Patience with Perennial Weeds

Many perennial weeds like quackgrass, Canada thistles, and milkweed are now emerging, and it's very tempting to want to knock them back right now. However, hasty action will only give temporary satisfaction now, as their extensive root systems will help them recover and regrow very quickly.

The best approach is to let them grow to their most sensitive stage before applying a systemic herbicide like glyphosate.

Quackgrass will likely reach its sensitive stage the earliest. Three to four leaves is the stage when quackgrass is most sensitive, but the trick is to have actively growing grass that will absorb the herbicide. If temperatures turn cold, the best advice is to wait until the quackgrass has returned to growth after a couple of warm days

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Figure 1. Quackgrass at sensitive stage for glyphosate

For broadleaf perennial weeds, the sensitive stages will be in late May, June and some should even wait until July. Wait until your weeds reach these stages:

• Canada Thistle - early flower bud
  
• Milkweed - flower bud
  
• Bindweed - full flower
  
• Nut sedge - first flower
  
• Vetch - full flower

Figure 2. Canada thistle at early flower bud

Figure 3. Milkweed at early flower bud

Figure 4. Field bindweed in full flower

Figure 5. Yellow nutsedge just before emergence of flower - wait!

Check the rates on the label for the weed you are controlling. There are 3 rate ranges for glyphosate: the lowest is for annual weeds, the mid-range is for Canada thistle and quackgrass, and the highest range is for other broadleaf perennials.

Apple & highbush blueberry growers can also use Lontrel for vetch control. Apply at the early flowering stage of vetch as a spot treatment.

Figure 6. Vetch in full flower

Caution: Avoid contact with crops or valuable trees like windbreaks when systemic herbicides are applied. Remember that these chemicals may circulate in your trees for several years if absorbed, and there is some research evidence that low levels of glyphosate in a plant makes it susceptible to disease. The best strategy is to avoid getting the herbicide on your trees. An alternative is to wick wipe with a 2% solution of Roundup to minimize both the amount of product used, and contact with trees.