Tall hawkeed, a new invasive plant at Black Earth Rettenmund Prairie

Last year’s drought is gone but its effects are still with us. Black Earth Rettenmund Prairie took a major hit and the whole preserve has been utterly transformed. Several serious invasive plants that were minor or nonexistent before are now taking over. These include smooth brome and quack grass, two of the most difficult nonnative grasses to get rid of.

However, the post today concerns tall hawkweed (Hieracium piloselloides), a native of Europe which we discovered at Rettenmund for the first time this year. Typical of most hawkweeds, this species forms a rosette of leaves at the base, sending up a flower stalk with clusters of small yellow flowers. It is clonal, and the whole colony spreads rapidly via rhizomes.

How do we know this is a new invader? The flora of Black Earth Rettenmund Prairie is very well known. The first detailed flora was done in the 1950s by Anderson, a Ph.D. student of Curtis. Robert  Reed from the Wisconsin DNR did another detailed flora in the early 1970s, and Rich Henderson did another in 1986 at the time the site was acquired by the Nature Conservancy. Sarah Kraszewski did another flora in 2005 for her Master’s thesis, and Kathie and I have done another since 2006. It is likely that if such a conspicuous species as tall hawkweed were here it would have been recorded.

Rettenmund Prairie is predominantly a dry prairie and last year many species took major hits from the drought. Especially the major prairie grasses such as little and big blue stem were killed, and there were large bare areas where they should have been. Tall hawkweed is a prolific seed producer and almost certainly colonized some of these bare areas. It is able to spread rapidly across dry, open fields and meadows and presumably became established last summer. It may have formed a few flowers but not enough for us to spot it.

Suddenly this year there are two large clones in the north unit, the legacy of last year's growth. One is on the knoll at top and the other is near the north lane and hence visible from Fesenfeld Road.

How do we get rid of it? According to Betty Czarapata’s book on Wisconsin invasive plants, it is a difficult plant to control. Since it is a perennial, hand pulling and mowing are a waste of time. Use of a broad-leafed herbicide is recommended, but this may be difficult since there are no leaves to spray on the flower stalk. Spot spraying each rosette will be necessary. However, herbicide residue will almost certainly transfer to the soil, slowing down the recolonization with native plants.

At present we are inclined to wait and see what happens. Perhaps if we have a couple of good wet summers, native plants, including the prairie grasses, will reestablish and outcompete the hawkweed. Otherwise…?