To Prevent Seed Set In Annual Weeds And To Control Perennial Weeds

On August 08, 2020

To Prevent Seed Set In Annual Weeds And To Control Perennial Weeds

Now is the time to think about controlling summer annual weeds prior to seed set in cropping situations where this is possible. Extension Weed Specialist Dwight Ligenfelter explains 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 are shed) 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. For example, we included yellow foxtail in the experiment. Yellow foxtail seed rain began in early August and continued into late October. Pigweed species can begin to produce mature seed by mid-August, while lambsquarters and ragweed generally do not mature until the month of September. For those fields that have 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.

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 produced 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 (i.e., glyphosate, 2,4-D, dicamba) in late September or early October so the herbicide will be transported to the roots for more effective control.

One more thing to think about is the establishment of a cover crop. If herbicides such as glyphosate, paraquat, 2,4-D, or dicamba are being used prior to cover crop establishment remember to consider the replanting interval. For glyphosate and paraquat, most cover crop can be planted after application. However, depending on the rate of 2,4-D applied, at least a week to four-weeks waiting period is necessary before establishing cover crops. If small grain covers are being established, some research suggests that a minimum delay of 7-10 days after application at rates of 1 pint/acre of 2,4-D ester.  Also, other studies have shown that certain clovers (e.g., red and crimson) and alfalfa may be established within the same constraints with 2,4-D ester. To be certain, a longer waiting period and rainfall help to reduce the potential for injury. Dicamba usually requires at least a couple weeks or more before certain cover crops can be planted and this also is directly related to the amount applied. Some have asked about the use of products such as Sharpen, Liberty, and Elevore as a burndown prior to cover crop planting. Currently, we do not have adequate data on this use for these products. However, from reading the labels, some suggestions can be made. Sharpen (1 fl oz) has no restrictions for small grains but has a 1 month wait period for other cover crops (species not specified); Liberty has a 70 day wait period for small grains, but canola can be planted anytime, and for Elevore, small grains, ryegrass, and canola can be planted after 14 days but there is a 9 month wait for clover and mustard species. Keep in mind, once some of these products (e.g., Sharpen) are applied, they cannot be harvested or grazed for livestock feed. Refer to appropriate herbicide labels for more details.

To Take Advantage Of Online Training For Various Credits


Over the next few weeks a number of on-line courses from Penn State Extension will be offered which provide a number of different categories of credits. Here is a list of some of the opportunities.


Certified Crop Advisor, Nutrient Management and Manure Hauler Offerings:

Agronomy Field Days – Live Webinar Events:

Manure Sampling and Nutrient Analysis online Friday, August 7 from 8:00 – 9:30am

  1. CCA NM Credit and 1.0 Act 38 NM Credit

Insect issues and planting green online Friday, August 14 from 8:00 – 9:30am

.05 CCA IPM Credit, .05 CCA CM Credit and 1 PC, 01, 18 Category Credit


Manure incorporation and soil health online Friday, August 21 from 8:00 – 9:30am

.05 CCA NM Credit, .05 CCA SW Credit, .05 Act 38 NM Credit and .05 Act 38 Manure Hauler Credit

Quote Of The Week: “I have found out in later years that we were very poor, but the glory of America is that we didn’t know it then.” Dwight D. Eisenhower