As the growing season progresses, there are certain weeds that will mature 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 this: By June, effective management of these weeds is no longer possible. Just because they are most noticeable this time of year does not mean it is the best time to manage them.

Here are some recommendations for proper herbicide application timing, so you can attack them in a timelier manner:

Buttercup. 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 early spring — mid-March to April. Herbicides such as  2,4-D (2-3 pints); 2,4-D (1 quart), plus dicamba (1 pint); metsulfuron/MSM (0.2-0.3 oz); or Crossbow (2-3 quarts) are commonly used to control buttercup.

POISON HEMLOCK: Poison hemlock is a biennial, so it forms a rosette in the fall. Herbicide applications are most effective when they are sprayed in fall when poison hemlock is in the rosette stage or before it bolts in the spring.

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

Roughstalk bluegrass. 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 is mostly spread by seed. As it matures, it typically turns reddish-tan and sets seed before summer. It then goes dormant until fall and remains green during winter. Roughstalk bluegrass can get up to 3 feet tall or more, and it is evident in crop fields. Once it starts to set seed, no management tactics, including herbicides, will provide adequate control.

ROUGHSTALK BLUEGRASS: This weed is becoming more of a problem in small-grain and forage fields across Pennsylvania and the region. It is a cool-season perennial with short stolons, but it mostly spreads by seed.

Studies from Michigan State University show that Osprey, Axial XL/Bold and PowerFlex HL can provide control. However, Osprey applied when the weed is 1 to 2 inches tall in fall or early spring provided the most consistent control (include nonionic surfactant (NIS), plus ammonium sulfate (AMS) in the spray mixture).

Fall treatments provide effective initial control — greater than 90% — from these products, but spring emergence leads to escapes by harvest. Late-spring applications are usually not as effective since the weed is too tall.

MANAGING PERENNIALS: By late May and June, the most optimal time to control perennial weeds such as Canada thistle, milkweed, hemp dogbane and others in a pasture will have passed. During late spring and summer, it is suggested to mow pastures a couple times to “set up” the weeds for a more effective herbicide application in fall.

Perennial weeds in pastures

By late May and June, the most optimal time to control perennial weeds such as Canada thistle, milkweed, hemp dogbane and others in a pasture will have passed.

During late spring and summer, we typically suggest mowing pastures a couple times or so to “set up” the weeds for a more effective herbicide application in fall. Time your late-season mowing to occur early enough to allow perennial weeds to grow about 12 to 15 inches or so (usually about a month after cutting) before an 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) from Sept. 1 through Oct. 15 or before a hard frost.

In northern areas of Pennsylvania, consider making the application before Oct. 1. An additional two- to four-week or more application window can exist for Canada thistle and quackgrass because of their cool-season habit of growth.

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