by David Kopec
Annual bluegrass (Poa annua) is the most prevalent grassy weed in all turfgrass settings. In the last 50 years of turfgrass science, more studies have been conducted on how to control it than any other turfgrass weed. Yet Poa annua is still here and probably will be for some time to come.
The mechanism for this amazing response was discovered at Pennsylvania State University in 2010 by a student named Jason LaMantia. That body of work is a major breakthrough in turfgrass genetics. That’s why Poa is such a survivor. A major reason for this fact comes from the survival strategy of Poa annua, which involves its basic reproductive scheme and the ability to switch between a flowering and a vegetative plant. Poa annua resulted from a natural cross between Poa infirma X Poa supina. Those plants were most likely sterile but as a chance event, the chromosomes spontaneously doubled in number, which resulted in a fertile plant. This fertile plant is Poa annua as we know it. And as we know, Poa annua makes seed. So why do we see so many different kinds of Poa and why is it so well adapted to wet, cold, dry, and hot environments? Poa annua can cross with other Poa annua plants to create new variant plants; in most cases it prefers to pollinate itself (pollen fertilizes the seeds of the same plant). When this occurs, individual genes in the plant become fixed in a “double base condition” in a hurry. After four or five generations of self-pollination, the plants are highly uniform genetically. Those that have the right combination of genes (for that environment) have an advantage over neighboring plants, whether they are other Poa annua plants or other turfgrass plants. In another turfgrass field 300 kilometers away, a collection of Poa annua plants will initially have different gene combinations, but quickly become genetically uniform in that environment for the same reason (inbreeding and gene fixation). The end result is a lot of Poa annua plants in a relatively short period of time, which quickly become highly adapted in the environments they are found in. When only a few Poa plants are alive in a very stressful environment, Poa annua then starts to “outcross” with the neighboring Poa plants. This outcrossing mixes up a new “batch” of unique gene combinations, which can then go through the self-pollinating process again, producing a new set of uniform plants that have a double dose of the “new” desirable genes. So Poa annua uses both systems to its advantage. Now for the second question: Why do some Poa annua plants produce abundant flower heads while some barely produce any? Again, Poa annua has a trick up its sleeve. The answer is in the height of cut of the turf. When Poa is mowed tall (3/4 inch or above or 2.0 cm or above) it produces seed heads abundantly. At low height of 1/4 inch 6.0 mm or less, it produces fewer seed head, and at 1/8 inch 2.75 mm or less, it often produces no seed heads. The partial reason for this is because at a high mowing height, the Poa has lots of leaf area to make food, enough for a robust plant with lots of food to make seed heads. So it flowers abundantly and then dies. At intermediate height, Poa annua plants are often smaller and may often have lesser amount of seed heads forming. That’s because the plant wants to stay alive longer in a vegetative state, under mowing stress. At very low heights of 2.5 mm (on greens) the Poa is mostly interested in “staying alive” vegetatively. At greens height it has to make more leaves and stems all the time in response to constant low mowing, As a consequence, it does not have a lot of energy to divert to seed heads. The last question is: Why does my Poa annua weed turn from wide leaf flowery types in the beginning, to non-flowery types after five years or so. If you are not yet amazed by Poa annua, well, here’s more! On a new turfgrass green that is low cut, the wide leaf, abundant Poa annua types which appear first, sense the continuous mowing pressure. In response, the Poa annua sends a chemical DNA message inside the stressed plant that tells the next generation to “grow lower.” This is repeated from generation to generation. So when you see Poa the next year, many of the plants look “smaller.” After several years at low mowing height, you end up with lower-growing types that flower very little. That is because the chemical message to mowing response gets passed to the next generation to “stay low.” It used to be a mystery why and how the low growing plants “suddenly” appeared in year five of a golf green. That’s the reason why. Lastly: Why can’t I buy a low-growing seeded cultivar of Poa annua? The reason is because when low growing Poa annua gets placed into a seed production field, the reverse DNA chemical message occurs. The desirable low-growing plants in the seed field sense that there is no mowing stress. As a result, the plants send a message for the next seed generation that “it’s okay and safe to be tall.” So after two to three years of seed production, the harvested seed produces wide leaf flowering types!
David M. Kopec is a turfgrass advisor with the University of Arizona in Tucson. For more information on the turfgrass genetics program, visit http://plantscience.psu.edu/directory/drh15.