There have been a number of calls from folks dismayed by the poor burndown of henbit. Indeed I have seen it in research as well. In general, burndowns of dicamba + glyphosate or Verdict + glyphosate have not been very effective on henbit. The good news is they have been effective on horseweed. Only folks who added Valor in with their burndown or used paraquat (Gramoxone Inteon) have had anything close to good henbit control this year.
Why is henbit so bad? I think a lot of it has to do with the early harvest last fall. Henbit was able to get established early with a heavy stand in many fields. This coupled with a wet late-February and early-March delayed burndown until henbit was flowering. Henbit when it is flowered is very hard to control as the plant has switched to the reproductive phase. In that phase it is moving photosynthate as well as any systemic herbicide to the flowers and not to vegetative growing points.
What can we do to control it? Move to herbicides that are more contact. Paraquat (Gramoxone Inteon) tankmixed with a photosystem II (PSII) inhibitor like Cotoran or Caparol in cotton or even metribuzin in soybeans should do a good job finishing the burndown of henbit. In corn one could go with paraquat tankmixed with atrazine. Atrazine at 32 oz/A tankmixed with 32 oz/A of COC should also do a good job before corn planting.
Fortunately, henbit is not that competitive and has at this stage stopped growing and will be producing seed and senescing by mid-April. From a competitive standpoint I am not that concerned with henbit typically, but Scott will tell you that the greenbridge to the crop increases the possibility of insect or mite issues later.
I have noticed as well, even in my lawn and garden. I tilled my garden last fall so I don’t know about early start there of henbit. I only sprayed it with Roundup, which alone is not that effective on henbit. I sprayed it 2 weeks ago and I don’t think it was blooming. But it not do much to it. But on my lawn I sprayed with Trimec (combo 2,4D and Dicamba) and it is very slow there or no activity at all. So, I’m not sure what is going on. But I suspect that above normal temps early fall may have contibuted.