Mark Hunt weather corner; 119 - An insight into light levels on the grass plant

Autumn/winter 2024/5 has been characterised with some significant weather, five named Storms including the extremely damaging Storms Darragh and Éowyn. In addition, we have had mild and humid weather over the Christmas period that caused significant disease activity.

Finally, January brought us snow and prolonged frost, quite an exception to today’s climate norm but more typical of how winter ‘used to be’.

There is one climatic parameter though that has quietly slipped under the radar, and that is light or, more accurately, a lack of it.

Reading the excellent December 2024 synopsis from The Met Office, tucked away in the multitude of graphs is ‘Sunshine Duration’ vs. the 1991-2020 average as a %. You can clearly see from the graph (below) how dull a month it was.

Let’s take this down to grass level

We can measure the amount of light that the grass plant can use using an Apogee PAR sensor fitted to a Davis weather station. PAR stands for photosynthetically-active radiation, and is the part of the light spectrum (shown above) that the grass plant utilises in order to power photosynthesis and therefore energy production. PAR light is quantified in a unit called the Daily Light Interval (DLI) and represents the total amount of PAR light over a 24-hour period, measured in mols per m2. Less sunshine means quite simply less PAR light available for the grass plants we maintain.

We have some pretty loose data which quantifies the minimum DLI requirement per grass species. In the pecking order, Poa annua is thought to grow the most efficiently at low light levels, hence its predominance in shaded areas, and so has the lowest DLI requirement, though that figure hasn’t been nailed down. Lolium perenne sits at 11 mols per m2 per day and Creeping Bentgrass 30 mols per m2. These figures aren’t written in stone though because we have a lot of variables including cutting height, type of cultivar (leaf habit) to name but two.

Below is a graph of the DLI levels in Central Scotland and England over autumn/winter 2024/5 to date with those two known DLI levels included. 

What this graph shows is, in my view, interesting. For Creeping Bentgrass, we dipped below the minimum DLI in the 3rd week of September, and for Perennial ryegrass, in the last week of October. Since then, the DLI has been below the minimum sufficiency level right up until the present day. So, our grass plants aren’t getting enough light and their energy production to fuel growth is compromised.

So, if your Lolium perenne sward is looking on the thin side lately, this is the driver behind it. A word of caution though, reaching for the fertiliser bag or bottle may not be helpful because it will force the grass plant to utilise its dwindling energy reserves. With low light levels, the grass plant will elongate and the cell walls will become thinner. Thinner cell walls means a plant less susceptible to wear and disease ingression. Maybe also it is time to consider extending the use of plant growth regulators to counteract this effect? The good news to finish off with is that the days are now extending and this allows the DLI to potentially increase, provided we have some sunshine of course.

visit: www.weatherstations.co.uk
Send your questions for Mark to: editor@pitchcare.com