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Here's a quick overview of the treatments ISN has found most successful for controlling knotweeds:

Milestone (aminopyralid)

  • Faster kill (~3 years), more off-target effects
  • Standard rate (3-7 oz/acre; 0.01%)
    • Using nonionic surfactant (standard rate)
  • Late spring (3-4 ft high) or late summer (at least 60 days before frost)
    • When treating in late summer, it is helpful to do a cut of the plants a month or so in advance ONLY IF safe disposal of the stems is possible
    • At some sites, repeat treatments (spring & summer) may be appropriate, so long as maximum application rates are not reached
  • Not aquatic approved
  • Note:  Tree sensitivity issues--some trees will die if their roots mingle with knotweed that is treated with Milestone.

Clearcast (imazamox)

  • Acceptable kill (~5 years), fewer off-target effects
  • Cocktail:
    • 5% Clearcast
    • 1-2% Methylated Seed Oil (MSO)
      • Not all MSOs are aquatic-approved!!  Check before applying to knotweed in areas with standing water
    • 1-2% glyphosate (aquatic-approved formulation)
  • Best sprayed in late summer/early fall
    • Spraying at least 60 days before the first frost is CRUCIAL--this date will vary by location (for northwest lower MI, it's early August)
  • Aquatic approved (if adjuvants are aquatic-approved)

 

We have applied both of these treatments as either foliar spray or as a "cut stem" application (cutting down stems and filling the stem cavity with herbicide solution), which is a great way to avoid drift and overspray, but is more time-intensive and contains the risk of stem disposal.

 

Questions?  Comments?  Leave a note!

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Also, please note that this is NOT a substitute for reading pesticide labels!  Make sure you're in compliance with laws and policies (and have landowner permission) before doing any control practices.

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Do you know what the die off percentage is from the year one treatment is for each species?

Also, after the plant has been sprayed when get the material that appears dead be cut, mowed, removed?

Thanks, K

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Unfortunately, the die-off percentage varies greatly by site (likely due to age etc.).  I would hazard an estimate of approximately 50% die-off in the first year with Milestone, and perhaps 30%  for Clearcast.  Since it's a multi-year project no matter what, we've been looking more at the long-term effects, where Milestone comes out ahead.

Truly dead plant material can be removed from the site as soon as it is bone-dry and therefore incapable of regenerating elsewhere.  We advise our landowners to wait until late fall or even winter/spring to take the guesswork out of it, but if they want to monitor the stems to see when they're properly desiccated.

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"Late spring" and "late summer" have much more to do with phenology than actual dates.   We consider late spring  to be when the knotweed is 3-4 ft high, and late summer as at least 60 days before frost.  

Here in northwest lower Michigan, that translates to mid-June and mid-August.  Saginaw Bay area may be different.

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Unfortunately, not really any updates.  We've moved away from doing cut-stem and into doing injections due to the complications of disposal, but we've only been doing injections for about a year, and don't have much to report yet.

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I only have one experience using Clearcast + Aquaneat for cut-stem. As Katie mentioned, we don't typically do this due to disposal issues. In this case, we worked with the landowner to properly cut, dry, and burn the the knotweed on their property.

Anyway, the treatment occurred in September and had really good results. I've attached a before and after picture. One is from September 2016 (before) and the other is from June 2017 (after). Hope that helps!

2116 N West-Bay Shore Dr (Belanger) 9-15-16.JPG

2116 N West-Bay Shore Dr (Belanger) 6-12-17.JPG

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That's encouraging. I'm looking for clarity on the concentration used with Clearcast for cut stem. When I've used aquaneat alone for cut stem, I've used 25%-50%, vs the usual 2%-4% for spraying and it is relatively cheap to use even at high concentrations. Clearcast is usually 5% volume-to-volume for spraying, so 6.4oz/gallon. At $300/gallon, spraying it is expensive enough. I can't imagine using it straight for cut stem given the price. So I'm looking for confirmation that the cocktail used for cut stem is the same as that used for spraying: that is, 5% Clearcast v-to-v, 1.5% aquaneat, and some MSO. Is that what you used for your cut stem application? 

Edited by Leslie Clark
remove quoted response

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