The waning days of summer are a good time to scout hayfields and pastures for biennial and perennial weeds, says William Curran, Penn State University weed scientist.

Problem biennials, such as burdock, poison hemlock, musk, bull and plumeless thistle, emerge from seed in mid-to late summer, overwinter and bolt and flower in the second year of their life cycle. Depending on time of year, mature and seedling plants might be present.

“Biennials are easiest to control while still small in the seedling or rosette stage of growth, so fall herbicide application can be quite effective,” Curran notes.

Shortening day lengths and cooler nights are also a signal to perennial weeds that winter is approaching. Now is when perennials more actively transport carbohydrates and sugars to underground storage structures such as rhizomes, tubers and roots. That enables them to survive winter and store necessary energy for beginning the next spring’s growth cycle.

“This is also the ideal time to attempt control with a systemic herbicide that travels with the carbohydrates and sugars to these underground structures where they can exert their mechanism of action on the foundation of these perennial plants,” he says.

To control warmer-season perennials like johnsongrass, horsenettle, groundcherry, wirestem muhly, pokeweed, Japanese knotweed and poison ivy, apply herbicide between Sept. 1 and Sept. 15. For weeds like hemp dogbane and bindweed, Curran advises making applications before Oct. 1. For quackgrass, other cool-season grasses and Canada thistle, try to make applications by Oct. 15.

“These suggested dates target central Pennsylvania, so adjust by a week or so forward or backward if you are south or north,” he says.

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