Managing Summer Annual Weeds Before Seed Set

By Blake Jackson

Now is the critical time to focus on controlling summer annual weeds before they go to seed. Limiting seed production is one of the most effective strategies for reducing the weed seed bank, which in turn decreases future weed pressure and the need for heavy herbicide inputs.

Fields following cereal grains like wheat, barley, or oats as well as certain vegetable crops such as sweet corn or snap beans provide good opportunities for preventing weed seed development. However, timing is essential to achieve success.

Weed Seed Production Timelines

• Giant foxtail: Begins shedding seed in late August, with peak dispersal from late September through October.
• Yellow foxtail: Seed drop can start as early as August and extend into late October.
• Pigweed species: Mature seed production usually starts by mid-August.
• Lambsquarters and ragweed: Seed maturity typically occurs in September.
• Palmer amaranth and waterhemp: These aggressive weeds must be monitored closely, as they often retain seed late into summer and fall. Unlike others, seeds are frequently spread by combines during harvest rather than dropping naturally.

To stop seed set, fields can be treated with herbicides or mowed once or twice. Glyphosate is highly effective against grasses, while plant growth regulators like 2,4-D or dicamba are better suited for broadleaf species.

For giant foxtail, even treatments applied by mid-September can significantly reduce seed output. If seed heads are visible, it’s important to check maturity levels and adjust control measures accordingly.

In alfalfa or pastures, where weeds often grow above the forage, clipping off immature seed heads with a brush-hog at a high setting is a useful tactic.

Stopping seed production now pays off next season roughly 80% of next year’s weeds come from seed dropped this fall. For more detailed guidance, growers can consult A Practical Guide for Integrated Weed Management in Mid-Atlantic Grain Crops (PDF).

Perennial broadleaf weeds are also active in small-grain stubble fields this time of year. Since plants are still in vegetative growth, herbicides mainly damage top growth.

A better approach is to mow soon, then allow regrowth before applying systemic herbicides like glyphosate, 2,4-D, or dicamba in late September or early October for more effective root control.

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