The new ultra-dwarf bermudagrass cultivars produce fine-textured turfs with an ability to withstand lower mowing heights. Thatch is a problem in vigorous, dwarf bermudagrass. Verticutting often reduces thatch. Groomers attached to mowers can provide daily thatch maintenance. Frequent and timely topdressing helps minimize thatch.

Dwarf bermudagrasses demand unique care

Dense, short turf requires a constant battle with thatch.

Beth Guertal, Ph.D., and Richard White, Ph.D.

New ultra-dwarf bermudagrass cultivars, such as Champion (right), produce many stolons that can build up into thatch without proper maintenance.

Perhaps the owners or managers at your Southern golf course want to rebuild your greens and the new dwarf bermudagrass cultivars have caught your eye. With their finer texture and ability to withstand lower mowing heights, ultra-dwarf bermudagrass cultivars will readily appeal to superintendents whose golfers want longer ball roll on better turf.

However, this increase in turf quality may come at a price -- the cost of increased maintenance or equipment to keep these new ultra-dwarfs in top shape.

These new thick, dwarf bermudagrasses have a strong tendency to produce thatch. Of course, this problem can be alleviated with proper horticultural practices, but it may require extra work.

Cultivar characteristics
At least five new dwarf cultivars of Cynodon species have become available to golf course superintendents in the past two years. They include Champion, FloraDwarf, Mini-Verde, MS-Supreme and TifEagle.

Generally, these cultivars produce shorter leaves than are found on the older bermudagrass cultivars used on greens: Tifdwarf and Tifgreen. Thus, they can withstand lower mowing heights.

However, the new dwarfs produce significantly more stolons than rhizomes, according to research at Texas A&M University. Champion produced about 900 milligrams of stolons compared with 4 milligrams of rhizomes per sample during a four-month period. Tifdwarf produced 750 milligrams of stolons and 200 milligrams of rhizomes per sample during the same period.

Also, shoot density of ultra-dwarf bermudagrasses (90 to 120 shoots per square inch) was greater than that of Tifdwarf (60 to 70 shoots per square inch) when mowed at 1/8 inch or less.

Thatch
In Texas A&M studies, Champion bermudagrass produced the most thatch, TifEagle the least, and Flora-Dwarf, Mini-Verde and Tifdwarf were intermediate in thatch accumulation during a four-month period after planting.

The combination of high shoot and stolon density, as well as high thatch production, demands frequent vertical mowing and topdressing to produce a smooth putting surface.

For example, 1996 measurements at Auburn University on bermudagrasses maintained at a 5/32-inch height revealed that TifEagle had a greater tendency to form thatch (to an average depth of 1.83 inches) than Tifdwarf (average depth of 1.68 inches).

That year, plots received minimal maintenance: no core aerification, infrequent topdressing and few vertical mowings. In 1997, however, plots were vigorously maintained, with four core aerifications per year, topdressing three times monthly and daily light verticutting via groomer reels on the greensmowers. When mowed at 1/8 inch in 1997, thatch depth decreased to 0.5 inch in both TifEagle and Tifdwarf.

Verticutting
Several golf course superintendents in the southeastern United States have experience with the new dwarf bermudagrasses, and most have been pleased with their quality and performance.

William Smith, CGCS and a 20-year GCSAA member, sprigged his TifEagle practice green at The Country Club of Columbus, Ga., in May 1997. The green was mowed to 5/32 inch after six weeks.

"Preventing thatch accumulation is going to be the key to success with TifEagle, but that has always been the key to quality bermuda greens," Smith says. "The increased maintenance will not be a problem in our program as the increased quality of the putting surfaces will be well worth the effort."

Superintendent Rod Lingle, CGCS and a 23-year GCSAA member, has both TifEagle and Champion on Memphis Country Club's course, and he is comparing them to existing Tifgreen greens.

Mowing is intensive: From the end of May until the beginning of October, the greens are double-cut seven days a week. Crews use walking greensmowers with sectional grooved front rollers and groomers at each mowing. Cutting height during this period ranges from 3/16 to 1/8 inch.

Lingle also lightly verticuts the greens when the grass is growing fastest, about three to four times a summer. Core aerification is done twice a year, in May and July.

Topdressing
"I consider light topdressing of bermudagrass greens to be one of the most effective tools in managing good putting surfaces," Lingle says. "During periods of rapid growth we will lightly topdress our greens once a week, and during slow growth we will stretch that interval to about one topdressing every three weeks."

