Killing Thistle

Discussion in 'Landscape Maintenance' started by Anthony Divit, Sep 30, 2018.

  1. Anthony Divit

    Anthony Divit LawnSite Member
    from Pa
    Messages: 143

    I use roundup quick pro and does that ok if I cut the tops of them off and spray it inside the stem. What are other people using and what is the results please?
  2. Delmarva Keith

    Delmarva Keith LawnSite Senior Member
    Messages: 638

    I have a similar problem with thistles in a landscape bed. Apparently the owner let them get out of control at some point and the root system is now gigantic. I’ve been working that patch for a couple years. It grows with rhizomes and is hard to kill. Where they emerge in the lawn near the bed I’ve been using triclopyr. In the bed they get a shot of gly. Topgrowth dies but more keep emerging here and there down the road. Seems to be one of those long term battles.

    Gly or triclopyr will kill young thistle plants. The size of the topgrowth doesn't seem to matter — it’s how old it is and how much is underground.
  3. RigglePLC

    RigglePLC LawnSite Fanatic
    Messages: 15,752

    Canada thistle is a difficult weed problem in beds and shrubbery. Deep tap roots and spreads mostly by underground rhizomes( which can spread out up to 6 feet). If you cannot spray it, cut it off frequently with pruners or rose nippers. Comes back from the rhizomes. Time consuming. Try to never let it get taller than 6 inches. Do not let the top send food to the roots. Got to get down and crawl under the bushes. The taller the top--the deeper the tap root. Dig out any that are easily accessible. Cut off any that are growing outward under the soil.
    I also suggest: cut off the main stem and inject it with Roundup or triclopyr. Drill a quarter inch hole in the cutoff stem and fill it with herbicide.
    Actually, as an experiment, try drilling a quarter inch hole in the stem at a slant and fill it with herbicide. Use a small hand sprayer--maybe there will be no need to cut it down.

    If it is possible--cover the good plants with garbage bags--spray the thistle--pull the bags off.
    hort101 and Delmarva Keith like this.
  4. That Guy Gary

    That Guy Gary LawnSite Silver Member
    Messages: 2,511

    When it's a well established plant in a lawn, and I mean one I spray and then come back next time to even more tops, I hit it hard in the fall. Triclopyr 3 way, quinclorac, and sulfentrazone if I have it on hand.

    In planter beds it's just a war of attrition for me, I've never tried to poison the root system but I have at least one property I think I will.
    hort101 and Delmarva Keith like this.
  5. RLeClair

    RLeClair LawnSite Member
    Messages: 24

    I use pelargonic acid (scythe) mixed with my gly. Seems to do a great job on all tap root type weeds: thistle, catseer, dandelion, chicory, etc. It has a synergistic effect with the gly by breaking down the cellulose and allowing the gly easier entry into the plant. Honestly, it works so well with gly, that I never mix a non-selective sprayer without it.

    Edit: the ratio of pelargonic acid to gly seems to be important, as you don't want too quick of a kill from the pelargonic acid, preventing the absorption of gly from killing the root. I use a 2:3 ratio.
    Last edited: Dec 5, 2018
    Delmarva Keith likes this.
  6. Mark Oomkes

    Mark Oomkes LawnSite Fanatic
    Messages: 18,264

    Why would you want to spray it inside the stem? Do you not understand how gly works?
  7. RLeClair

    RLeClair LawnSite Member
    Messages: 24

    I suspect that spraying it on a cut stem would help with its absorption, as gly must enter the plant to inhibit the production of amino acids. From what I understand, damaged foliage aids in this. Hell, a botanist at one of my old jobs claimed that simply grinding a weed under your boot will make absorption more effective.
  8. Mark Oomkes

    Mark Oomkes LawnSite Fanatic
    Messages: 18,264

    Do you understand how gly kills a plant?
  9. RLeClair

    RLeClair LawnSite Member
    Messages: 24

    Sure, absorbed gly blocks the usage of EPSPS, ultimately preventing the production of amino acids and various metabolites. How do you think it kills the plant?
    Last edited: Dec 6, 2018
  10. Delmarva Keith

    Delmarva Keith LawnSite Senior Member
    Messages: 638

    Applying gly to cut stems works. E.g.,
    Gly is transported symplastically to the meristems same as spraying on foliage and when the plant tries to grow the plant dies.

    Damaging plant leaves before or after spraying breaks down the leaf cuticle and can enhance uptake.

    I’ve brushed gly on cut stems of tough to kill weeds (like phragmites and poison ivy) and and it works. The challenge for large thistle plants is getting enough gly to all the growing points of the plant. What’s underground can be huge and once it has been there a long while and grown all over it’s hard to kill.
    KerbDMK, That Guy Gary and RLeClair like this.

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