No Mow May

by | May 1, 2023 | Science Communications

In the PNW, April showers bring May showers. Our rainy springs play a vital role for our ecology, and as the days get longer and warmer, the grass starts to not so much crawl as leap up from the ground. (Queue the sound of thousands of roaring lawn mowers).  Around the same time, bumble bee queens and other native bees emerge from underground as temperatures rise, looking for their first meal in months. Plants, however, are dependent on day length for changing their phenology (shifting from vegetative to flowering). See last month’s newsletter for how plants can tell when spring has arrived! As climate change leads to warmer temperatures, this has led to bees emerging before flowering begins, with some pretty dire consequences.  Enter “No Mow May,” a slick slogan like Movember, but for your lawn, that originated in the UK in 2019. Instead of raising awareness for men’s health, Now Mow May aims to bring attention to the plight of native pollinators in a landscape increasingly bereft of vital floral resources, especially early in the spring. One study published by scientists in Wisconsin in 2020 compared bee diversity and abundance in un-mowed yards versus routinely mowed city parkland. The results showed a dramatic difference, with mowed lands housing fewer flowers and bees than untouched lawns. The study was picked up by news outlets, and the movement has gained national attention. There’s a catch. A bee taxonomist from the University of Minnesota posted an article questioning the identifications of the bees in the study. Long story short, the paper was eventually retracted, but not before it was cited widely in the news. This kind of scenario is always unfortunate, as public understanding of and confidence in scientists has declined in recent years, although it is encouraging that science is the most trusted institution according to this Pew Charitable Trust survey.  
Fortunately, there are other papers on the impact of mowing on flowers, and by extension bees. In Massachusetts, researchers found that yards mowed every 2 or 3 weeks had more flowers and bees than yards mowed every week. Bees were most abundant in yards mowed every 2 weeks, but more diverse in yards mowed every 3 weeks. It was proposed that grass can actually obscure some flowers when it grows too high, but more studies are needed to test that hypothesis. It is important to remember that the ecological context of each study site is unique. Washington is not Massachusetts, and different bees occur in different areas and can range dramatically in size. Carpenter bees (Xylocopa spp.), for example, can be 20mm be, whereas Fairy Bees (Perdita minima) is only 2mm long. I’m 6 feet tall and change, and I imagine that a person 60 feet tall might use the environment very differently than I do! So, to mow or not to mow? Here is a great video that addresses that question. Warning! It is over an hour long! Along with the paper from Massachusetts I alluded to above, the advice is that a good (maybe better?) approach than No Mow May could be Slow Mow May. Also, choice of turf grass species is a big part of the equation. Fine Fescue grasses deal better with infrequent mowing than some of the thick Bluegrasses (sorry banjo lovers). Letting grass grow high and then chopping it all off stresses plants, as lots of energy is spent producing flowers (yes, grass flowers) and seeds. Depleted lawns tend to have more invasive species (the common showy flowers in urban landscapes), and although dandelions and buttercups are important food for bees, they are competitors with native plants too.  Despite best intentions, making choices when nature is involved is often complicated. Research supports that Slow Mow May can help bees through the starving time of spring, and so it is likely worth not running the mower (and burning the gas) every week. Additionally, consider devoting a portion of your yard-space to native plant species that bloom early, such as osoberry and bleeding hearts, or even non-native species, as long as they aren’t invasive! 

By former SHADOW Education and Restoration Lead, Nathan LeClear