Please go away ⎯ you are a perennial problem

Editor's note: In case you missed it, here is Dr. Beth Guertal's Verdure column published in the November issue of GCM.

This month we are going to get into the weeds. That is, into the common winter annual weed, annual bluegrass (Poa annua species). The problem: Is Poa annua truly an annual, or are there other species out there, populations that express themselves in a perennial nature? First hints of this were mentioned in research from 1957, and the concept of perennial Poa was firmly established in 1971 by Vic Gibeault, Ph.D., who clearly showed that perennial Poa varieties are more prostrate, darker green, have few seedheads and produce seed that do not require a dormancy period. So we now recognize Poa annua var. annua (annual type) and Poa annua var. reptans (perennial type).

But it’s not just one or the other. A wide variety of Poa varieties are trotting around the southeastern United States, some of a completely perennial nature, some that are true annuals, and a whole bunch that occupy some “in-between” state of quasi-perennial. The excellent turfgrass weed scientists at Auburn (Scott McElroy, Ph.D., and R.H. Walker, Ph.D.) decided to collect a batch of them and look at how environmental cues (temperature and photoperiod) affect seed germination. Knowing the environment in which Poa germinates is important, because that environment is directly linked to the effectiveness of a pre-emergent herbicide program.

McElroy collected eight annual bluegrass ecotypes (an ecotype would be a particular Poa annua that is adapted to a specific environment) from various golf course fairways and putting greens. These ecotypes varied widely in appearance, with some thought to be a “suspected perennial” and others of an “annual nature.” Seeds were collected from all the plants and then used for the laboratory germination studies.

Six photoperiods (0/24, 8/16, 10/14, 12/12, 14/10, and 18/6 hours day/night) and three temperature regimes (102 F/84 F, 84 F/66 F and 66 F/50 F [39 C/29 C, 29/19 C and 19 C/10 C] day/night) were evaluated. These were all evaluated as combinations, so there were six photoperiods × three day/night temperatures × eight Poa annua ecotypes, a total of 144 treatments in the study. The entire experiment was done twice. Collected data was the number of germinated seed after three weeks.

So what did they find? First, maximum germination in all annual bluegrasses was found at the 66 F/50 F day/night temperatures. Second, as the day/night temperatures increased, germination decreased, but the degree to which germination was affected varied with the annual bluegrass ecotype. The perennial or “suspected perennial” annual bluegrasses had better germination at 84 F/66 F day/night temperatures (~86 percent), while the germination of the annual ecotypes was lower (79 percent). At the highest temperature range (102 F/84 F day/night), 1 percent to 8 percent of the annual bluegrass germinated, and one perennial type had around 20 percent germination. Thus, some of the selected annual bluegrass did not behave at all like a cool-season annual and germinated at temperatures previously thought of as too hot.

The response of the annual bluegrass to photoperiod was not as clear-cut. In fact, there was no strong trend of increasing or decreasing germination as the hours of day/night were varied. The one distinct finding was that complete darkness did not inhibit germination. Across all the annual bluegrass ecotypes, average percent germination in complete darkness was 57 percent. The authors noted that some researchers have suggested that a dense turfgrass canopy can prevent annual bluegrass germination. McElroy and his co-authors have shown that reduced light from the turf canopy does not limit P. annua germination, but the dense canopy may be useful in creating seedling competition (the creeping bentgrass will crowd out Poa seedlings).

Source: McElroy, J.S., R.H. Walker, G.R. Wehtje and E. van Santen. 2004. Annual bluegrass (Poa annua) populations exhibit variation in germination response to temperature, photoperiod and fenarimol. Weed Science 52:47-52.

Beth Guertal, Ph.D., is a professor in the department of agronomy and soils at Auburn University in Auburn, Ala., and the incoming editor-in-chief for the Agronomy Society of America. She is a 17-year member of GCSAA.