Scientist plucks out the challenges of springtime weeds

URBANA, IL -- 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 the bright yellow flowers of two species. Yellow rocket and cressleaf groundsel (a.k.a. butterweed) both produce bright yellow flowers and are common across much of the southern half of Illinois.

University of Illinois weed scientist Aaron Hager explains the difference between butterweed (Packera glabella) and yellow rocket (Barbarea vulgaris), both winter annual species.

“Most of the yellow-flowered plants currently in fields are butterweed, which is in the same plant family as daisies. The flowers are bright yellow and grouped in clusters on several flowering stalks of the plant. Seeds are easily dispersed via wind due to the fluffy white hairs that catch the breeze,” he says.

Yellow rocket is a winter annual species in the mustard plant family. Flowers are produced on spike-like stalks and consist of four petals that form a cross. Seed pods are about 1 inch long and nearly square in cross section.

Hager points out that because plants are at the flowering stage in many fields, farmers should not skimp on burndown herbicides to control seed spread.

“The yellow flowers mean the plants are close to completing their lifecycle, and their sheer size will make them more challenging to control compared to when the plants were still in the vegetative stages. Complete control is important to reduce seed production which will be helpful for many future seasons,” Hager says.

Butterweed and yellow rocket are not the only weeds farmers must contend with now. Marestail (Conyza canadensis), also in the daisy family, produces small white flowers and numerous tufted seeds at maturity. Farmers know it as one of the most challenging weeds to control prior to planting no-till soybean.

“Already this season, some have reported poor marestail control following applications of glyphosate plus 2,4-D. Poor control can be caused by several factors, including large plant size and resistance to glyphosate,” Hager explains.

Hager says to avoid relying solely on 2,4-D for glyphosate-resistant marestail.

“Adding Sharpen or metribuzin to glyphosate plus 2,4-D can improve marestail control. Include MSO with Sharpen and be sure to adhere to planting intervals in treated fields where another soil-applied PPO inhibitor will be used. Glufosinate (Liberty, Interline, etc.) or Gramoxone SL are other options to control marestail before planting,” Hager says.

Control is often improved when these products are tank-mixed with metribuzin and 2,4-D.  Both glufosinate and Gramoxone are contact herbicides, so Hager suggests adjusting application equipment (nozzles, spray volume, etc.) to ensure thorough spray coverage.

Another option to control emerged marestail is tillage. Hager says farmers should delay tillage until field conditions are suitable and till deep enough to completely uproot all existing vegetation.

More information on the biology and control of these spring weeds can be found at http://bulletin.ipm.illinois.edu/?p=3589 and http://bulletin.ipm.illinois.edu/?p=3584.