Invasive Plants of Wisconsin
|Lepidium latifolium L.
Lepidium latifolium is a new invasive species in Wisconsin (June 2007) and we have a lot to learn about how it will behave in our area. It is a relatively large herbaceous plant, reported to reach 2 meters in height, though the plants of the single Wisconsin site are mostly in the 1 - 1.3 meter range. Leaves are simple, alternate and usually toothed, but some leaves may be entire, according to the available literature. Lower leaves tend to be broader (up to 4 cm wide) and may be longer petioled than the narrower upper leaves. Flowers are very small (4-5 mm wide in full flower) and white with 4 petals, and many flowers are borne close together in a panicled inflorescence which may be 30 cm or more in length. Fruits are flattened in one plane and only slightly longer than wide in the other 2 dimensions, therefore nearly circular in side-view (see above). They are about 2 mm in length. The below-ground structure appears superficially to be a rhizome, but the literature indicates that it is a root and careful examination suggests that it is so. In any event, the plants are perennial and spread vigorously from the below-ground structures to form large populations.
There are not many large members of this family (the Brassicaceae or mustard family) in flower at this time of year. The large size of Lepidium latifolium (3-4 feet tall or more), small white flowers with four petals and six stamens, glabrous (smooth) stems and short flattened fruits should separate them from other mustards in mid-summer. The above photos may help in identification.
The first Wisconsin record of Lepidium latifolium of which I am aware is in Green Bay, growing in disturbed soils at and near a complex of warehouses and storage yards, and spreading out to a nearby road. The distance between sites occupied suggests that the plants have spread both vegetatively and by seed. This population was discovered on about June 30 and is being treated by the invasive species team of the Brown County Extension Horticulture office. You can find more information at the Brown County UW-extension web site
The literature indicates that pulling or digging the plants is not very effective, because if the roots are left in the soil they will simple resprout, even if broken into relatively small pieces. Some success has been achieved with herbicides, and readers are advised to check reliable web sites for specific herbicides, methods and safety precautions.