Impact of Lawn Mowing Height & Frequency on Plant Root Growth

At the beginning of last month I posted an article on this blogsite titled Growing Lawn Without Watering. In response, one of the readers (Peter B.) commented: “Liked your article on grass …. I was under the impression the more frequently the grass was cut (at maximum height setting without catcher) the stronger the root system … maybe this is an urban myth?”

Judi Earl is an Holistic Management Certified Educator based at Guyra who has a strong scientific background in pasture growth and the interaction of pasture and animals. She and her business partner Lewis Kahn collectively own and operate Agriculture Information and Monitoring Services. I contacted Judi and sought her thoughts in respect of Peter B’s question, and her enlightening response is below:

“There is huge potential to improve water cycling if city folk realised the impact they could potentially make by improving all their collective small patches of land.

I've no doubt that mowing at maximum height enhances root growth and improves the persistence, vigour and health of lawn species. On average for pasture grasses cutting, or grazing, to leave a residual height around 5cm will not result in any damage to root systems. Leaving this amount of green leaf enables the plants to regenerate leaf from energy derived from photosynthesis without the need to draw energy for regrowth from the roots or crown. Obviously the actual ideal height for cutting will vary between species some may be able to be cut lower than 5cm without damaging root biomass and some will likely be favoured by higher cutting but 5cm from my reading appears to be the average.

Certainly in the defoliation experiment I conducted some years ago defoliation to 3cm height reduced root biomass. Plants cut to this height every 2 weeks had about half the root biomass of those cut every 4 weeks and these had half the root biomass of those cut every 8 weeks. I had 8 species in the experiment and this effect was apparent in every one. By cutting less intensively (leaving more residual) plants actually grow faster and may be cut more often without impeding root growth. This has all the benefits you so nicely describe in your blog.

If it happens that you get good soaking rain to really wet the soil profile to depth during the growing season you could potentially take the opportunity to give the lawn a good close trim. This will enhance the turnover of root material (organic matter) in the soil and with good soil moisture conditions the regrowth ability of leaf material and new roots will be enhanced.

That's what I do with my lawn although I have a sort of leader follower thing happening occasionally. I often get a couple of days grazing for my old horse every now and then and then follow the next day with the mower. The mower also spreads the manure nicely which speeds up its breakdown and evens out the height. I've been doing this for a few years now and this spring my lawn isn't aware of any drought and I've a whole new suite of different species appearing.”

My thanks go to Judi for sharing her knowledge, and also for giving me permission to publish her comments. If anyone would like further information you can contact Judi via email at judiearl@auzzie.net.