The spring struggle with Poa annua

The first exciting signs of spring on a golf course are unmistakable — the turf breaking dormancy, bunkers being raked and reshaped, flagsticks returning to the cups and tee markers reappearing in their proper places. However, there is no more definitive sign of spring than the emergence of Poa annua. 

Its flowering seedheads sway in the breeze, almost as if the plant is waving to golfers. For many, however, this sight serves as a grim reminder that the season for controlling this persistent weed has officially begun.

For permanent cool-season turfgrasses like creeping bentgrass, perennial ryegrass and Kentucky bluegrass, pre-emergent control of P. annua has been marginal at best. Postemergent control for several years was limited to ethofumesate primarily used on perennial ryegrass fairways. A new herbicide, bispyribac-sodium (Velocity), which is an ALS (acetolactate synthase) inhibitor like sulfonylurea, has come to the market showing effectiveness for controlling P. annua in creeping bentgrass.

Traditionally, on warm-season turfs like bermudagrass, dinitroaniline herbicides have been used effectively for preemergent control of P. annua, while the triazines and pronamide have been used for both pre- and postemergent control. Recently, a new class of herbicides — sulfonylurea — has been developed that are effective for P. annua control on bermudagrass and other selected warm-season grasses.  

Although sulfonylurea herbicides have been around since the late 1970s, the new products have high activity at low rates with extremely low environmental risk. It appears that sulfonylurea chemistry can be effectively altered to target specific weeds. The potential exists within this herbicide class for future product development. 

Although these new herbicides show great promise, potential variability in controlling P. annua exists. One cause of variability may be due to learning how to use them. Rate, timing, application methods and environmental conditions often influence efficacy.  

However, the greatest variability in control will be due to P. annua itself. With repeated use of the dinitroanilines, triazines and pronamide, P. annua resistance or tolerance has occurred. In addition, a population shift from annual to more perennial species has taken place.

The effectiveness of new herbicide technology will be based on knowing why — and what type — of P. annua is present. Globally, P. annua is adapted to a wide range of climatic conditions. It is one of the most widely dispersed in the world, being found on all seven continents. Regionally, P. annua behaves primarily as a true annual, while the further you move into the northern United States and Canada, a more perennial behavior occurs.  

On a smaller scale, variability exists on golf courses. Studies have found that P. annua varies considerably from greens, fairways and roughs on a single golf course in the temperate regions of the United States. Gene flow (the transfer of genes from one population to another) can occur among greens but be restricted among fairways. The potential for “blending” and “isolation” increases the potential for herbicide resistance or tolerance.  

P. annua is a formidable opponent. If every herbicide that was labeled for P. annua control in the last 70 years worked, P. annua would be on the endangered species list. The key to long-term control is realizing why it is there in the first place. Management practices targeted for making conditions less favorable for P. annua will in turn make herbicide applications more efficient and effective.

Spring on the golf course brings a duality. While the greening of the turf signal a long-awaited homecoming for golfers, the emergence of P. annua serves as the official starting gun for turf management teams.