Discourage Warm-Season Grass Encroachment Into Putting Greens

Warm-season grasses, like bermudagrass can outcompete cool-season grasses during the summer months and cause serious encroachment issues.
Warm-season grass encroachment in cool-season putting greens is unattractive, especially during winter months when warm-season turns light brown.
Warm-season grass encroachment can cause playability issues.
A combination of mechanical and chemical edging can help suppress warm-season grass encroachment into cool-season putting greens.

Producing and maintaining cool-season putting greens that are free of contamination from warm-season grasses growing immediately adjacent to the greens is a significant challenge. Warm-season grasses – such as bermudagrass, kikuyugrass, seashore paspalum and zoysiagrass – will outcompete cool-season grasses like bentgrass and Poa annua during the hot summer months. This is because the warm-season grasses are able to fix carbon dioxide more efficiently than cool-season grasses under hot temperatures. Consequently, warm-season grass growing in putting green collars or approaches often will grow into a putting surface occupied by cool-season grasses. This presents a problem for both aesthetics and playability.

When warm-season grasses encroach into a cool-season putting green it can result in several negative consequences, such as:

Poor aesthetics, especially in the winter when the warm-season grass turns a brown color during dormancy.
Mower scalping often occurs when there are warm-season grasses along the green perimeter due to differing growth rates.
Aeration or vertical mowing practices may spread the warm-season grass into the interior of the green, causing greater impacts on ball roll.
Once a putting green is fully contaminated by warm-season grasses, addressing the issue can be costly and will cause significant disruption to golfers.

Applying preemergence herbicide into the slit created by the edger is one method to bolster the prevention of warm-season grass encroachment.

Even with the best prevention methods in place, warm-season grass encroachment may still occur, but here are several methods courses have utilized to successfully limit encroachment issues.

1. Utilize a combination of mechanical and chemical edging – Use a tool that resembles a pizza cutter to sever encroaching stolons. Some superintendents have found that outfitting a mechanical edger with a thin blade is more effective and will cut deeper, potentially cutting underground rhizomes as well. Immediately follow the edging operation by spraying herbicide into the slit created by the edger. A preemergence herbicide such as siduron has demonstrated efficacy when utilizing this strategy. Repeat edging weekly during spring and summer months and apply the herbicide according to label directions.

2. Establish a cool-season grass buffer – Install ryegrass or a combination ryegrass and bluegrass band around the putting greens. Installing one or two sod rows of cool-season turf around putting greens can act as a buffer between the warm-season grass in the green surrounds and the cool-season putting green turf. Making fall applications of glyphosate on this collar is a popular strategy to suppress encroaching bermudagrass. Ryegrass is then seeded into the sprayed area to re-establish turf cover.

It is essential to have a proactive plan in place to prevent, or at least suppress, warm-season grass encroachment into bentgrass or Poa annua putting greens. While no method is perfect, the absence of any strategy will result in encroachment issues and potentially lead to contamination within the green, which will likely mean complete regrassing at significant cost. Utilize one of the strategies offered here or develop your own unique method to curb warm-season grass invasion.