Time to Spot Treat Perennial Weeds

June is the ideal time to scout fields for perennial weeds, and spot treat them to prevent major problems.

Timing: Many broadleaf perennials are approaching the sensitive stage for systemic herbicides (eg. glyphosate). For best results, apply at these stages:

• Canada Thistle at early flower bud (Fig 1)
• Milkweed at flower bud (Fig 2)
• Field Bindweed in full flower (Fig 3)

Quackgrass needs to be actively growing with at least 3 to 4 new leaves to absorb the herbicide. If it is larger than this, but still growing, it's still susceptible.

Rates: Note that there are 4 concentrations of glyphosate on the market this year, which affects the product rate for each type of weed. So Read the Label! For example, there are 3 rate ranges for glyphosate products with an active ingredient concentration of 360 g/L:

• Annual weeds: 0.75 to 3.5 L/ha
  
• Canada thistle and quackgrass: 2.5 to 7.0 L/ha
  
• Perennial broadleaf weeds: 7 to 12 L/ha
  
• Use the high end of the rate range where weeds are larger and growth is dense.

Caution: Avoid contacting desired plants, including low branches and root suckers. Glyphosate will circulate in trees for several years if absorbed. Research suggests that low levels of glyphosate in a plant makes it susceptible to disease. Wick wipers will minimize contact with the crop, and shields can protect low hanging branches.

Prevention: Essential to success in controlling perennial weeds, especially in perennial crops:

• Scout for weeds along the edges of fields, in fencerows and ditch banks.
  
• Prevent seeds by mowing or using a systemic herbicide before flowering.
  
• Clean up fields for new plantings by spot treating perennial weed patches.

Control of perennial weeds is generally a 2-3 year proposition. Investing some time each year will keep these pests to a manageable level in your fields.

Figure 1. Canada Thistle at early flower bud

Figure 2. Milkweed at flower bud

Figure 3. Field Bindweed in full flower

Figure 4: Quackgrass stopped growing due to dry conditions.

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