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To mow or not to mow?

To mow or not to mow?

The problem of climate change is no longer limited to complex scientific models and researchers’ predictions. It is reflected in a number of phenomena which are beginning to have significant impact on our everyday lives. We are facing such dangers as the creeping loss of biodiversity in our immediate environment or the catastrophic draughts caused by rising air temperatures and changes in rainfall patterns. These threats are global, but countermeasures against them can also be undertaken locally. Can changing the approach towards mowing lawns help the environment, and, if so, is it a feasible solution? Dr Joanna Kajzer-Bonk and Dr Justyna Kierat from the JU Faculty of Biology shed more light on this issue.

The maintenance of green spaces has recently become a frequent subject of public debate. According to the first of two competing approaches, lawns should be kept short, mown frequently, and consist only of grass, whereas proponents of the second one think that lawns should be cut only once or twice per season, so that diverse species of plants could develop. The first option is often preferred for aesthetic reasons, although in recent years dry weather often results in the neatly trimmed lawns left with patches of dried grass and bare earth. On the other hand, infrequently mown lawns have a number of advantages other than aesthetic ones.

The latest research [4] has shown that six years after the intensity of mowing is reduced to 1-2 times per season, lawns became much more biologically diverse (the number of plant species growing there increased by 30 percent) and included more species typical of meadows. So reducing mowing is an easy and costless way of increasing plant diversity. This triggers a chain of events, as a more diverse set of plant species provides a better source of food and shelter for pollinators and many other organisms [5, 6]. Protection of plant and animal habitats is especially important today, when species diversity is decreasing at an alarming rate, and so is the size of many of their populations, which is sometimes even referred to as the “sixth mass extinction” [7,8,9]. In this context, special attention should be paid to invertebrates, which are among the main beneficiaries of extensively (less frequently) mown lawns, along with plants themselves, and, at the same time, one of those groups of organisms whose populations are most rapidly shrinking. It would seem than an urban lawn is too small an area to have a significant impact on environmental protection, but actually, in a strongly transformed urban environment [10,11], it is the lawns that offer one of the few possible sanctuaries of biodiversity [12, 13]. Besides these advantages, reducing the frequency of mowing leads to curbing the emission of fumes, including greenhouse gases, as well as noise [6].

The infrequently mown lawns, besides making a “bad” impression, are often accused of providing a breeding ground for ticks and pests as well as spreading allergens. The validity of these fears is disputable. The metanalysis of studies on lawns with different mowing regimes in the USA has shown that intense mowing can increase (!) the probability of occurrence of insects considered as pests, as well as allergenic plants [6]. Besides, the presence of allergens in environment is inevitable, and the more sterile the conditions around us are, the more allergy-prone we become. It ought to be remembered that climate changes [14,15] are among the chief factors increasing the risk of allergies and so is stress, which is effectively reduced by the presence of green spaces [16]. The problem of ticks and the diseases they carry is indeed growing, which is probably at least partially the result of climate change [17]. An extensively mown lawn can be indeed inhabited by ticks, but frequent mowing doesn’t guarantee their absence. On the other hand, an unmown lawn allows greater diversity of organisms and mechanisms controlling the population of ticks [18]. The argument related to ticks becomes irrelevant in the case of roadside greenery, unsuitable for leisure activities. 

A mini-experiment

Can less frequent mowing have a direct effect on local climate conditions? Trees and bushes are already known to have such a positive impact [19]. Can replacing a neatly trimmed lawn with a multispecies plant community be beneficial for the local climate? We hadn’t found any research on that issue, so one of us decided to conduct a small experiment.

