Late-season Control of Horseweed: Preventing Seed Dispersal

Posted: July 27, 2016

Horseweed in Pennsylvania is beginning to flower and will start dropping seed in the coming weeks – learn why it’s important to remove any remaining mature plants now.

While the best time to manage horseweed is prior to planting, some stubborn plants can remain in and around fields through the late-season. Potential reasons for this include 1) glyphosate- and ALS-resistance, 2) small stands persisting on field edges, roads, and buildings, and 3) horseweed that are cut off during small grain harvest will recover, branch out, and likely be even more difficult to control. Large plants (over 6 inches) have shown to be challenging to control chemically, and plants over 12 inches may require mechanical means of removal. However, control is still important, as a few remaining plants can have notable impacts on yield especially under dry weather. Furthermore, due to the tendency of horseweed seeds to spread widely in the wind, eliminating remaining plants before they shed seed will help prevent larger infestations come fall.

In PA, horseweed that remain in late July may be starting to flower. Seed dispersal typically begins in August and continues as plants mature. In central PA, we have observed plants in growth stages varying from 4-foot tall vegetative to flowering (see photos below taken 7/26/2016). Small white and yellow flowers are arranged in a large panicle. Seeds are 1/16-1/4 inch long, with white bristles that allow them to be carried by the wind. A tall horseweed plant produces up to 200,000 seeds that spread rapidly among neighboring fields and farms; a previous study at the University of Illinois reported that at 20 feet from the source plant, a single plant can deposit 12,500 seeds per square yard, and 125 seeds per square yard at 400 feet away. Germination rates reach up to 80%, with higher germination in no-till fields.


With herbicide control being largely ineffective on very large, late-season plants, growers aiming to eliminate individual mature horseweed should manually pull them using a hoe, weed hook, or by hand. Pulled plants should be moved out of the field to prevent regrowth or seed drop. While late-season control measures are labor-intensive, eliminating remaining horseweed is an important measure for preventing seed dispersal and new infestations especially in no-till fields. Just a few plants can produce enough to infest an entire field in a couple of seasons. Manual removal this year could save significant money, time, and labor in future years.


Contact Information

Annie Klodd
  • Extension Associate
Phone: 814-863-7643