Turf MD: In a growth drag race, Poa wins
In the northern U.S., spring is the time for rejuvenation. Winter is gray and dark, but with the arrival of spring, colors are generally warm and bright. The vibrant and fresh colors are due to the flowers and trees that bloom during spring. Although the colors associated with flowers and trees include yellows, orangey reds, pinks and every shade of light brown, turfgrasses provide a palette of colors, mainly different shades of green.
One of the most vibrant greens on a golf course in the early spring is annual bluegrass (Poa annua). The vibrant light-green to green color often contrasts the semi-dormant to bluish color of creeping bentgrass.
On putting greens, Poa grows at a relatively rapid vertical rate. In contrast, creeping bentgrass (agrostis stolonifera) grows at a much slower rate; often to the point of appearing not to be growing at all. Due to differential color differences between annual bluegrass and creeping bentgrass, the greater density and vertical growth rate of the annual bluegrass results in a blotchy appearance. Additionally, the difference in growth rates makes the greens bumpy due to Poa’s faster growth rate.
In some cases, the appearance of Poa patches or increased number of patches may appear what may seem like overnight. Greens that appeared solid creeping bentgrass in early November may appear coming out of winter like Poa invaded them. Poa seed germination begins mid-fall, which is often visually difficult to detect.
Greens in the fall may appear relatively clean (low Poa populations), but once greens emerge from winter, something happens; relatively small Poa patches appear.
A common explanation is that annual bluegrass colonizes ball marks. Although this can happen, Poa does not need a ball mark to become established. Research has found that Poa could germinate in complete darkness at a level relatively close to that observed under light. Although most weeds that produce small seeds need light to germinate, one weed — Poa— apparently does not. Although not the only factor (and disturbance is still important), the ability to germinate under a dense turf canopy is a competitive advantage of Poa.
To smooth out the differences in growth rates, the temptation exists to push creeping bentgrass to get it growing more rapidly. Creeping bentgrass, however, is on a different biological clock than Poa. Research on root and shoot growth reports that creeping bentgrass growth comes later in the spring compared to Poa.
If you have developed a solid fertilization program, don’t try to adjust it to “jump start” the creeping bentgrass by giving it subsequent elevated shots of nitrogen. Bentgrass doesn’t want excessive nitrogen, and the impact can actually be detrimental to root growth. The excessive amounts of nitrogen applied at the time of root growth may stunt growth in favor of pushing leaf growth. The implications will be felt later in the year. And in actuality, you are most likely favoring more growth from Poa.
Practices to smooth out the growth rate differences of the two turfgrasses include plant growth regulator applications like trinexapac-ethyl, which start when foliar growth starts. In early spring, this would be timed with Poa foliar growth. In combination with light rolling, the putting green will be smoother, and differences in growth will be less noticeable.
Light mechanical practices like brushing, verticutting and topdressing should help improve creeping bentgrass leaf texture. Once consistently warmer temperatures arrive, the differences in growth will disappear. By allowing creeping bentgrass to start growing on its own, you will have a healthier turf through the coming summer months.
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