Issue 2, May 4, 2015

What's That Yellow Weed in Bloom?

Not everything you see that's yellow this time of year is dandelion, or daffodils or forsythia for that matter. Some other species featuring vibrant shades of yellows are currently or soon will be prevalent in roadsides, fields, and gardens particularly in rural settings.

Butterweed, also known as cressleaf groundsel, is additionally known as Senecio by many. The latin name was Senecio glabellus for many years but was changed in the last few years to Packera glabella. Other common names include ragwort and squawweed.  Regardless of what you may call it, this is a weed we have been seeing much more frequently in recent years although it is native to North America. It tends to prefer cool, wet conditions and is common in fields and roadsides. It has been known to find its way into nearby landscape beds.

Butterweed Senecio Packera cressleaf groundsel in bloom.

Butterweed is a winter annual that grows erect on a hollow, succulent, smooth stem from a basal rosette. Stems are typically green but can have a reddish coloring as well, often in vertical stripes. Stems can reach 3 ft. in height. Leaves along the stem are deeply lobed, smooth, and often glossy. The flowers of this aster are somewhat distinct. Appearing in clusters at the end of the stems, they are bright yellow or golden. Unopened flowers are somewhat rounded in shape. Slightly lighter colored outer ray petals number 5 to 15 and surround the slightly darker colored disk florets. The number of petals is useful in differentiating between this species and other yellow weeds in bloom at this time. The seed heads are dandelion-like puffballs which are disseminated by the wind.   

Common groundsel (Senecio vulgaris) is similar but much smaller at only 4-12 inches tall. This winter or summer annual weed is erect and much branched. The leaves of common groundsel are dark green, lobed, and 2 to 6 inches long. The flowers make this plant fairly easy to identify: they are yellow, about 1/2 inch wide, and are borne in corymbs nearly 3 inches wide. Flower heads are composed of several yellow disk flowers. This weed is often found in fields, nurseries, and landscapes. This species, like butterweed, is also known as Ragwort.

Yellow rocket (Barbarea vulgaris) is a winter annual, biennial, or sometimes short-lived perennial mustard, also known as winter cress, bitter cress, and rocket cress. The basal rosette leaves are dark green, thick, and glossy. Young leaves are rounded, sometimes with a heart-shaped base. Mature leaves have a distinct lobe pattern with the terminal lobe being heart-shaped. The second year, 3 to 8 flowering stems are produced and are much like that of broccoli, which is a related species. The stems are very branched at the top. The flowers are bright yellow with only four petals. The flowers appear on spike-like racemes. They form pyramidal clusters at the ends of branches. Plants can reach 2-3 ft. in height but can tolerate mowing. There are many similar weeds in the mustard family but they bloom later in the year. 

Yellow Rocket.

These weeds can be controlled by hand removal using a dandelion fork. Be sure to bag up pulled weeds for disposal so that flowers do not continue to develop, producing viable seed. Mowing can be properly timed such that flower and seed production is prevented.
Postemergent herbicides such as carfentrazone, MCPA, MCPP, dicamba, 2, 4-D, can be applied. Treating plants when they are young and actively growing (in the fall) would be best.

Weeds of the Northeast. (1997). Richard H. Uva, Joseph C. Neal, and Joseph M. DiTomaso. Ithaca, NY: Cornell University Press.
(Michelle Wiesbrook)

Michelle Wiesbrook

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