Grass needs light, but how much is too much?

Like the other plant health inputs, water, nutrition and air, grass needs light. And like all things in life its not that simple. Each of these inputs has an optimum or 'Goldie Locks' zone. For water; too little and the plant withers and dies, too much and it becomes waterlogged and dies. The same is true of all the inputs, there is a Goldie Locks zone where its 'just right' for the plant. Grass will be able to handle a certain degree of drift from 'just right' but it will generally add additional stress up to the point that the plant is damaged/killed.

Plants respond naturally to excessive light, purple Anthocyanins can be seen in Spring or Autumn

People also have a 'Goldie Locks zone' for sunlight, the British Skin Foundation recommends "daily sunlight exposure for 10-15 minutes between April and September" for lighter skin types and "25-40 minutes" for darker skin types. This puts us in that optimum zone where we have enough Vitamin D production to support healthy bones and immune responses, and not so much that we are getting burned and raising risks of skin cancer. On hot days like today its probably best to stay out of the sun where possible!

For most of the plant health inputs, we have many options available to help us stay in the Goldie Locks zone, wetting agents, aeration and incorporation of sand in constructions to affect moisture levels. All manner of different fertiliser technologies to adjust available nutrition. And for us hats and sun cream to keep excessive UV light off.

But for turf we don't have many option available to help control the stress excessive light levels can bring. And its difficult to isolate the effects of light stress in a real world examples, as it so often has an impact at the same time as other, related stresses. Namely drought stress and heat stress which we all know can drastically impact turf health.

Whilst plants can react naturally to light stress in a limited way, production of pigments such as carotenoids and anthocyanins to decrease photosynthetic stress, these, like all biological reactions are 'expensive' for the plant meaning they are putting resources into creating those substances instead of root growth or other processes essential for health. These naturally occurring pigments also take time to produce and can only do so much to help.

Ryder, the turf pigment technology, has been proven to increase turf quality as seen in the graph below. This can relieve one of the stresses, which combined can make turf more susceptible to disease pressures, including anthracnose.

The effect of Ryder applications across the Summer season, with soil moisture held at 15-20% VMC

Too much light!

Its easy to think that high summer, when the mercury is touching late 30C is the only time its relevant to apply a turf pigment technology. Like this week for example! But studies have supported what we see in turf naturally. We see purple Anthocyanins produced by some grasses in Spring and Autumn when light levels are still high but growing conditions are not optimum for the plant to deal with excess light.

Different grass cultivars will react differently, and in different situations. The graphs below show how some Creeping bentgrass performed in an Ohio State University. Aneta Studzińska , M.S study.

Plants need light to photosynthesize and so grow, so we might expect that the more light we subject the grass to the more it would photosynthesize? But the graphs show that the amount of CO2 uptake (increasing up the left graph axis) which is essentially a measure of how much photosynthesis is going on (fixing CO2 into sugars - hence taking away CO2) does not keep increasing with increasing light intensity (that Goldie Locks zone again!).

No light = no CO2 taken up - as no photosynthesis is happening

In the first graph the plant is in a healthy unstressed situation, but we can see that at a certain point the curve flattens off and increasing light intensity does not increase the photosynthesis 'output' of the grass plant. Its reached the maximum of what it can achieve in that tiny little plant factory, more light on top will not make additional sugars but may damage the grass.

In this second graph the grass is stressed, this may be because its winter and outside of our Goldie Locks zone for growth, or due to stresses imparted as part of a maintenance programme, or even excessive heat like we're seeing this week. We can see that the intensity of light required to flatten off the curve in the second graph is a lot lower, so it doesn't take as much light to take the grass out of the productive zone and into stress/damage territory.


So whilst on long hot summer days pigment technologies are a great option for reducing light stress. Later in the year when the flip flops and sunglasses are long forgotten, it's still worth considering protecting your turf from excess light stress.

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