It’s Too Late for Weed Control

As the growing season progresses, there are certain weeds that have matured beyond their prime for effective management. We typically get calls this time of year inquiring about recommendations for control of weeds such as poison hemlock, buttercup, roughstalk bluegrass, perennial pasture weeds, and others. The simple answer is – no, now is not the time for effective management of these weeds. Just because they are most noticeable this time of year, does not mean it is the best time to manage them. Below are some considerations about proper herbicide application timing for these weeds.

 
Buttercup infested pasture. (Penn State Weed Science, D. Lingenfelter)

Buttercup management: Do not wait until the pasture is “yellow” with buttercup flowers to spray. In general, the best time to apply herbicides to control buttercup is in the early spring (Mid-March thru April). Herbicides such as 2,4-D (2-3 pints/acre), 2,4-D (1 quart) + dicamba (1 pint), metsulfuron/MSM (0.2-0.3 oz), or Crossbow (2-3 quarts) are commonly used to control buttercup.

 
Poison hemlock at flowering stage. (Source: D. Lingenfelter, Penn State Weed Science)

Poison hemlock control: Poison hemlock is a biennial, so it forms a rosette in the fall. Herbicide applications are most effective when they are sprayed in the fall when poison hemlock is in the rosette stage or before it bolts in the spring. Effective herbicides include 2,4-D + dicamba, Crossbow (2,4-D+triclopyr), or glyphosate as a spot treatment. When the weed is flowering and 3-7 feet tall, herbicide applications are not effective.

 
Mature roughstalk bluegrass seedheads in wheat (Penn State Extension, J. Graybill)

Roughstalk bluegrass management: This weed is becoming more of a problem in small grain and forage fields across the state. It is a cool-season perennial with short stolons, but it mostly spreads by seed. As it matures, it typically turns reddish-tan and sets seed prior to summer. It then goes dormant until the fall and remains green during the winter months. Roughstalk bluegrass can get up to 3 feet tall or more and is very evident in crop fields. Once it starts to set seed, no management tactics, especially herbicides will provide adequate control. Fall and early spring are the best times to apply effective herbicides. In studies at Michigan State University, Osprey, Axial XL/Bold, and PowerFlex HL provided control of roughstak bluegrass. However, Osprey applied to roughstalk (1-2 inches tall) in the fall or early spring provided the most consistent control.

 
Problem weeds in a pasture setting. (Source: Penn State Extension)

Perennial weeds in pastures: Is now the best time to spray?: In general, the answer is no. Now is not the most optimal time to control perennial weeds such as Canada thistle, milkweed, hemp dogbane, and others in a pasture. During the late spring and summer, we typically suggest mowing pastures a couple times or so to “setup” the weeds for a more effective herbicide application in the fall. Time the late season mowing to occur early enough to allow perennial weeds to grow about 12-15 inches or so (usually about a month after cutting) before a herbicide application in the fall. Warm-season perennials such as horsenettle, hemp dogbane, common milkweed, smooth groundcherry, pokeweed, bindweed, poison ivy, and brambles should be sprayed with a systemic herbicide (such as 2,4-D, dicamba, triclopyr, etc.) from September 1 through October 15 or before a hard frost. In general, applications by October 1 may be more effective. In northern areas of Pennsylvania, consider making the application before October 1. An additional 2 to 4-week or more application window can exist for Canada thistle and quackgrass, because of their cool-season habit of growth.