Why has American Burnweed escaped from my preemergent herbicide application?

Jeff Derr, Professor of Weed Science and Director of Hampton Roads AREC, and Mike Goatley, Professor and Turfgrass Extension Specialist

The following was an inquiry from a lawn care operator in Williamsburg, VA.

QUESTION (LCO): Here in late spring in eastern Virginia we have clients complaining that our pre-emergent herbicide treatment failed this year on the basis of the presence of American burnweed that is being found on sports fields, golf courses, and home lawns throughout the area. However, there is no evidence of any crabgrass on any of the sites I have inspected, so I believe our spring pre-emergent herbicide worked.

Another issue I believe that helped the burnweed be so problematic this year has been persistently-wet soils in this area that restricted regular mowing; many of these clients could not mow for 7-10 days at a time because of how wet the soils have been. From my experience the more one can mow, the better the ‘control” of the burnweed.

I am looking to justify that what we sprayed did work this year, but it appears that with burnweed PRE herbicides are often not successful. I have done some research online and the summary I get is that the burnweed will germinate in the thatch layer above the pre-emergence chemistry, and therefore, its germinating seed missed the pre-emergent herbicide layer in the soil. Is this a reasonable explanation?

American burnweed flowering

RESPONSE (Dr. Goatley): This scenario is a very common occurrence with American burnweed (Erechtites hieracifolia), and this weed seems to increase in population each year in the Mid-Atlantic. It is in the Asteraceae family (Daisy family) and produces ‘parachute seed’ that are easily wind dispersed - causing the seed to remain on top of the soil/turf canopy surface, and negating most PRE herbicide control. This weed is also often called ‘fireweed’ and is one of the first weeds to appear in disturbed soils, either from burning or tillage. It is very easy to hand-pull from the soil, but its numbers are often too great to remove by hand.

American burnweed

RESPONSE (Dr. Derr): American burnweed is a summer annual is an upright growing plant that can get pretty tall (up to 3-ft or higher at maturity). It appears usually in late April to mid-May so it often has not germinated when glyphosate or other POST herbicides are applied over dormant warm-season turf during the winter. Preemergence crabgrass herbicides like prodiamine, pendimethalin, ... etc., provide poor control of this weed; however, isoxaben provides fair control. Probably best to go after this weed, when it is small and actively growing, with a 3-way postemergence broadleaf herbicide.