Summer's Pastime: Poa annua Management
For me, Baseball’s All Star Game marks the midpoint of summer. It is at this time that annual bluegrass (Poa annua) management becomes challenging. The difficulty arises in a large part to rising soil temperatures. As average soil temperatures rise above 70o Fahrenheit, the root system ceases to grow and root dieback begins to occur. As soil temperatures continue to climb root loss rapidly increases. Eventually we get to a point where the functioning root system is shallow. Managing shallow rooted annual bluegrass turf is challenging because the margin for error is so small. The challenge associated with annual bluegrass is best demonstrated with water management.
Figure 1. Annual bluegrass green wilting in the early afternoon.
With other cool season turfgrasses like creeping bentgrass and Kentucky bluegrass for example, signs of moisture stress are reflected in a bluish tint to the turf. That bluish color is an indicator the turf is wilting. If water is applied during this time the creeping bentgrass or Kentucky bluegrass quickly recovers. Timing of irrigation can be based on the wilting signs. In the case of annual bluegrass, the wilting sign, or bluish turf occurs briefly, that subsequently it turns brown. The quick progression from green to blue to brown is so rapid that not watering quickly at the right time can result in considerable brown or dying turf. Thus, watering based on the appearance of a “wilting sign” is so temporary that the annual bluegrass is well on its way to dying.
Figure 2. Annual bluegrass green suffering from wet wilt.
Conversely, if we combine warm to hot air temperatures with a shallow annual bluegrass root system, the occurrence of a thunderstorm where multiple inches of rain fall, annual bluegrass will rapidly die. Often described as wet wilt, the saturation of the soil profile results in insufficient oxygen levels often referred to as wet wilt. Respiration requires oxygen to create the necessary energy. Root respiration is important in nutrient and water uptake. From an energy standpoint a turf growing on a saturated anaerobic root zone would have 1/18th the energy that it would have under aerobic conditions. Often mottled or chlorotic turf that occurs on anaerobic root zones does not have enough energy to actively take up water and nutrients. Wet wilt is most often associated with annual bluegrass because of the restricted and shallow root system that is most predisposed to the creation of an anaerobic soil condition.
From a water management perspective we need a careful balance. On one hand good drainage both surface and subsurface is needed to remove excess water from an annual bluegrass fairway or green, and good air movement (tree removal) around the turf to promote higher evapotranspiration (ET) rates (more drying). At the same time moisture monitoring is need to account for ET rates.
High temperatures also weaken annual bluegrass turfs. High temperatures contribute the depletion and waste of energy stored within the annual bluegrass plant. High temperatures can also directly impact the internal workings of the plant. Often we see this decline in plant health as a loss of wear tolerance, turf density and increased disease pressure to name a few. Integrated programs incorporating practices that minimize stress and protecting the plant from biotic stresses is required. Disease prevention is a key to any summer stress management of annual bluegrass. Some of these disease management programs can be found at Agronomic Programs
Anthracnose is often associated with annual bluegrass turf during periods of summer stress.
Maintaining high quality annual bluegrass turf through the summer is challenging. However, annual bluegrass is not much different than a baseball team heading down the home stretch; you need to stay on top of your game.
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: