By Dwight Lingenfelter
As we proceed through the growing season, we are receiving calls about controlling pokeweed in field crops. Certain residual herbicides (atrazine, metribuzin, Authority, Canopy, Python) applied near crop planting will provide about 85-90% control of pokeweed seedlings but will not control established pokeweed plants coming from the crown of the taproot. Pokeweed seedlings can emerge throughout the growing season so residual herbicides can help to reduce the populations. From our studies at Penn State, glyphosate and dicamba-containing treatments provided the best control of pokeweed. Applying glyphosate mid to late summer (mid-June to mid-July) is more effective than in the spring due to greater translocation during flowering.
In corn, pokeweed can be controlled with several POST herbicides, including glyphosate, 2,4-D, dicamba, Status, and Callisto + atrazine. Tank-mixing provides the best control. These herbicides can provide at least 80% control by the end of the season.
In soybean, similar POST control can be achieved; however, there are fewer effective options than in corn. Glyphosate is most effective (90% control) and should be used as a foundation of spray programs when controlling pokeweed in soybeans. The ALS-inhibitor herbicides (Classic, Synchrony, FirstRate, Harmony, Raptor, etc.) provide 60% control or less when sprayed alone and should be used in combination with glyphosate if possible. The contact herbicides (Reflex, Cobra, Cadet, Liberty, etc.) only provide initial “burn" but then the weed recovers. Xtend and Enlist E3 soybean varieties and their associated herbicide options (dicamba and 2,4-D choline, respectively) can also be used to control pokeweed.
Also, when spraying POST applications, be sure to use higher spray volumes (15-20 GPA) to get adequate coverage of the overall weed and try to position the spray boom high enough over the weed so it's not dragging across the foliage which negatively impacts spray coverage. In non-GM soybeans, controlling pokeweed is more difficult due to fewer effective herbicide options. However, if possible, the use of a wiper or sponge applicator system that contains a 50:50 solution of glyphosate and water can be wiped across the taller pokeweed plants while avoiding the lower soybean stand can provide some pokeweed suppression. This type of application might need to be done a few times during the season to allow a height differential between the pokeweed plants and the soybeans.
Source : psu.edu