Birds foot trefoil: a difficult invader

This is peak season for birds foot trefoil (Lotus corniculatus), a pesky ag weed that won't seem to go away. In contrast to many of the other ag weeds, this is a perennial, with a growth habit that makes it difficult to eradicate. For years we tried to eradicate it by pulling. Although pulling works for a biennial such as sweet clover, it doesn't work for a perennial, since you can never get all the underground viable fragments. After about 10 years trying various approaches, I think we have finally worked out a procedure that works.

If you drive any road in Madison right now, you are seeing birds foot trefoil. This is a plant that "likes to be mowed", and actually benefits by roadside or median strip mowing. What mowing does is cut down the plants that might cast shade on birds foot. Because of its low growth habit, birds foot is not affected.

Note that birds foot has never been planted in these median strips. It is an escapee from ag fields, where it is sometimes planted as a forage crop for pasture, hay, or silage. It has become a serious invader in our area. Do not let its "pretty" color fool you. Nothing that is this difficult to eradicate is pretty.

At Pleasant Valley Conservancy birds foot is found primarily in our planted prairies, especially Pocket, Ridge, and Toby's. Not surprisingly, we also see it occasionally in the middle of our service road. Until it flowers it is almost impossible to spot from a distance, but once it flowers it can be easily seen. The patch in the photo below is from the Pocket Prairie, one of our "prime" problem sites. I spotted this easily from about 100 feet away.

We are fairly certain that birds foot moved in from the adjacent farm, where it had once been used for sheep.

In our early days, we were ignorant of the biology of this species, and the printed information about control was sketchy. Several web sites recommend mowing to a height of less than 2 inches for several years, which seems foolish, since this would also effectively suppress all native species. As we discovered by digging, a single plant (such as the one shown in the Pocket Prairie photo) has a deep tap root from which a large number of separate stems grow, crawling out over the ground. If you grab each of these flowering stems and lift it up, you will discover that they all arise from the single tap root in the middle. This gave me the idea for how to control it.

For 3 or 4 years now we have been using on herbaceous plants an herbicide mixture formulated for basal bark work. This is 20% Garlon 4 diluted in bark oil. It turns out that spraying with this mixture at the bases of herbaceous plants will kill the whole plant, and with care the adjacent native species are unaffected since the herbicide is confined to just a narrow "spritz" area. This technique works beautifully for legumes such as alfalfa, multi-stemmed sweet clover, and birds foot trefoil. (It also works for other herbaceous weeds such as mullein and burdock.)

With birds foot trefoil, the technique is to grab some of the individual stems and lift them so that the center of the colony can be found. A few spritzes in the center is all that it takes to eliminate the whole plant. The photo to the left shows a dead birds foot plant adjacent to a thriving colony of black-eyed Susan. The larger photo below shows how the various individual "shoots" thread their ways back to the tap root.

Treated plants do not recover either the same or subsequent year and thus are eradicated.

This technique also works for small plants that are just getting started, although one must be careful not to "overspritz."

Our approach is to wait until flowering time and then canvas the whole area of a problem prairie. Because no digging or mowing are involved, it takes little time to deal with a single colony. I did 8 separate colonies in the Pocket Prairie in less than 15 minutes.