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Fall Weed Management

Weeds to “hit” this time of year

  • Perennials:  Canada thistle, even if it has already seeded; Field bindweed; Common tansy; Dalmatian & Yellow toadflax; Spotted & Russian knapweed; Whitetop/Hoary cress; Leafy spurge; and Oxeye daisy.
  • Biennials: Common mullein; Black henbane; Bull thistle; Musk thistle; Diffuse knapweed; and Houndstongue.
  • Annuals:  seedlings of Chamomile & Downy brome/cheatgrass

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Following information by Sharon Bokan, Boulder County Extension, from CSU Small Acreage Management Fall 2019 Newsletter
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For some weeds, fall is a good time for weed management. As always, you want to identity the weeds first. Not all management strategies work for all weeds. The life cycle of the plant will help determine what management methods will work best.

 For Winter Annual Weeds: NOTE THEIR LOCATIONS

Winter annual weeds, such as cheatgrass, will be germinating soon. While it’s a little late to put a pre-emergent down for cheatgrass (late July and August are better), you can be noting where you see it emerging for a possible winter herbicide application. The other winter annual weeds won’t germinate for a while yet so there’s not much you can do other than note their locations and numbers. You’ll need to deal with them in the spring.

 For Summer Annual Weeds: BAG AND REMOVE

Summer annual weeds, such as kochia and scentless chamomile, are nearing the end of their growth for this year. Spraying them at this point is a waste of herbicide and money as they are already producing seeds. The only option you have at this point is to remove the plants and their seeds, bagging them so you aren’t adding more seed to the soil for next year.

 For Biennial Weeds: Hand Pull or Spray

Biennials, such as Scotch thistle, are forming or have formed their rosettes for next year. You can hand pull them, hoe, undercut or spray them. If you undercut the plant, be sure to pull the plant out of the ground. Just under-cutting the plant will not kill it and it is likely to re-root. “Organic” herbicides (containing acetic acid, citric acid or clove or cinnamon oils) will kill the smaller plants. Larger plants may require a couple of applications. The appropriate (based on which weed it is) “synthetic” herbicide can also be used. Remove and bag any flower stalks that still have flowers or seeds.

 For Perennial Weeds: Hand Pull or Spray

Perennials, such as oxeye daisy and Canada thistle, are storing energy for winter, much like bears and other wildlife. Perennial weeds are more apt to take in an herbicide, so fall is a good time to spray them. If you can, remove and bag any flower stalks and seeds to reduce seed spreading.

A bit on herbicides

Don’t assume that the herbicide you purchased several years ago for your diffuse knapweeds will also work on your toadflax. Certain herbicides work better on certain weeds. To be the most effective and do the least harm to surrounding plants, learn what weeds you have and what management techniques and herbicides work best on them.

If your herbicides are several years old and have been exposed to alternating hot and cold temperatures out in your shed or barn, they may lose some of their effectiveness. They are chemicals and chemicals have expiration dates and break down over time and in extreme temperatures. If you notice that your weeds are not affected by an application, it may be that your herbicide has lost its’ effectiveness.

If you have herbicides that you are no longer using, find a Household Hazardous Waste Collection site in your area.

Always read and follow the label on the herbicides, whether “organic” or “synthetic.” They are both meant to kill plants and you want to be sure you kill the weeds and not your forage plants or injure your-self. The label provides information on any surfactant needed, personal protective equipment, timing of spraying, and grazing or haying restrictions.

 

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