I've been reviewing the stewardship files for Black Earth Rettenmund Prairie since the site was first acquired by the Nature Conservancy in 1986. This is part of an environmental history study I am doing on this high quality State Natural Area.
At the time TNC acquired the site, sweet clover was recognized as a problem. Soon after acquisition, work began to control it. (Wild parsnip was also recognized as a problem, but diligent work by Tom Wise and others led to its eradication.)
A variety of methods for sweet clover control have been tried, and as far as I can tell from the records, every summer since 1986 TNC staff, contractors, and/or volunteer work parties have been either hand pulling or mowing it. Has progress been made?
Kathie and I took over management of Black Earth Rettenmund Prairie in 2001, motivated mainly by the still persistent sweet clover problem. Since then we have hand pulled and mowed infestations, either by diligent volunteers or by hired contractors. (We have also made major efforts to make sure that no flowering sweet clover plants were able to go on and make seed.) We were of course sanguine that it would not take us long to get this problem under control. Well...!
The photo below shows the massive sweet clover patch that was in the Saddle area in 2005, about 19 years after TNC started sweet clover control.
In 2005 it took three brush cutters two full days
to mow just the Saddle area. A few years later it was again bad enough that it took two people most of one day to deal with. Since then we have not had to do any mowing, but major hand pulling work continues from mid June through the end of September.
There is of course the role of prescribed burns. As most people know, sweet clover seeds are very persistent, and the data in the literature indicate that seeds can remain alive in the soil for longer than 30 years. Also, the very hard seed coat of this legume is loosened by heat, so a good hot burn is almost guaranteed to bring on a flush of plants. Since sweet clover is a biennial, one generally sees this flush two years after a burn. Interestingly, we had a hot fire in the Saddle area on 4 April 2004, which probably brought on a flush of first-year plants that year and led to the major flush of second year plants shown in this 2005 photo.
What can be done?
People have fiddled around with the timing of burns, with the idea that if you burn late in the spring, after the sweet clover plants have already appeared, you will kill them off. However, twice TNC tried this trick, in 1994 (a June 4th burn) and 2002 (an April 30th burn). These may have cut back the infestation, but obviously not enough. Perhaps you have to burn late for many years in succession. But that would likely have deleterious effects on lots of the spring prairie flora.
The point of this blog post is to inquire whether anyone anywhere in the Midwest has been able to "eradicate" sweet clover with some method that does not involve herbicide. (Herbicide is hardly advisable for use in a high-quality prairie remnant like Black Earth Rettenmund Prairie.)
If so, I'd like to know the technique.
Returning to my environmental history study: having long-term stewardship of a site such as Black Earth Rettenmund Prairie is tremendously valuable, as it provides a baseline for other sites. And it points up the importance of keeping detailed records of a site.