Golf greens and shade stress

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Nearly all cool-season golf and some warm-season golf is played on creeping bentgrass (Agrostis stolonifera) greens, and in many cases, these greens are subjected to some level of shade stress.

While some superintendents have unlimited use of chain saws, or what a previous supervisor called “Thunder” and “Lightning,” many superintendents have restricted access to them as shade management tools. In this case, management of shaded creeping bentgrass greens can be difficult, time-consuming and input-intensive. Shaded greens often require irrigation, fertility, cultural and chemical regimes that differ from full-sun greens. Even with all of this effort, the results may still be subpar.

Creeping bentgrass is an excellent putting green turfgrass, but is there another turf that will provide better playing conditions in shade?

Velvet bentgrass

Velvet bentgrass (A. canina) is a possible alternative that may have similar or lower input requirements than creeping bentgrass. Currently used only in the northeastern and northwestern areas of the United States, velvet bentgrass was originally planted as South German bentgrass (75% colonial bentgrass, 15% velvet bentgrass, 1% creeping bentgrass, plus many impurities) in the early 1900s and saw extensive use on golf courses for the early half of the 20th century (5).

As new seeded cultivars of creeping bentgrass were released, velvet bentgrass fell out of favor, possibly because management practices were not well understood (3). Management of velvet bentgrass differs slightly from that of creeping bentgrass because of velvet bentgrass’s high density and extremely fine texture. Overwatering and overfertilizing were also possible reasons for its decline in popularity.

Why take another look at velvet bentgrass? If it did not work in the second half of the 20th century, why would it now?

Thanks to a resurgence of velvet bentgrass research in the 1990s and beyond, we now know much more about its proper management in modern golf turf situations (2). New velvet bentgrass cultivars, such as SR 7200, Legendary and Vesper, exhibit a more desirable dark-green color than the earlier cultivars (2, 11). Modern velvet bentgrass cultivars also have comparable, or better, wear tolerance (4, 12) and require no more nitrogen than creeping bentgrass (1, 7). Velvet bentgrass also uses less water than other bentgrass species (6) and is a top performer in low-input fairways (13). Recent research has shown that velvet bentgrass maintains acceptable putting green turf quality on soils outside the coastal regions of the northern United States (7, 8).

Most important, velvet bentgrass may be more shade-tolerant than creeping bentgrass (10), with preliminary tests showing SR 7200 velvet bentgrass to be more shade-tolerant than Penncross creeping bentgrass at fairway/tee height (personal observation).

Because of our improved understanding of velvet bentgrass input requirements and the improved color of modern cultivars, velvet bentgrass can be properly managed to provide playing conditions and appearance equal to those of creeping bentgrass. Therefore, the goal of this research was to compare the relative shade tolerance of these two bentgrass species in an effort to determine the best bentgrass for shaded putting green situations.

Our experiment

Vesper velvet bentgrass and Tyee creeping bentgrass were established on a native soil push-up green at the O.J. Noer Turfgrass Research and Education Facility near Madison, Wis. Two hoop houses (each 30 feet wide × 45 feet long × 15 feet high; 9.1 × 13.7 × 4.5 meters) were constructed above the green to provide support for shade cloth simulating 80% shade. Treatments of bentgrass species, nitrogen rates and Primo Maxx (trinexapac-ethyl, Syngenta) rates were arranged in a randomized complete block, strip-split plot design with four replications.

Bentgrass species served as whole-plot treatments, and each was randomly split to receive one of two trinexapac-ethyl rates (0.0 and 0.125 ounce/1,000 square feet; 0.0 and 0.38 liter/hectare). Three annual nitrogen rates (1, 2 and 4 pounds/1,000 square feet; 49, 98 and 196 kilograms/hectare) were stripped across the bentgrass species and trinexapac-ethyl rates. Plots were maintained at 0.125-inch (3.2-mm) height of cut using a Toro walk-behind reel mower and irrigated at 40% ET replacement. Nitrogen treatments were applied at 14-day intervals, using urea as the nitrogen source, and trinexapac-ethyl was applied every 28 days, both beginning for each growing season on May 15, 2009, and May 3, 2010.

Black shade cloth was installed over both hoop houses from May 10 to Oct. 25, 2009, and from May 13 to Oct. 18, 2010, to reduce photosynthetically active radiation (PAR) by approximately 80%. The installment and removal dates corresponded with area of spring tree leaf development and fall tree leaf senescence. Of particular note is that 80% shade cloth provided more than 80% shade when compared with calculated 80% of full sun PAR values (Figure 1) (9).

Nitrogen rate effect

The amount of nitrogen fertilizer had a greater effect than Primo applications on turf quality of both bentgrasses. Fertilization at the high rate of 4 pounds/1,000 square feet (196 kilograms/hectare) resulted in turf declining to less-than-acceptable quality by midsummer of the first year, while the other fertilization rates of 1 and 2 pounds/1,000 square feet (49 and 98 kilograms/hectare) held acceptable quality through early September (Figure 2).

Declines in turf quality in the second year were less severe, but all rates ended with unacceptable quality. The best fertility rate for both species under 80% shade was 1 pound nitrogen/1,000 square feet. This confirms that fertility inputs must be reduced for shaded turf. Overfertilization results in turfgrass plants depleting their carbohydrate reserves and becoming unable to generate adequate replacements.

Bentgrass research
In the first year of the study, plots fertilized at the lower rates (left, 1 pound of nitrogen/1,000 square feet; middle, 2 pounds of nitrogen/1,000 square feet) had acceptable turf quality through early September, but plots treated at the high rate of nitrogen (right, 4 pounds of nitrogen/1,000 square feet) had less-than-acceptable quality by midsummer.

As turf quality decreased throughout the trial, so did clipping yield. Used as an indicator of turf health, clipping yield confirmed the unacceptable turf quality of higher fertility rates, with 1 and 2 pounds nitrogen/1,000 square feet producing greater clipping yields than 4 pounds nitrogen/1,000 square feet.

Ball roll distances (green speed) were mostly similar at all fertility rates in the first year. When differences did occur in both years, ball roll distances increased as nitrogen rate increased. This is the opposite of what other researchers reported for velvet bentgrass in full sun (8) because, in this trial, the turf treated at rates of 2 and 4 pounds/1,000 square feet had much lower percent ground cover and, therefore, less turf resistance and higher ball roll distances than the turf treated with 1 pound nitrogen /1,000 square feet.

Addition of trinexapac-ethyl

Shaded bentgrass treated with trinexapac-ethyl had higher quality and held that quality for a longer duration than non-treated turf in the first year. This was true for both bentgrass species. In the second year, when trinexapac-ethyl treatments differed, turf treated with trinexapac-ethyl had significantly higher turf quality than the non-treated turf. In the second year, the highest turf quality was achieved with 1 pound nitrogen/1,000 square feet and the addition of trinexapac-ethyl. No differences in turf quality response by bentgrass species is good information for superintendents because trinexapac-ethyl product labels do not differentiate between bentgrass species.

Trinexapac-ethyl did not have a consistent effect on clipping yield, because turf quality for most treatments declined quickly and was greatly influenced by fertility rate. Shortening the trinexapac-ethyl application interval could have resulted in a more consistent clipping yield response and, likely, increased turf quality ratings. Trinexapac-ethyl also did not have a consistent effect on ball roll distance because turf uniformity was poor.

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