited for putting green uniformity compared to the early GA inhibitor.

TANK MIXING PGRS

During summer stress, superintendents may use predominately liquid fertilizer sources and spoon feed nutrients to bermudagrass and creeping bentgrass greens. Typical liquid fertilizations are applied every 7 to 14 days at approximately 0.1 to 0.2 lbs. N/1000 sq. ft. This may also be a convenient time to incorporate regular PGR applications at 7 to 14 day intervals in the spray program. Reduced PGR increments from monthly rates may be an effective strategy to consider for helping reduce sprayer operations.

If these fertilization increments don't work for you, another option is to tank mix PGRs with fungicides to still achieve a spoon feeding PGR program. Because disease pressure for creeping bentgrass increases during the summer, you may need to apply fungicides every 10 to 14 days. Also, tank mixing two PGRs simultaneously has become a popular trend in putting green maintenance. It is critical to read all labels or consult an extension specialist before mixing chemicals.

Suppressing the most problematic winter annual weed on golf courses, Poa annua, is possible by combining GA inhibitors with Proxy, a compound that decomposes to release ethylene. Synergistic growth suppression of the desired turf may occur from simultaneous GA inhibition, such as from Primo, and the induction of ethylene within the plant from Proxy. Proxy may accelerate leaf senescence, resulting in turfgrass color fading from chlorophyll breakdown. However, mixing Proxy with Primo reduces discoloration and prevents thinning of creeping bentgrass. Monthly applications of Proxy with two or three Primo applications at lower rates will likely be an effective regimen. More research is needed to determine the reaction of this method on bentgrass greens and Poa annua greens.

PUTTING PGRS IN PERSPECTIVE

Overall, PGR use will continue to be a fundamental part of putting greens and intensive turfgrass maintenance. Labeled rates are important guidelines for incorporating PGRs into your management programs. However, dispersing these rates into incremental applications may be effective for bentgrass and bermudagrass putting greens maintenance to reduce phytotoxicity, rebound growth, and sprayer operations.

McCullough is a graduate assistant, McCarty, Ph. D., is a professor of turfgrass science, and Liu, Ph. D., is an associate professor in the Department of Horticulture at Clemson University (Clemson, S.C.).

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