Sweet clover starting!

Many prairies are plagued with sweet clover, either yellow (Melilotus officinalis) or white (M. albus). Because of its long-lived endurance in the seed bank, you don't eradicate sweet clover very easily.

We will be spending a lot of time from now until late July on this pesky plant. Yesterday, we had a volunteer group at Black Earth Rettenmund Prairie and spent about half the time hand pulling M. albus, the species present there. Fortunately, the recent rains made it relatively "pullable", so we did not need to dig it with our Parsnip Predators.

This week we'll also be starting sweet clover work at Pleasant Valley Conservancy, where we have both species. M. officinalis is flowering now, making it easy to spot, and especially important to get. Later we will have to deal with M. albus, which is more common there.

The key to sweet clover control is to keep the plants from setting seed. Since they are biennial, once they flower they die. Now, in the early stages of development, it is not necessary to bag and remove the plants. Later, when they have already started to make seeds, it is necessary to take them off the site.

How long will sweet clover seeds persist in the soil? The gossip says 30 years, but I doubt whether there is much data to support this number. We behave as if the seed bank will be around forever, and assume that every summer starting now pulling sweet clover will be a main chore.

Sweet clover was introduced in Wisconsin as a pasture crop in the 19th century, so even if it isn't planted much any more, it has a long history here.

The seed coat of sweet clover is very hard, making it possible to remain dormant for many years. Burning will scarify the seeds and stimulate germination (the late Virginia Kline of the UW-Madison did some pioneering research on the role of fire in sweet clover development). I have seen large flushes of first-year sweet clover plants in areas burned for the first time. Sometimes removal of the second-year plants (the first-year plants are almost impossible to pull) will almost eradicate such flushes, but there will undoubtedly be seeds that were not scarified and will germinate another year. Long-term persistence is essential.