growing degree days to determine the date to spray our greens to suppress and or reduce the amount of poa annua seed heads that are produced. As you know, the seeding of poa annua in the spring to early summer can lead to ball roll not being as smooth as we would like to see on our golf greens. The production of more seed heads also increase the seed bank for poa annua. Millions of seeds are produced and can be viable in the soil for years just waiting for some thinning of turf to encourage more poa to germinate. Seed heads are not just produced on greens surfaces, they are produced in the 100's of millions underneath the mass canopy of trees that run along our fairways and roughs.
Growing degree days are formulas which take the average temperatures for the day and subtracts them from the base number used.
I.E. The base number for poa annua seed head suppression is 32 degrees. The high was 38 and the low was 28 yesterday at our weather station. so the mean/avg temperature for the day was 33. Subtracting 33-the base of 32 = 1 growing degree day. This number is then added to an accumulating total that begins normally around the end of the 2nd to 3rd week of February. If the number would be negative for a day which it could if the mean temperature is below 32, you have no growing degree days for that date. Currently we are at 344 growing degree days. The range to spray is normally around 300 growing degree days, more or less depending upon the year. The spray we use suppresses or reduces seed heads from about 50-80% depending upon if your timing was correct. I sprayed 1/2 our greens on Friday and the other 1/2 on Saturday morning. The poa annua on a green, the more potential for seed head production. This is one of the reasons we are trying to encourage sun on our greens. Bent grass is a sun loving plant and poa annua likes it shady and moist.
Growing degree days models are also used for other weeds such as crabgrass germination and broadleaf weed applications. Bluegrass weevils and grub control of beetles are a couple of insects that can be monitored using growing degree day models. Farming relies heavily upon growing degree day models as well in planting of crops and applying pest control measures.
We will make a second application of the growth regulator products twenty one days after the first application. The spray we use is a mixture of Primo which is our main growth regulator product for greens and Proxy which reduces the seed head formation. With early season aeration in March, our seed head suppression application usually falls around the time of aeration. It can delay healing slightly but I think it might only delay it a couple of days at the most. This season, a majority of our aeration was finished five days before the application of this product mixture.
Seed heads that are produced do have a tendency to be shorter and tighter and do not effect ball roll since the greens are regulated. Ball roll in general should improve as the aeration heals and the plant growth regulators PGR begin to do their work.