The following chart illustrates the growth rate on my putting greens and the effects of plant growth regulators. Prior to the spring growth surge my growth rate was at about 0.4 (This number really doesn't mean anything to anyone but myself. It is based on the clippings we collect on our greens mower each time we cut. It gives us a constant to compare data). In late April the growth rate surged to 2.5 or almost 6x the growth I was seeing in the early spring. After my first application of Primo Maxx at the label rate of 2ml/100m2 the growth rates dropped down to about 1 (our ideal clipping yield based on observed stress and consistency). This growth rate was sustained for about a week when we made our second Primo Maxx application again at the label rate. This application was based on the Growing Degree Day model that can be seen here. After this second application the growth rate continued to drop back down to the levels that we saw in early spring or 6 times less that the late April Growth rates! We wend from cutting every other day back to every 4 days! This is a huge time saver especially when you have a crew of 3!
Now the manufacturers of Primo Maxx and research only claim a 50% reduction in vertical growth. The reason I think that we are seeing the further reduction is the fact that the Poa annua is going to seed. We actually observed the first seed head 2 weeks earlier than normal this year on the 5th of May. When the Poa goes to seed it uses all its energy to produce the seed heads. I think that it is this stress that is causing the growth rate to drop so far down. We also can't forget that the past 3 weeks have been remarkably dry and hot (for West Coast standards) so this could also be a contributing cause of the reduced growth rates.
What I really like about this scenario is that the slowed growth rates we are seeing allow us to keep the greens extremely consistent day to day as well and very fast despite the effects of the seed head on ball roll. In previous years we normally made our first Primo Maxx application mid-May so we were usually a bit late to really get control of the growth on the putting greens.
Another thing monitoring growth rates shows me is the effects of fertilizer applications. When I saw the first seed heads I upped the N rates by 50% to help replenish the carbohydrate reserves in the stressed Poa. Despite the large increase in fertility the growth rate continued to drop. It is my hope that the plants are using this energy to push through the seed head cycle and remain healthy and strong going into June instead of growing vertically like usual (but who really knows?).
Time will tell if this low growth rate is here to stay or not. I think that by mid June the rates will jump back up into the 1 range when the seed heads finally leave.