The Turf Doc: An annual bluegrass summer

The period from late May to early June results in an interesting change in annual bluegrass growth and, indirectly, its management. Through May, annual bluegrass (Poa annua) has the ability to flower and produce seed under low mowing heights.

During the period of profuse seedhead production, annual bluegrass uses up to 35 percent of its top growth based on dry matter for seed production. The annual seed production on a golf green is between 150,000 to 650,000 seeds per meter squared per year (177,000 to 767,000 seeds/square yard/year). Almost all of the seed is produced through the period of May through June.

The seed that is produced contributes to the soil seed bank. Some annual bluegrass may germinate quickly, which is referred to as a transient seed bank, but normally this would occur where summers are mild like the very northern United States. Most of the seed produced enters the seed bank and remains dormant. For the seed to germinate, an after-ripening period is needed. In the case of annual bluegrass, its seed requires a period of high summertime temperatures to trigger its germination in late summer.

An important aspect in managing annual bluegrass in the summer is coring, normally with small or quadra-tines, to relieve surface compaction and allow for oxygen exchange into the root system. The requirement of the annual bluegrass seed bank to undergo an after-ripening period greatly reduces the potential for seed to germinate in the core-opened areas.

It’s important to note that small-tine coring relieves surface compaction, but after annual bluegrass has completed most of its flowering stage, there is a loss of a deep significant root system. The amount of energy that annual bluegrass spends to produce inflorescences, and thus seed, comes at the expense of root development and growth.

The image of annual bluegrass as it moves into the summer months is a plant that has a restricted or shallow root system, especially on compacted soils. Combine a restricted shallow root system with summertime temperatures, then add in thunderstorms generating inches of rain, and the result is annual bluegrass that will rapidly die. The saturation of the soil profile results in insufficient oxygen levels.

Often mottled or chlorotic annual bluegrass turf symptoms occur because not enough energy is being produced by the plant to actively take up water and nutrients. This condition is called ‘wet wilt’ and is most often associated with annual bluegrass.

Specific keys to managing annual bluegrass is through proper water management. Recognizing the changing growth habit (restricted root system), watering needs to be adjusted. Allowing creeping bentgrass, for example, to undergo wilt (bluish tint) prior to applying water would not be successful with annual bluegrass. The bluish wilt stage for annual bluegrass, from my experience, lasts about five seconds before it rapidly enters drought.

Regarding top growth, annual bluegrass aggressively tillers under optimum growing conditions found in spring. As summer approaches and conditions become less favorable for growth, new tillers are produced, but not nearly at the rate when conditions are favorable. Research has found that slowing senescence is targeting pests, specifically pathogens through fungicide programs. Critical in maintaining an annual bluegrass turf is a well-designed disease control program.

The growth of annual bluegrass rapidly changes as the season moves into summer. A key to its management is recognizing the changing nature of annual bluegrass. Given that most of the changes are relatively negative in nature, it is critical for superintendents to stay on top of their annual bluegrass management programs.