Sweet clover and prairie restoration

We are just finishing our "sweet clover season." This species, Melilotus alba, is a real problem in prairie restoration, but rarely a problem elsewhere. The photo here (by Matt Ziehr) shows a "wall of sweet clover" at Black Earth Rettenmund Prairie, with intern Emily just starting to attach it. Every prairie restoration activity that I know of in southern Wisconsin deals with it in a major way. At Black Earth Rettenmund we have spent about 150 paid hours dealing with this weed, in addition to some significant volunteer hours.

Strangely, sweet clover does not appear as a significant problem on any of the Wisconsin lists of baddies. It does not appear on the recent list of noxious weeds that is making the rounds through the Wisconsin DNR as a runup to the new proposed noxious weed law. My written note at the public comment session last winter suggesting that it be added was ignored.

Why is sweet clover ignored when it is clearly such a major problem? First, it is an ag crop, and may still be planted commercially. But the main reason, I believe, is that sweet clover is unique in being stimulated by prescribed burns. Prairie people are the major burners, and a single good burn can bring out a major "wall of sweet clover." In the absence of burns, it hardly shows up.

The reason for this is that the sweet clover seed is able to remain dormant for long periods of time, and is stimulated to germinate by fire. There has been significant research on the relation between fire and sweet clover, but anyone managing prairies could understand this even in the absence of careful field research.

The encouraging thing here is that if one eventually flushes out all the dormant seeds, and keeps pulling all the new plants before they set seed, one should be able to eradicate this pest. At least that is our hope, and keeps us moving ahead on our control efforts.

How long will sweet clover seed remain dormant in the soil? Some authorities say 30 years, but hopefully the major infestations will be greatly reduced in fewer years. We'll see.