June 19, 2024

I, Science

The science magazine of Imperial College

Can leaving grass unmown have a measurable effect on biodiversity? Katie Porter investigates.

No Mow May is gaining traction. What was once a niche movement, created by the botanical charity Plantlife in 2019, has grown: a survey conducted by Plantlife in 2022 found that 92.4% of respondents had left their lawns untouched for May, up from 78.8% the previous year. As the name suggests, the initiative encourages gardeners to resist mowing their grass for the entire month of May to allow wild plants, including weeds, to flourish, pollinators to feed, and soils to be protected, which helps preserve the garden’s biodiversity. As of now, the UK only has half of its natural biodiversity left, which puts it in the bottom 10% of countries globally in terms of remaining biodiversity. Hence, incentives such as No Mow May are vital to regain some lost or struggling species and the ecosystem services they provide. 

If the urge to cut the grass remains too strong, or you are unaware of the ample benefits of partaking in this movement, let us convince you why we urgently need to ditch the boring monoculture lawn and celebrate a wilder, biologically thriving garden. We set out in Hyde Park to conduct a series of mini-experiments to investigate if the species richness (number of different species), and abundance (the count of individuals per species) is, in fact, more significant in unmowed areas compared to mowed.  

Hyde Park has not officially decided to partake in No Mow May, but appears to have areas of grassland left untouched, as pictured below, making it the perfect location for such an experiment. It was also intriguing for us to see a snapshot of the biodiversity in green spaces in a major city, perhaps areas that we expect to lack biodiversity.  

So, with that, we set off to do some very mediocre fieldwork. Our initial plan to use a white sheet to attract insects failed miserably. We found that, at the end of the time, neither sheet had attracted any insects, despite us being able to see them scurrying around visibly. This idea was therefore abandoned with a new approach devised: we would instead create a 2x2m quadrat, set a timer for 5 minutes and document each insect we saw- arguably not the most scientific method, at least not when counting insects that have a tendency to try to evade you, but the results were compelling enough for us to persist. We repeated the experiment three times in each patch to improve reliability, as good scientists would.  

The final results were startling, the unmowed area consistently had greater abundance and diversity. However, as the field researchers, these results did not come as a surprise; we had consistently struggled to record the insects in the unmowed patch rapidly enough but had great difficulties finding a single insect in the 5 minutes in the mowed patch. The mean results are also depicted in the table below.  

 Mowed Unmowed 
Insect Abundance 28 
Species Diversity 10 

Although this was only a brief experiment, it does give genuine insight into the differences in biodiversity of an unmown lawn compared to a mown. It is, perhaps, a microcosm of what is occurring across the country and supports more rigorous studies that have come before, such as that of Del Toro and Ribbons (2020), who found No Mow May lawns have a fivefold higher bee abundance compared with regularly mowed areas.  

As we reach the end of May, it is vital to consider what comes next. Whilst it is brilliant that gardens are given a month to flourish undisturbed, it is not enough. More urgently needs to be done in the midst of the biodiversity crisis; it does not suffice to resist mowing for one month only. Instead, there needs to be a broader societal shift, where unkempt lawns are celebrated for their variety and natural wonder, and miserable plain grass lawns become a thing of the past, one where it is seen as a positive to let your grassland grow wild and free all summer long. Ultimately, in terms of encouraging biodiversity, the longer and larger the areas left to do as nature intended, the better. After all, if we are truly serious about helping the planet, it really does begin at home (or a local park…) 

Text and images by Katie Porter and Troy O’Donohue.