April showers Brings May Flowers...
We normally do not associate flowering with turfgrasses except in one case, the profuse flowering of annual bluegrass (Poa annua) in May. Poa annua flowering is not the beautiful type of flowering associated with ornamentals. For golfers, seed heads disrupt the aesthetics, uniformity and playability of greens and fairways. Annual bluegrass can produce 75,000 to 225,000 flowering tillers per meter squared per year that will result in 150,000 to 650,000 seeds per meter squared per year. What makes this flowering event even more amazing is the plants ability to throw up a seed head at less than an 1/8th of an inch. Annual bluegrass races to complete its life cycle from germinating in the fall climaxing in the spring with profuse seed head production prior to dying in the face of summer stress.
Although it germinates in the fall, annual bluegrass patches are most evident in early spring. These patches seem to appear overnight. Often associated with disturbances have occurred like ball marks, and divots. Although true, annual bluegrass it has the ability to germinate in darkness2. Thus just a small amount of space in the turf canopy, or maybe none at all, and annual bluegrass can germinate and gain a foot hold.
Annual bluegrass is considered an annual plant. However, as the cultural intensity increases (lower heights of cut, fertilization, irrigation, pest control, etc.) the annual type takes on a perennial nature. Perennials are characterized by a more stoloniferous growth habit, produce more tillers and shoots, have a deeper root system, and produce fewer seedheads when compared to the annual.
A question however that has never really been thoroughly explained is how does Poa annua evolve from an annual type that initially invades an athletic field or a putting green to a Poa annua that exhibits perennial type characteristics?
Reasons given for the rise in perennial types include being due to environment, cultural practices. Some research points to the rapid rate and turnover of tillers of annual Poa annua could actually account for the perennial nature of greens. However, a fascinating study was done at Penn State3 University that shed light on to how perennially might arise.
To better understand the perennial nature of Poa annua on greens LaMantia and Huff characterized both the annual and perennial type of phenotypes in their Poa annua collection. Focusing on inflorescence differences, they found when crossing putting green type (perennial) by annual type that the subsequent progeny did not follow expected Mendelian ratios that would be expected with single gene characteristics. Furthermore when green type (perennial) by green type (perennial) Poa annua crosses were conducted subsequent generations past the first generation resulted in annual type of Poa annua.
What this could mean is that the perennial type of Poa annua found on greens will revert back to annual types. The authors hypothesized that the reason is that close mowing induces an epigenetic effect on gene regulation. What this means is that the functionality of the DNA may be affected (in this case by mowing) giving the perennial type of biotype but the underlying DNA is not changed (the genetics). As long as the mowing stress is present the Poa annua on greens will tend to exhibit the perennial traits, but once mowing is removed eventually the Poa annua reverts to an annual. What the Penn State researchers found to me is so cool - Poa annua can be whatever it wants to be.
So how do we control or manage this formidable opponent? No one thing practice is going to control annual bluegrass. The first question is to ask, is why is it there in the first place? By identify why annual bluegrass is there in the first place will lead to corrective strategies to minimize re-infestation. You may be able to kill annual bluegrass but if you do not change the reasons why it is there in the first place annual bluegrass will be back. Making conditions less favorable for annual bluegrass growth and more favorable for the desired turfgrass is not easy and requires careful management. Extreme measures taken to rid annual bluegrass from a stand like not applying certain nutrients or eliminating watering rarely succeeds, because the practices do little to promote the desired turfgrass species health.
References
1. Fenner, M. 1985. Seed Ecology. Chapman and Hall, New York, NY
2. LaMantia, J.M. and D.R. Huff. 2011. Instability of the Greens-Type phenotype in Poa annua L. Crop Science 51: 1784-1792.
3. McElroy, J.S. R.H. Walker, and G.R. Wehrje. 2004. Annual bluegrass (Poa annua) populations exhibit variation in germination response to temperature, photoperiod, and fenarimol. Weed Science 57:47-52
About the author
Dr. Karl Danneberger is a professor of Turfgrass Science at The Ohio State University. Dr. Danneberger's contact information can be found here. You may also follow Dr. Danneberger on Twitter: