Butterweed or Cressleaf Groundsel (Packera glabella formerly Senecio glabellus) is one weed that I have seen more of in no-till cropping systems in Missouri over the past two seasons. I'm not going to say it's a new weed to Missouri but it is one I think we are starting to encounter more frequently. There is some information that indicates that cressleaf groundsel thrives in moist, saturated soils so this would explain its increased occurrence over the past several years.
Cresslea groundsel is a winter annual. It germinates in the fall, grows throughout the winter months, and is flowering right now in many areas of the state (Figure 1). Cressleaf groundsel initially forms a basal rosette of leaves in the fall and winter months. Rosette leaves are highly variable in shape and deeply lobed or notched. Lobes are not initially apparent on seedlings but become more apparent as the plants take on a rosette growth habit. Cressleaf groundsel rosettes might be confused with those of yellow rocket, but the lobes of cressleaf groundsel leaves are arranged oppositely from one another while those of yellow rocket are arranged alternately. As the plants begin to mature in the early spring, flowering stems are produced. Stems are capable of reaching as much as 3 feet in height and are light green in color with conspicuous red veins running the length of the stem (Figure 2). Stems are also hollow, thick, and succulent. Leaves are arranged alternately along the flowering stem and become progressively smaller towards the upper portions of the plant. Like the rosette leaves, the mature stem leaves are deeply lobed and these lobes each have serrated, or toothed, margins (Figure 3). The leaves and stems of cressleaf groundsel are without hairs and are also poisonous to grazing animals.
Many bright yellow "golden" flowers are produced on the ends of the central stems. Individual flowers consist of inner (disk) and outer (ray) petals, although the outer ray petals are the most conspicuous (Figure 4). Cressleaf groundsel will generally have anywhere from 5 to 15 outer ray petals that are bright yellow in color and these petals surround the inner disk florets which are more golden yellow in color. The number of ray petals in cressleaf groundsel helps to distinguish it from any of the mustard species which only have 4 yellow petals per flower. Individual flowers are approximately ½ to 1 inch in diameter and are grouped together in clusters. Plants eventually produce many "puffball", dandelion-like seedheads. The seed within these puffballs are small and reddish-brown in color, with each seed having a white feathery pappus that facilitates wind dispersal.
Recent research conducted at the University of Illinois by Dr. Aaron Hager has shown that many of our common residual fall herbicide programs like Canopy EX, Valor, or Princep will provide good control of cressleaf groundsel. Additionally, their research has shown that either fall or spring applications of glyphosate or glyphosate plus 2, 4-D will provide good control of this weed. However, like most winter annuals, control of cressleaf groundsel is best achieved prior to flowering so timely spring applications of these herbicides will be vital to the level of control achieved.
REVISED: October 2, 2015