What is the best time to apply a contact postemergence herbicide to an annual weed?

What is the best time to apply a contact postemergence herbicide to an annual weed?

• A. Treat annual broadleaf weeds when they are large.
• B. Treat annual broadleaf weeds when they are small, generally less than 4 inches tall; treat annual grasses before tillering.
• C. Treat annual grasses after tillering.
• D. Treat weeds during dormancy.

Answer: (B)

The key idea is that contact postemergence herbicides need to touch and be absorbed by active leaf tissue to kill the plant. Because of this, applying them when weeds are still young and have exposed, tender leaves yields the best results. For annual broadleaf weeds, using the spray when they are small—generally under about 4 inches tall—provides enough leaf area for thorough coverage and rapid absorption, leading to effective kill. If these weeds get larger, their tougher tissues and greater surface area can reduce contact efficiency and overall control. For annual grasses, applying before tillering means the grass is still simple in structure with less biomass and fewer leaves for the herbicide to reach. Early growth stages allow the product to contact more leaf surface and, if possible, affect the growing points. After tillering, more complex growth and thicker tissue can make control more difficult with a contact product. So, treating small broadleaf weeds and applying to grasses before tillering aligns with how these products work and maximizes the chance of a complete kill.

The key idea is that contact postemergence herbicides need to touch and be absorbed by active leaf tissue to kill the plant. Because of this, applying them when weeds are still young and have exposed, tender leaves yields the best results.

For annual broadleaf weeds, using the spray when they are small—generally under about 4 inches tall—provides enough leaf area for thorough coverage and rapid absorption, leading to effective kill. If these weeds get larger, their tougher tissues and greater surface area can reduce contact efficiency and overall control.

For annual grasses, applying before tillering means the grass is still simple in structure with less biomass and fewer leaves for the herbicide to reach. Early growth stages allow the product to contact more leaf surface and, if possible, affect the growing points. After tillering, more complex growth and thicker tissue can make control more difficult with a contact product.

So, treating small broadleaf weeds and applying to grasses before tillering aligns with how these products work and maximizes the chance of a complete kill.