By Barry Bessent
The grass isn’t always greener on the other side of the fairway. Sometimes, it’s a menace. This is true of the poa annua grass on Sonoran desert golf courses. Poa annua (poa a.) is an annual bluegrass that is used as a turf grass on some courses. For example, Pebble Beach Golf Links in California has one variety of poa a. on its greens. However, on most courses it is considered a weed (definition a weed is any plant growing where it is not supposed to be). When it grows on a course overseeded with rye grass or other cool season grasses it is a weed and must be controlled because it affects the playability of the course.
Overseeding dormant turf grass on golf courses enhances both the playability and aesthetic beauty of the course in the winter. To save money many courses do not overseed the roughs. Tees and fairways are seeded with a cool season grass, typically a variety of perennial rye grass, and greens with a combination of a perennial rye grass and poa trivialis or bent grass. It is lighter green than most turf grasses (almost yellow) and easily identified in overseeded areas. Poa a. is a bunch grass like rye grass in that it has multiple leaf blades growing from a central point but is a much courser plant, providing a much harsher playing surface. On greens poa a. grows at a faster rate than the other cool season grasses so the greens roll smooth in the mornings after being mowed but in the afternoons can be very bumpy. Regardless of where poa a. may appear, it must be dealt with immediately because it propagates and spreads quickly.
A single poa annua plant is capable of producing 1000 to 2000 seeds in a season. Even when mowed to green heights of 3/16” or less they are capable of producing over 300 seeds per plant. These can remain viable in the soil for up to 6 years and start growing when the conditions are right. Mowing only serves to spread the seed once the seed head has formed. These seeds are easily spread by birds, machinery or humans. Once started poa a. spreads very quickly and can take over a turf area. There are several methods used by golf course personnel to control poa a.
Healthy turf is the first line of defense, because it limits the amount of sunlight that hits the ground and poa a. seed. Poa a. thrives in areas where turf is thin. Pre-emergent chemicals are applied to non-over seeded turf areas. These chemicals are soaked into the soil with irrigation or rain water, and kill the plant as it breaks through the seed coat. Pre-emergent in the soil breaks down over time and additional applications have to be made, as poa a. grows in surges. Here in the Sonoran Desert we cycle between cold days and nights to periods on warm days and cold nights. Warm days can trigger a new surge of poa a. growth. The poa a. plants that survive in the non over seeded rough can be treated directly with selective herbicides without harm to the dormant Bermuda grass. These herbicides can’t be used on poa a. in the overseeded areas because they would also kill the rye grass in the fairways. In the overseeded areas chemicals are used to suppress the growth of the poa a. These chemicals have varying degrees of success. It is a constant battle but, to provide the best playing conditions we can it is being addressed almost daily.