A Sea of Purple

April 7, 2000
The mild winter weather may have allowed for reduced home-heating bills, but it also allowed many winter and early spring annual weeds to thrive in fields not disturbed by fall tillage. One person called earlier in the week and said the fields near her house looked "like a sea of purple" with all the henbit growing there. With the large, healthy stand of winter and early summer annual weeds already present in many no-till fields, burndown herbicide selection and application rate will be critical to successfully control these weeds prior to planting. Table 8 contains herbicide efficacy ratings for many popular burndown herbicides against many winter and early summer annual weed species. This table is taken from the 2000 Illinois Agricultural Pest Management Handbook, with some minor modifications.

Some of the most common broadleaf weed species currently in undisturbed fields include henbit, purple deadnettle, chickweeds, mustards, knotweed, and butterweed. A few of these can be difficult to identify correctly.

Henbit (Lamium amplexicaule) and purple deadnettle (L. purpureum) are close relatives; both exist as winter annuals and both have square stems. Henbit is more commonly found throughout Illinois, while purple deadnettle appears more often in the southern half of the state. The lower leaves of henbit are petiolate (attached to the stem with petioles), while the upper leaves grasp the stem (i.e., lack petioles).

The upper leaves of purple deadnettle, however, are attached to the stem with petioles, are more triangular than those of henbit, and are less deeply lobed. As the name implies, purple deadnettle has a distinctive reddish to purple coloration of the foliage and stem.

Mustard (Brassicaceae) is a family of many species, but several dominate the Illinois scene. Mustards are often divided by flower color. Wild mustard and yellow rocket have showy, yellow flowers, while shepherd's-purse, field pennycress, and pepperweeds (Virginia and field) have smaller, white flowers. Illinois has many mustards, however, and these do not cover the "waterfront" by any means. One plant that may often be mistaken for a mustard is butterweed (Senecio glabellus), which belongs to the Aster (Asteraceae) family.

Butterweed has bright yellow flowers and exists as a winter annual, so it often flowers close to the time the true mustards flower.

All mustard species have flowers consisting of four petals (either yellow or white),

while butterweed has a disk array of petals. The stem of butterweed is hollow

and often has a reddish or purple color.

--Aaron Hager and Christy Sprague

Author: Aaron Hager Christy Sprague