Experimental lawn in Podgórze District of Kraków (Photo: Joanna Kajzer-Bonk)

I wanted to check if and how an unmown lawn can regulate temperature and humidity. In May last year, I managed to persuade lawn mowers in one of Kraków districts to spare part of a lawn (photo above/below), which became a “feeder” for pollinators (such as bumble bees, orange tips, and common brimstones, which I spotted feeding on blooming dead-nettle). I used this opportunity to carry out a little experiment. On May 18-19 I put temperature and humidity meters one metre apart from each other on the ground in the mown and unmown part of the lawn. The results are presented below:

1)    In the mown part of the lawn, the temperature between 9 a.m. and 6 p.m. was up to 11°C higher.
2)    Conversely, the humidity in the mown part of the lawn was up to 25 percent lower.
3)    Temperature in the mown part of the lawn was less stable – at night it was slightly lower than in the unmown part and hence the amplitude there was much higher.

To put it short, the unmown lawn turned out to be a more stable habitat with a lower temperature and lower humidity fluctuations, which provides a great chance for reducing overly hot temperature in cities. The planned large scale studies are expected to confirm the results of this pilot test.   

* The measurements were performed using iButton® temperature/humidity logger; the measured parameters were: temperature (°C) / humidity (RH%); measuring range - temperature: -20 do +85 °C; measuring range - RH: 0 – 100%; accuracy: 0.5°C; resolution: 0.5°C. The devices were protected against sunlight.

A viable alternative?

Wildflower meadows grown from special seed mixtures have been proposed as an intermediate solution between a frequently mown “sterile” lawn and grass cut only one or two times a year. This idea seems to offer an aesthetic, colourful and biodiverse alternative to a lawn. In spite of appearances, infrequent mowing doesn’t result in an inferior version of a wildflower meadow. On the contrary, the less frequently mown lawns are in some respects an even better solution, being much more cost-effective and requiring less effort (in 2017 the Municipality of Kraków spent 750 thousand zlotys to create 10 hectares of wildflower meadows).

An urban lawn with death-nettle, shepherd's purse, and mallow. This feeder for polinators couldn't be saved from mowing (Photo: Joanna Kajzer-Bonk)

Creating a wildflower meadow requires the removal of turf, which is especially damaging for older lawns, rich in soil fauna, or, alternatively, sowing seeds into a pre-existing lawn, which, however, doesn’t bring such spectacular aesthetic results. In addition, the seed mixtures often include plant species that don’t naturally grow in Poland, which can have very serious negative consequences. Foreign species of plants introduced to local habitats can pose a threat to the ecosystem, as was the case with Canadian goldenrod or Himalayan balsam. Initially sown in gardens for their aesthetic value and ability to attract bees, they spread to the wild and began to outcompete native plants. Besides, the origin of seeds is significant even in the case of native plants. A native species of clover whose seeds have been imported from Australia can have an utterly different level of resistance to Polish climate conditions and pests.

A compromise?

A good way to reconcile the preferences of people who prefer neatly trimmed lawns with those of wild nature lovers would be to divide green areas such as parks into typical leisure spaces with short cut grass and parts with less human interference. Such partial or sequential mowing is becoming more and more popular [20]. It consists in leaving unmown grass in selected places, e.g. around trees and bushes or in green areas located far away from infrastructure. Additionally, mowing patterns in different parts of lawns differ in time and frequency, depending on the intended result, terrain type, its function, and the needs of its users. This strategy makes the lawn a mosaic of diverse species, sources of food and shelters for animals. Currently, mowing is usually conducted in the way most convenient for the contractor, which means that grass in an entire estate/housing area is cut in a short period of time. Such large area mowing means a death sentence for small organisms. This year, the Municipality of Kraków decided to experimentally spare some lawns from mowing, cutting short only the strips of grass adjacent to roads as well as walking or bicycle lanes. We would like to encourage all green space managers to reduce the frequency of mowing and, consequently, create wildflower meadows for free, using the gene pool of local plants whose seeds are spread by wind and animals. The saved money can be spent to buy up the (still remaining) green spaces.

The original text by Dr Joanna Kajzer-Bonk and Dr Justyna Kierat was published on the website Nauka dla Przyrody.  

 

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