Topdressing at 0.02 inch, or a light dusting two to three times a month during the summer, will most likely provide a smooth putting surface and reduce thatch accumulation.

Overseeding woes
High shoot and stolon density and thatch may make overseeding difficult in autumn. The seeds of cool-season grasses may need help penetrating this organic barrier so they can germinate in a suitable environment.

The large seed of perennial ryegrass (Lolium perenne) is particularly difficult to work into the dense canopy of closely mowed bermudagrass golf greens. Perennial ryegrass, at 30 pounds per 1,000 square feet, was less successful than Poa trivialis overseeded at 15 pounds of seed per 1,000 square feet at Texas A&M University.

P. trivialis, a small-seeded grass that established well through the dense turf, provided better winter quality than perennial ryegrass. Mixtures of perennial ryegrass and Poa trivialis may be required to provide wear tolerance on high-traffic golf courses.

Raising mowing heights from 0.125 inch to 0.188 inch or more during establishment of overseeded Poa trivialis and perennial ryegrass will increase the likelihood of success. This may seem like simplistic advice, but overseeding failures have occurred because mowing heights were not raised to allow the overseeded grass to establish.

Because creeping bentgrass (Agrostis palustris) seed is small and can penetrate the dense bermudagrass canopy, some superintendents have used it in overseeding mixtures on golf greens mowed at 0.125 inch or less. But creeping bentgrass contributes little to the quality of overseeding until late winter and spring, according to research at Texas A&M University.

Also, creeping bentgrasses create too much competition during spring transition back to bermudagrass. Golf course superintendents should use great care in developing overseeding mixtures for use on the new dwarf bermudagrasses.

Studies at Texas A&M University indicate that vertical mowing and topdressing twice a month provided more uniform overseeding grass establishment than heavy vertical mowing and topdressing just at overseeding. Aggressive vertical mowing immediately before overseeding decreased the uniformity of overseeded Poa trivialis because of substantial germination in verticut grooves.

At Auburn, research plots overseeded with 100-percent Poa trivialis germinated well. The CC of Columbus also overseeded with 100-percent Poa trivialis, which germinated more uniformly and had a denser initial stand on the TifEagle green than on comparable Tifgreen or Tifdwarf overseeded surfaces.

Transition
A cultivar's lateral spread is important during spring transition and recovery from injury caused by traffic, stress and pests. Lateral spread differs among the greens-type bermudagrasses.

MS Supreme has had more rapid lateral coverage than Tifdwarf and Champion, whereas FloraDwarf, Mini-Verde and TifEagle have an intermediate rate of lateral spread from plugs.

Bermudagrasses' aggressive stolons can be tamed with grooming rollers attached to mowers.

Fertilization
Nitrogen fertilization will also be a key ingredient in management of the new bermudagrasses. Initial information from studies at Texas A&M University indicate that 10 to 14 pounds of nitrogen per 1,000 square feet will be required annually to maintain acceptable putting green quality year round (with a Poa trivialis overseeding).

Nitrogen at 18 pounds per year produced the best winter quality but may contribute to winter injury and excessive thatch formation. Total annual applications of 6 pounds of nitrogen per 1,000 square feet did not provide acceptable turf quality at the Texas research site.

The payoff
Superintendents who are willing to take on the management needs of these grasses will be rewarded with turf quality that often exceeds that of Tifdwarf or Tifgreen.

In three years of color and quality ratings at Auburn University, where the grasses are maintained at 1/8 or 5/32 inch, TifEagle has consistently ranked near the top of the rating scales, scoring high in both color and quality. TifEagle keeps its green color later into the fall than Tifgreen, although in spring greenup, ratings did not differ from Tifgreen or Tifdwarf.

As the new cultivars are used in more locations, we’ll learn more about their special cultural needs. For now, superintendents should consider the new ultra-dwarf bermudagrasses only if they have the equipment, budget and labor to properly care for them.

Such care will also include frequent and timely topdressing and verticutting or grooming. Thatch management is the key.

References

1. Beard, J.B. 1996. Bermudagrass breakthrough: New cultivars for Southern putting greens. Golf Course Management 64(12):58-62.
2. Westmoreland, J.W. 1998. Another option for Southern greens. Golf Course Management 66(3):64-67.

Beth Guertal, Ph.D., is associate professor of agronomy and soils at Auburn University in Alabama.
Richard White, Ph.D., is associate professor of turfgrass science and physiology at Texas A&M University in College Station.