Whilst writing this at the end of April, we are usually just about to embark on the main Poa annua var. reptans seedhead flush which causes varying degrees of disruption to the spring putting surface, dependent on sward composition, cutting height and nutrition, to name but three dependent factors.
On high ryegrass-content pitches, seeding Poa annua (and particularly the annual biotype) can provide an aesthetic and localised wear problem with its pale leaf colour, prolific seedhead production and shallow rooting habit.
Although it is true that you will find an annual biotype plant of Poa annua seeding in any month of the year on rough and fairway height of cut turf, at greens height it often kicks off seeding with a vengeance once we hit around 100GDD cumulative from January 1st (using a 6°C base temperature for the calculation). This number does vary between geographical locations mind.
As an aside, I often look for another flowering plant as a bio-indicator. Danish Sea Scurvy (Cochlearia danica), a salt-loving plant (halophyte) that lives in the salt spread zone along the side of our roads. When I see the Danish Sea Scurvy flowering, I know Poa annua var. annua isn't far behind. Read about the highly successful, Danish Sea Scurvy here (https://www.bbc.co.uk/news/uk-wales-21123964).
This year, I had it in my mind that the perennial biotype seedhead flush started earlier, so I decided to compare spring 2022 and 2023 from a cumulative GDD perspective, up to and including the end of April.
It is unusual for two years to be alike, so I was amazed at how similar 2022 and 2023 actually are given the very different weather scenarios we have endured. Not least because of the very cold start to March brought about by the Sudden Stratospheric Warming event that I talked about in my last column.
Growth-Degree-Days is of course just measuring minimum and maximum air temperature and not taking into account other factors like moisture and light, but even so, from a purely-temperature perspective, 2022 and 2023 are extremely closely-aligned.
Below is a graph of the cumulative GDD from my default location at The Oxfordshire Golf Club, Thame using data from the Davis Vantage Pro2 6820 weather station on site. A big thanks to the course manager, Sean Wilson, for allowing me to use this data.
I have worked with Sean for over twenty years, and we know on his site that the annual Poa biotype starts seeding at around 100GDD cumulative starting from January 1st and the perennial at around 180GDD cumulative. When you look at the trace of the two years above, there are clear differences, but at these two GDD points, the dates are near identical.
In the past, this used to be the very end of April, start of May, but our warming winters are giving Poa annua a head start. Food for thought there I think when it comes to plant growth regulator timing.
Mark is well respected in the turfcare community and welcomes your questions. Send them to: email@example.com