Perennial Weed Control in Grass Pastures and Hayfields

By Blake Jackson

A common question among pasture and hayfield managers is the best timing and herbicide choice for controlling perennial weeds like Canada thistle, milkweed, hemp dogbane, horsenettle, and quackgrass. The most effective control typically occurs in late summer to fall using systemic herbicides, but there’s more to consider.

Weed Biology and Timing

In fall, perennial weeds actively move sugars and carbohydrates to their roots and underground structures to survive winter and fuel spring growth. This natural process helps systemic herbicides-such as 2,4-D, dicamba, triclopyr, and glyphosate-move into root systems for better control.

However, many perennials flower and produce seeds by mid-summer, so delaying treatment until fall can allow seed spread.

To manage this, mowing pastures 1-2 times during the growing season helps reduce seed production and encourages new weed growth, which is more vulnerable to fall herbicide applications. After mowing, wait about 4-6 weeks for weeds to regrow 12-15 inches before spraying.

The ideal herbicide window is generally early September through October, adjusted regionally. Warm-season perennials like horsenettle and milkweed should be sprayed by mid-October, before frost. Cool season weeds like Canada thistle and quackgrass can be treated even after light frosts.

Herbicide Choices

Broadleaf weeds respond well to 2,4-D and dicamba, while triclopyr and metsulfuron are options for tougher weeds. Mixing herbicides can improve control across different species. For instance:

• Canada thistle: Stinger or 2,4-D + dicamba (higher rates)
• Hemp dogbane: Starane Ultra, Yukon, or Crossbow (high rates)
• Horsenettle: Crossbow (bud to bloom or late summer)
• Milkweed: 2,4-D + dicamba or Crossbow (suppression likely, not full control)

Additional Tips

Avoid mixing herbicides with liquid nitrogen fertilizer during hot, dry conditions to prevent crop damage. Wiper applicators can be used to target taller weeds but are generally less effective than spraying.

Always follow product-specific grazing and haying restrictions to protect livestock health.

For detailed recommendations, consult product labels and the 2025 Penn State Agronomy Guide.

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