Mowing seems like a simple act—gas up the tractor and go! But for savvy citizen scientists, questions about timing, frequency, and possible impact on other species (vertebrate and invertebrate alike) start to cut through the air like the swirling steel blades. We know that grassland habitats are important for the birds we love to watch throughout the summer. Bluebirds and swallows in particular prefer the view from their nest box or tree cavity to be an open, grassy expanse complete with wildflowers and insects. And in order to maintain this picturesque “early-successional” habitat for our avian friends, mowing becomes a necessity. [Prescribed fires can have the same effect, but for most landowners, mowing is the most realistic option.] But what about the other critters that also call this home? To become better habitat stewards, we need to take a closer look at this complex web of life.
Before we get into the nitty-gritty details of a mowing schedule, let’s first take a peek at some grassland inhabitants and visitors. Click through the slideshow below to meet some familiar and unfamiliar meadow-goers.
Two Good Reasons To Delay Mowing
For most of these inhabitants and visitors, a defined mowing schedule is not critical. However, there are two groups of inhabitants that are extremely sensitive to a well-timed mow: (1) ground-nesting birds and (2) monarch butterflies. For most of the United States, it is safe to mow by early or mid-August, but check out the specific dates of the nesting season in your region here. By late summer, most birds will have completed their breeding cycle and second broods will have fledged.
Monarchs And Milkweed
When taking into consideration monarch butterflies and their life cycle, the date for mowing becomes a bit trickier. In recent years, monarch populations have suffered a precipitous decline . One hypothesis is that milkweed (Asclepias spp.) availability has been reduced due to industrialized agriculture. Monarch butterflies require milkweed for egg-laying and larval development, and with fewer milkweed plants, the remaining populations struggle to find suitable breeding habitat .
Monarchs spend several months breeding in milkweed habitat. In northern latitudes, breeding tends to run from June-August with migratory departure starting around mid-August. By the first frost, most monarchs have left their breeding grounds. In southern latitudes, departure for overwintering sites might not occur until September or October. To avoid cutting milkweed that has developing eggs or caterpillars, or eliminating it as a food source during migration, it is best to wait to mow in northern ranges until after the first frost or after late October/early November, whichever comes first. In southern parts of the range, it is recommended to wait until late December or early January, well past when monarchs have arrived at their wintering grounds and no longer need the milkweed as a food source.
These dates seem pretty clear cut; however, a recent research study conducted in New York’s Hudson Valley adds an interesting twist . The research team oversaw timed mows in early and late July and in mid-August to determine if milkweed regeneration would provide a more continuously suitable habitat for monarch eggs and caterpillars than an un-mowed control area. The swaths that were mowed in early and late July significantly extended the monarch breeding season and increased their overall reproduction. Mowing that occurred in mid-August had no impact on the monarch population.
While we would discourage mowing before August 1 out of caution for ground-nesting birds, a mow at the start of August, for those living in northern latitudes similar to upstate New York, might be something to consider. There will be some loss of monarchs in their juvenile stages, but as recent research points out, monarchs prefer the milkweed regrowth for laying eggs, and any loss is more than replaced in an extended breeding season and in egg density on the regenerated plants . Additional research with different milkweed varieties and in different regions is necessary, but the implications are significant for the struggling monarch butterflies.
With all these variables in mind, when will you mow this season?
1. Davis A.K., Dyer L.A. 2015. Long-term trends in eastern North American monarch butterflies: A collection of studies focusing on spring, summer, and fall dynamics. Ann. Entomol. Soc. Am. 1-3.
2. Pleasants J. M., Oberhauser K. S. 2012. Milkweed loss in agricultural fields because of herbicide use: Effect on the monarch butterfly population. Insect Conserv. Divers. 6: 135–144.
3. Fischer S.J., Williams, E.H., Brower, L.P., and P.A. Palmiotto. 2015. Enhancing Monarch butterfly reproduction by mowing fields of common milkweed. Am. Midle. Nat. 173: 229-240.