By Mark Loux, Ohio State University Extension

Dandelion seems to be on the increase in some fields, as noted in a video last summer and CORN article last fall. Fall is the optimum time of year to reduce dandelion populations with herbicides, so we expect them to become more of a problem in fields that are not treated in the fall at least occasionally.  If history is any indicator, other causes can include oversimplification of herbicide programs in soybeans, omission of residual herbicides, and delaying burndown herbicides until later in spring. All of these occurred during the first few years of RoundupReady soybeans, and we had some dense stands of dandelions that developed in late 1990s. We again have some very effective weed management platforms for soybeans, and the possibility of the same happening. In addition, while POST applications of glufosinate have broad-spectrum activity on annual weeds, they are not that effective on dandelion and other perennials, which can allow some of these weeds to get more of a foothold. 

In the absence of fall herbicide treatments especially, control of established dandelions in corn and soybeans will require effective burndown and POST herbicides. Do not expect adequate control of established dandelions from just the burndown or just the POST. Where the burndown herbicides are relatively ineffective for whatever reason, the POST herbicides will also likely be less effective. It’s also necessary to include residual herbicides to help control the dandelion seedlings that can emerge after planting from seeds produced last fall and this spring. Some things to consider:

- Our experience with dandelion over the years has shown that the effectiveness of spring burndown herbicides on dandelion can be extremely variable.  We conducted research with Purdue and Penn State back in about 2000, where we applied glyphosate and glyphosate + 2,4-D weekly from early April to early May. Control generally improved with the later applications. The glyphosate + 2,4-D was more effective, but under some colder than normal conditions in late April, control decreased considerably. We suggest avoiding applications during this type of weather. 

- Most effective burndown will generally result from combinations of glyphosate with 2,4-D or dicamba. Increasing rates of these products can improve control, but will not overcome the negative effects of cold weather. The addition of a chlorimuron-containing product can help in soybeans. In corn, products or mixtures that contain higher rates of atrazine and mesotrione have also been effective for suppression well into the growing season (adding 2,4-D can help).

- The addition of herbicides with contact activity to the burndown can make it appear as though control has been improved, but can actually reduce the activity of the systemic herbicides, and plants regrow sooner in some cases. This includes sulfentrazone, flumioxazin, saflufenacil, and metribuzin, and glufosinate. 

- POST followup in soybeans should include glyphosate, and as with burndown, mixing with 2,4-D (Enlist) or dicamba (Xtend/XtendiMax) will be most effective. We would suggest including glyphosate with glufosinate in LLGT27 soybeans (resistant to both of these herbicides). Some of these same options apply to corn, along with mixtures that contain mesotrione. Adding a high rate of Classic in soybeans may also help any of these.

- After doing battle with dandelion this coming season, make a plan to apply herbicides this fall, when a little money goes a long way.