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I have ongoing problems with weedwacking around my trees. I notice that on a lot of trees in government tended - and even on other private property - have a brown ring circle of very thin lawn which I assume is caused by the use of a herbicide - but the trees seem very healthy. (And in addition to reducing the amount of maintenance around the trees I wonder if this is actually better for the trees then the risks of the bark being damaged by a weed wacker?)

Can anyone provide any advice on how to create this "dead grass/kikuyu zone" underneath trees while minimizing the harm? What herbicides are typically used to do this, how to apply it? Are there special mitigations I can take to reduce the risk to the trees? Is glyphosate appropriate here or do I need to go with something else? (I have selectively used glyphosate around my property, and ask because "its the devil I know")

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    In "lawn" you could presumably use a monocot specific grass-killer - which would leave any broadleafed weeds untouched, so you'd have to kill them some other way. Not what I'd do, but folks that want a spray "solution" generally can't be bothered to mulch. They do make "tree mulch rings" pre-formed if you'd consider that. I'd suggest the "permeable and degradable ones" rather then the rubber sort. You could also use a pre-emergent to prevent any new seedlings from sprouting there.
    – Ecnerwal
    May 8, 2022 at 2:27
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    The Arbor Day Foundation (and other sources) recommend a ring of wood chip mulch around trees. They specifically call out "to prevent mower damage" as one of the reasons. The mulch also feeds the tree and helps reduce compaction. You might want to look into that option instead of using chemicals.
    – Jurp
    May 8, 2022 at 11:52

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Yes, glyphosate is the one to go for. As well as preventing strimmer (what you call "weedwacker") damage, maintaining a vegetation-free circle around the trunk is particularly beneficial for young trees to help them establish. In the words of the RHS here:

Weeds, lawns and other vegetation intercept water before it reaches the roots of newly planted trees and shrubs. Keep a vegetation-free circle at least 1.2m (4ft) in diameter around the plant for its first three years to help avoid this problem. The circle can be kept weed free through hoeing, using a mulching mat or use of contact or systemic weedkillers. Laying mulch over this circle is also helpful, although take care to leave a collar of 10cm (4in) around the woody stems that is free of mulch, to prevent the risk of rotting the bark.

EDIT. To summarise, if you think your established trees may be susceptible to strimmer damage, spray off (using glyphosate) a mowing strip (about six inches wide) around the trunk. For young trees, follow the RHS advice given above.

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  • The Arbor Day Foundation agrees that a grass-free area around a tree is very beneficial, but recommends mulch only. Bare-ground around a tree does nothing to help the tree (especially a young one), and the repeated weeding with a hoe disturbs/kills the tree's feeder roots. Why use chemicals when not using them is better for the tree and reduces your work? Reference: arborday.org/trees/tips
    – Jurp
    May 9, 2022 at 2:47
  • @Jurp - I'm not anti-mulch. But the question was about choice of herbicide to avoid strimmer damage to trees. Also, before applying mulch you need to kill/remove a circle of grass around the tree. The easiest way to do this is with glyphosate. And the easiest way of removing weeds/grass growing through the mulch (which they probably will) is to spot treat with glyphosate. My ideal personal preference would be a combination of mulch and spot treatment with glyphosate. If I couldn't be bothered with mulch, I'd be happy to keep the circle of bare soil weed-free with glyphosate.
    – Peter4075
    May 9, 2022 at 7:46
  • You actually do NOT need to kill the grass before mulching with wood chips - a layer three inches thick will kill all grass under it. Where you would need to remove it is within four inches or so of the trunk because the mulch needs to be thinner at that point. The easiest way to do that is by hand. Wood chip mulch renewed annually (three-four inches the first year, 1 inch every spring thereafter) will not have weeds coming through it because no weed seeds will germinate under the mulch.
    – Jurp
    May 9, 2022 at 20:37
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As a natural alternative, you might consider planting a shade-loving ornamental plant that crowds out weeds, such as deadnettle. That should grow nicely around trees without harmful chemicals.

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