Milkweed Madness

August 30, 2015

Nature Notes by Bill Dunson

If you manage a field, you face a conundrum in deciding your specific goals and if, when, and how often to mow. If you want to retain the field, you must eventually mow it, but preferably no more than once per year to minimize negative effects on wildlife. Otherwise woody vegetation will eventually move in and convert the field into a woodland. But the speed of this succession is highly variable in different locations. Mowing more than once per year usually converts the field into short grasses which are minimally useful to wildlife. But when do you mow? A traditional approach is to wait until late August after all birds have nested. This has the drawback that it disrupts the replacement of the early cool season grasses (such as fescue) by warm season grasses (which is stimulated by earlier mowing), and limits the amount of food and cover that will be in the field during the non-growing season. Another issue is what the impact would be on milkweeds. We have large numbers of the common milkweed, Aesclepias syriaca, and want to keep them healthy for monarch butterflies.

You would think that mowing would be bad for milkweeds and monarchs. I have found this not to be true because monarchs in our area primarily pass through in late summer and early fall. By August, milkweeds which are not mowed are past blooming, are dry and senescent, and suitable as food for milkweed tussock moth caterpillars, but not monarchs. If the fields are mowed in mid-June to early July, the milkweeds quickly regrow and some are blooming within about 5-6 weeks. There are then many young tender leaves suitable as food for monarch caterpillars. The photo taken Aug. 23 shows a field mowed on July 15; some plants are blooming and there are many tender young plants. It is likely that if open field-nesting birds such as meadowlarks are disrupted by early mowing, they will quickly re-nest.

Adult monarchs feed on the nectar from milkweed flowers, as do other butterflies such as this female black swallowtail. Monarchs also derive nectar from a variety of other flowers such as this ironweed. Many such nectar sources other than milkweeds are found not in the mowed grasslands, but around the wetter edges which are mowed less frequently. We have found that mowing as infrequently as every 3-5 years can be effective in maintaining a bio-diverse field attractive to insects and birds. Hand cutting of woody stems and selective mowing of problem areas with invasive infestations such as Canada thistle can help maintain these natural flower gardens as nectar sources. Burning in late winter can sometimes also be beneficial in controlling unwanted plants and in releasing nutrients from dead plant material, depending on your goals.

I urge you to consider the possibilities for milkweed cultivation in your local situation, either from the natural growth in fields, or plantings in your yard. Experiment to see what works best in your circumstances. Try cutting some plants down at different times to see if the subsequent re-growth provides a useful means of providing better food for our marvelous monarch butterflies.

Bill Dunson

About Bill Dunson

Bill Dunson, born in rural Georgia, skipped 12th grade and went directly to Yale. Bill subsequent-ly received a PhD in Zoology from the University of Michigan, studying softshell turtles. Bill is Professor Emeritus of Pennsylvania State University thanks to a career spent entirely at that institution, teaching and doing research on the physio-logical ecology and ecotoxiciology of reptiles, amphibians and fish. Always curious about nature, Bill has dedicated his life to learning and sharing his knowledge with others. He has served on many advisory boards here in Southwest Florida to preserve the water that gives life to our region.

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