Weed Seeds This Fall Mean More Weeds Next Spring

Now is the time to think about controlling summer annual weeds prior to seed set in cropping situations where possible. Preventing seed production is important for driving down the weed seed bank and reducing the need for weed control inputs (i.e., herbicides). It is rather easy to prevent weed seed production following a cereal grain such as wheat, barley, or oats, as well as some vegetable crops such as sweet corn or snap beans. Proper timing of the control practice is essential in preventing seed production. In general, below is a summary of estimated seed drop for various weed species:

• Giant foxtail: late August and peak seed rain usually occurs from late September through the month of October
• Yellow foxtail: early August and continues into late October
• Pigweed species: begin to produce mature seed by mid-August
• Lambsquarters and ragweed: generally, do not mature until the month of September
• Palmer amaranth or waterhemp: Make sure to monitor them routinely over the next couple of months and control any regrowth or new seedlings before they set seed. Palmer amaranth plants notoriously retain their seeds late into the summer and fall, and thus seeds don’t necessarily fall to the ground upon maturity but are usually spread via the combine.

To prevent seed production, fields can be sprayed with an effective herbicide or mowed once or twice. Glyphosate is particularly effective at stopping grass growth and reproduction. The plant growth regulators (2,4-D and dicamba) would probably be a better choice for broadleaf weeds. With giant foxtail, even treating the field by mid-September can greatly reduce seed production. If seed heads are present, check suspect fields to determine how advanced flowering and seed rain are and time control practices accordingly. In alfalfa or pastures, if weeds are taller than the forage, consider running a brush-hog at a high setting to clip off the immature weed seed heads above the forage canopy.

Taking the time to prevent seed production this year can make a big difference next year. About 80% of weeds next season come from weed seed this fall. For more information on weed emergence, weed seed set, and seedbank dynamics, refer to "A Practical Guide for Integrated Weed Management in Mid-Atlantic Grain Crops." (PDF)

In addition, many perennial broadleaves are evident in these same small-grain stubble fields. The challenge with perennial weeds at this time of year is the fact they are still in the vegetative and reproductive phases. Therefore, most of the plant sugars are not being significantly transported to the roots, and a herbicide application now will mostly only impact the top-growth. One consideration would be to mow those fields soon to prevent seed production and allow regrowth to occur. Then apply an effective systemic herbicide (ie, glyphosate, 2,4-D, dicamba) in late September or early October so the herbicide will be transported to the roots for more effective control.