Don’t Allow Weeds to Set Seed

By Dwight Lingenfelter

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). In particular, it is 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.

Several years ago, we conducted an experiment at Rock Springs looking at giant foxtail seed “rain" (when seed matures and drops to the ground) as influenced by date of emergence. Foxtail emerged from the middle of May to the middle of June in the experiment. In the end, the date of emergence did not matter. The later emerging grasses flowered at the same time as the earlier plants. With foxtail, this phenomenon is controlled more by day length (short day plants) than temperature. In both years, mature foxtail seeds were not produced until late August and peak seed rain occurred from late September through the month of October. Other species can be different.

Below is a summary of estimated seed drop for various weed species:

• Giant foxtail: late August and peak seed rain occurred from late September through the month of October
• Yellow foxtail: began in early August and continued 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 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 would have greatly reduced seed production in our experiment. If seed heads are present, check suspect fields to determine how advanced flowering and seed rain are and time control practices accordingly. 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.

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.