Purple flowers in the Fields
The two winter annual weed species producing the "purple patches" are henbit and purple deadnettle. Although close relatives taxonomically, henbit and purple deadnettle are distinctively different.
"Flowering indicates henbit is close to completing its life cycle and will likely be more difficult to control with burndown herbicides, but this does not imply that no attempt should be made to control existing plants prior to corn or soybean planting," "These weeds are known to be hosts for a number of insect and disease pests, and mature seeds can survive in the soil seedbank for several years. Planting into dense patches of these species can be challenging and could result in poor seed placement."
Preplant tillage or herbicides can provide good to excellent control of existing henbit and purple deadnettle. In general, 2,4-D and dicamba are weak on henbit. Glyphosate can provide good control, but application rates should be at close to 1.1 lb ae/acre for these mature plants. Combining glyphosate and 2,4-D or adding these two herbicides to other residual herbicides is a popular broad-spectrum burndown.