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A number of eco-friendly turf management sources say something along the lines of

Increasing grass height only 1/8 inch adds about 300 square feet more energy-collecting leaf surface for each 1,000 square feet of lawn. So, it has a greater ability to produce its own food and less need for fertilizer.1

and

If you are currently mowing at less than 3” every 1/8” of grass blade added increases leaf surface area by 30%. - from the Perfect Earth Project Toxin-Free Lawn and Landscape Manual available here.

Does this have to do with the fact that the first couple of inches are non- or low-photosynthesizing crowns or stems? Wouldn't having greater leaf area up high lead to shading out a grass plant's own or neighbor's leaves?

  • Photosynthesis takes place in daylight as well as sunlight, see here rookieparenting.com/do-plants-breathe-science-experiment so the more leaf surface there is, in theory, the more energy the plant produces during photosynthesis, and the more nutrients its able to make for itself. – Bamboo Dec 10 '19 at 21:31
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A couple of factors to keep in mind:

Grass leaves tend to grow out close to horizontally, particularly at ground level in the crown. So if we raise the height of the cut by 1 unit (inch, cm. and so on) the increase in length of a leaf at an angle of 10 degrees to the horizontal would be 1.0 ./ sind(10) = 5.76 units, (Ground Control please check my math)

also the angle of incidence of light onto leaves varies during the day so lower leaves are rarely shaded for long.

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This sounds like woo-woo for the mathematically challenged.

The total amount of energy falling on 1000 sq ft of lawn is (amazingly enough) 1000 sq ft's worth of energy.

If you increase the height of the grass by another half inch, you don't magically get another 1200 sq ft's worth of energy from somewhere. Some of the grass is just shading other parts of the grass.

Granted that growing conditions are different in the UK and the USA, but in the UK the maximum cut height of most household lawn mowers is about 2 1/2 inches, and the minimum is usually less than 1 inch. The grass grows fine cut like that.

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  • Really, Alephzero? Depends on the mower; I used to cut grass at around 4 inches in height with no difficulty at all. At six inches, it was a bit more difficult and required a certain technique with a hover mower, but still doable and not that unusual either in May when its growing fast and its only getting mown once a week. – Bamboo Dec 10 '19 at 21:26
  • Leaving the grass longer is currently a common recommendation in many lawn turf management guides. It has to do with reducing heat/water stress by shading the soil to minimize evaporation and by allowing grass to support longer root systems which can help grass deal with dry conditions. I am a strong adherent to the practice, but I question the above claim. I wonder if it is just one of those unsubstantiated claims that gets perpetuated without question. – That Idiot Dec 10 '19 at 21:29
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    This claim has the inherent assumption, that no light makes it to the ground. Which is false. Grass is not a photovoltaic plate. Longer leaves can actually increase the amount of sunlight taken up, simply because it is a rather chaotic pattern of leaves and so they might catch light, that wasn't caught by another leaf before. – elPolloLoco Dec 11 '19 at 9:36

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