I struggled to control glyphosate-resistant marestail (horseweed) in my no-till pumpkin field last summer. I was reminded of the problem when I saw a recent article in the Michigan Vegetable Newsletter that reported on testing for herbicide-resistant weeds in the state last fall.
Of 14 samples received, 13 were confirmed resistant to various herbicides and eight were cases that had not been previously confirmed in the counties in which they were sampled.
Most of the samples tested were from fields with soybean rotations (pigweeds, ragweeds, marestail/horseweed and common lambsquarters). The most common resistances found were to glyphosate, but there were some new reports of resistance to other commonly used herbicides.
While the article discussed herbicide-resistant weeds in Michigan, I know that we are experiencing the same problems here in Pennsylvania.
I realize that not all vegetable growers have soybean rotations (or field corn for that matter) and you may be wondering how this applies to you.
Glyphosate is a commonly used herbicide for burndown of fields prior to planting vegetables, particularly in no-till. And an herbicide-resistant weed does not care whether it is growing in a soybean field or a pumpkin field, nor will the seeds stop from spreading into any available field, vegetable or field crop.
I expect these resistant weeds to become more of an issue as time goes on.
Last year, the glyphosate-resistant marestail in my no-till pumpkin fields was only stunted briefly by glyphosate applied when the rye cover was burned down prior to planting, and was troublesome in the planting the rest of the season.
Hand weeding managed to keep things mostly under control, but in a large field this would probably not be a realistic control method (although it often is the only available method).
Remember, a weedy field will create problems through competition with the crop plants as well as reduced coverage of insecticides and fungicides.
What was my plan to minimize the problem this year?
After consulting with Dwight Lingenfelter, weed specialist at Penn State, the research farm tech made an application of glyphosate and Liberty prior to planting my rye field last fall and an application of 2,4-D to the established rye early this spring.
Note: Be sure any herbicide applications you make are within the preplant label requirements for your crop.
My marestail population was excessive last season because the field I was in had not had any agronomic crops on it for several years and the marestail population and seed bank were able to build up.
Depending upon weed management practices and plant populations on your farm, you may be experiencing management issues as well.
I cannot stress enough the importance of scouting your fields in the fall to know what weeds were present that could be a problem in a field the next season.
Examine control options for weeds seen last year in the various crops you plan to grow this year and match the fields with crops that will give you the best control options. Hand weeding is probably not the best control option to plan on.
Another potentially troublesome weed pair is glyphosate-resistant Palmer amaranth and waterhemp. Dwight told me that growers have been made aware of these weeds and many are taking steps to manage them in their fields.
However, there are still instances in which these species are a problem in a field and seeds from a neighbor’s crop could spread into your planned pumpkin field.
Management of these two weeds in no-till pumpkin starts with Curbit at planting (Command is not effective) followed up three to four weeks later by an application of Dual in the row middles.
While Sandea will control most pigweeds, there are Palmer populations resistant to this herbicide, so evaluate the effectiveness of any herbicides applied to know if resistance is becoming an issue on your farm.
A Note About Drift
My final note regarding herbicides relates to drift. Every season I receive calls with questions regarding herbicide damage to a crop about this time of year.
Most often the crop is tomatoes, either in a tunnel or greenhouse, and the grower has a neighbor that did a preplant burndown treatment on their corn field.
Sadly, there is not much that can be done to remedy herbicide damage to tomatoes. My best answer is to tell the grower that the plants will either grow out of the damage or not.
I do not like being so vague, but unfortunately there really is no clear way to tell what will happen.
Sometimes the damage is severe enough to require replacing the plants.
I have heard of products that can be applied to snap plants out of herbicide damage.
I have no direct experience with any product for this purpose and, again, it will depend upon the herbicide that the tomatoes were exposed to as well as the dose to predict if recovery is possible.
The best option I can give is to work with your neighbors and discuss with them that you are growing tomatoes, a crop very sensitive to herbicides, so hopefully they will follow all precautions to prevent drift onto your crop.
