Act now to control fall weeds

For most crops, September marks the beginning of the end of the growing season. However, it isn’t time to put our management abilities on autopilot.

Now your attention should be turned to cleaning up weed issues in hay crops and open fields. About 80% of next season’s weeds come from this fall’s seed, so getting good control now and reducing weed spread is key.

When it comes to forage stands, weed control starts with proper timing. Although the temptation is to spray immediately after a cutting, weeds need to regrow enough leaf foliage to allow for proper spray coverage. Summer annuals such as ragweed, pigweeds, lambsquarters and smartweeds need several days to recover before spaying, while it’s better to allow perennial weeds to grow about 12-15 inches before making an herbicide application.

Warm-season perennials such as horsenettle, hemp dogbane, milkweed, smooth groundcherry, pokeweed, bindweed, poison ivy and brambles should be sprayed with systemic herbicides in September before a hard frost, while Canada thistle and quackgrass can be sprayed into October. However, for most summer annual grasses, such as crabgrass, the control window has passed. Commonly used herbicides that are generally effective on the above weeds include 2,4-D and/or dicamba. Triclopyr or metsulfuron can be used as well, although there are some crop rotation restrictions with these products. A combination of products may be necessary for a mixture of weeds or difficult to control plants.

Cool season grassy weeds, such as Quackgrass, are unfortunately some of the hardest to control in grass forages. The best option is to apply glyphosate, either by spot spraying or with a wiper applicator. And if forage stands are severely declining, fall is the best time to terminate the stand for rotation to another crop next year.

Now is also the time to control summer annual weeds prior to seed set for cereal grains, silage crops and summer vegetable crops.

Many summer annual weeds begin to drop seed in late August and September, with some grasses like foxtail continuing into October. Additionally, some weeds like Palmer amaranth retain their seeds into the fall, allowing the seeds to be spread via the combine.

To prevent seed production, fields can be sprayed with an effective herbicide or mowed once or twice. Glyphosate is typically effective at stopping grass growth and plant growth regulators, such as 2,4-D and dicamba, are generally better for broadleaf weeds.

Perennial weeds should be treated similar to what I described above: Mow fields soon to prevent seed production and allow regrowth to occur, then apply an effective systemic herbicide in late September or early October to allow herbicides to transport to the roots to achieve the most effective control.

Ultimately, a lot of next year’s weed problems can depend on what you do this fall, so invest the time now to get things right for next season before bringing this one to an end.

Zach Larson is the Penn State Extension agronomy educator for Blair County. He can be reached at (814) 414-0582.


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