This has been a good year for sweet clover. That’s not to say there are walls of it everywhere at Pleasant Valley and Black Earth Prairie, as in the years I remember from 2005-2010. But even after “clearing” section after section, five foot flowering plants, even eight-foot giants, seem to elevate out of nowhere overnight, many times in sweltering prairies, often on steep and slippery road cuts. White sweet clover (Melilotus alba), or Honey Clover, was brought to North America from Eurasia in the mid 1600s as a forage crop. Beekeepers helped spread it, and its nitrogen-fixing abilities helped farmers enrich their soils. Today, in Wisconsin, it is listed as “ecologically invasive.” (Yellow sweet clover blooms earlier than white and is equally to be removed from natural areas.)

Tom was particularly religious about removing every last sweet clover blossom from our natural areas. He often said that because continued annual stewardship relies on money and knowledgeable personnel, it can never be assured. Even more, sweet clover is a very aggressive biennial. It likes fire, and its seeds remain viable in the soil for up to 30 years. He said that if we ignore the “stragglers,” all of a sudden it seems, due to the characteristics of exponential growth, the last doubling “turns the site from tolerable to ruined and hopeless.” That “all-of-a-sudden” Tom called the “tipping point.” And he was quick to add that even if we were able to eliminate sweet clover completely, we would still have to be vigilant of more brought in by animals, humans and road-side mowing machines. In his honor, here is Tom’s haiku on tipping points:

Tipping Point

growth, exponential
today it’s tolerable
tomorrow, ruined

We typically pull or cut sweet clover. Once seed set begins, plants must be bagged and removed. Large areas that are mowed must be cut before seed set, and the areas revisited for survivors. We no longer have to mow large areas of sweet clover at Black Earth Prairie except occasionally in an adjacent privately owned area. But sweet clover remains what Tom called “a very expensive weed to deal with.” Amanda visited and revisited (and revisited) Black Earth Prairie this year, always finding more and more sweet clover to pull. She insists it’s now all gone. Once and for all…. We’ll see.

A Story

So, is it a mirage that sweet clover seems to be growing overnight, or might we write a story about what may be contributing to its flourishing? We know that carbon dioxide levels today are the highest they’ve been in millions of years. In pre-industrial times, the carbon dioxide level was 280 parts per million (ppm). In 2022, the level was 417 ppm (NOAA). We also know that sweet clover nectar is 52% sugar, vs 20% for most other flowers. Given that it takes six molecules of carbon dioxide to make one molecule of glucose, it can be reasoned that higher carbon dioxide levels favor plants that efficiently take in and utilize this gas. Of course, as they always say, more research is needed.