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Issue No. 7, Article 4/May 20, 2011

Yellow Replaces Purple

The springtime color scheme provided by winter annual weed species in many no-till fields has shifted from the hearty purple of flowering henbit and purple deadnettle to sunny yellow, contributed primarily by two species. Yellow rocket and cressleaf groundsel (also known as butterweed) both produce bright yellow flowers and are common across much of the southern half of Illinois. Although their flower colors are similar, the plants are separate species.

Yellow rocket (Barbarea vulgaris) is a winter annual species in the mustard (Brassicaceae) plant family. The plant can produce numerous stems, which grow from a basal crown surrounded by a rosette of deep green leaves. These basal leaves range in length from about 2 to 8 inches and have a large terminal lobe that is somewhat heart-shaped at its base. Stem leaves are arranged alternately and become progressively shorter toward the top of the plant. Flowers are produced on spikelike racemes and consist of four petals that form a cross. Seed pods (siliques) are about 1 inch long and nearly square in cross-section.

Yellow rocket in flower.

The basal leaves of yellow rocket have a large terminal lobe.

Yellow rocket flowers consist of four petals forming the shape of a cross.

Cressleaf groundsel (Packera glabella), or butterweed, emerges predominantly during the fall months. After emergence, rosettes form before overwintering. The rosette leaves have petioles that connect the leaves to the stem. Often the undersides of the rosette leaves are deep purple. Bolting (stem elongation), flowering, and seed production occur the following spring, often in May.

Cressleaf groundsel (butterweed) in flower.

The stem of cressleaf groundsel (butterweed) is smooth, green to purple in color, and hollow.

The upper leaves of cressleaf groundsel (butterweed) are irregularly shaped.

Cressleaf groundsel (butterweed) flowers are grouped in clusters on flowering stalks.

The stem of butterweed is glaborous and hollow. After bolting, petioles are absent from leaves on the upper part of the plant. The leaves are pubescent, generally irregular in shape, and deeply cut to the midrib. The elongated stem often has a purplish tint. A member of the Asteraceae family, butterweed produces two types of composite flowers. The outside portion of the flower contains ray florets, while the center part contains disk florets. The bright-yellow flowers are grouped in clusters on several flowering stalks of the plant. Seeds are easily disseminated by wind due to the white hairs (pappus) on the apex of the achene.--Aaron Hager

Aaron Hager

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