Yes, it’s busy right now, but start thinking about controlling summer weeds now before seeds start to set.

Preventing seed production is important to drive down the weed seed bank and to reduce the need for weed control inputs.

It is easy to prevent weed seed production following a cereal grain such as wheat, barley or oats, as well as some vegetable crops like sweet corn or snap beans. Proper timing of a control practice is essential in preventing seed production.

Here 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 October.

Yellow foxtail. Early August, and it continues into late October.

Pigweed. It begins to produce mature seed by mid-August.

Lambsquarters and ragweed. Generally, these do not mature until the month of September.

Palmer amaranth or waterhemp. Monitor these regularly over the next couple of months, and control any regrowth or new seedlings before they set seed. Palmer amaranth plants will retain their seeds late into summer and fall, and thus seeds don’t necessarily fall to the ground upon maturity, but they are usually spread via the combine.

How 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.

When it comes to giant foxtail, even treating the field by mid-September can greatly reduce seed production. If seed heads are present, check fields to determine how advanced flowering and seed rain are, and time your 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.

Of particular importance is Japanese stiltgrass. It is an aggressive annual species that will die off in fall. However, it is best to prevent as much seed production before it dies.

Unlike crabgrass and other grassy weeds, stiltgrass flowers and sets seeds much later, from mid-September through October. When you start seeing seed heads — but before they drop their seeds — mow as low as possible to destroy them.

Spot-spraying with glyphosate can help if the seeds have not progressed too far into the maturation phase. Currently, there are no labeled post herbicides that can effectively and selectively control stiltgrass without injuring desirable forage grasses. Prowl H2O or Satellite HydroCap can be applied in early spring to control germinating seedlings of stiltgrass and other weedy grasses such as crabgrass and foxtails.

In addition, many perennial broadleaves may be evident in small-grain stubble fields. The challenge with perennial weeds 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 an herbicide application now will mostly only affect the top growth.

One consideration would be to mow those fields to prevent seed production and allow regrowth to occur. Then, apply an effective systemic herbicide (glyphosate, 2,4-D, dicamba) in late September or early October, so the herbicide will be transported to the roots for more effective control.

Get ahead of future problems

About 80% of weeds next season will 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.” It can be found at growiwm.org.

Lingenfelter is an Extension associate of weed science with Penn State Cooperative Extension.