Grass strimmers and brush cutters kill more young and ornamental trees than most gardeners realise — not through anything dramatic, but through repeated, low-level bark damage that the tree quietly fails to recover from. The fix is cheap and takes a few minutes per tree.
Why strimmer damage kills trees: girdling explained
A tree moves water up from the roots through the wood (xylem) and moves sugars made in the leaves back down to the roots through a thin layer just under the bark (the phloem/cambium). That layer is only a few millimetres thick, and it sits right at the height a strimmer head naturally swings past — usually 5-20 cm above ground level.
When spinning nylon line hits bark repeatedly, it doesn’t need to cut deep. It just needs to strip that thin cambium layer away. Once that ring of tissue is broken all the way around the trunk — a process called girdling — the roots stop receiving sugars from the leaves. The tree can still pull water up for a while, so it may look fine for a full growing season, sometimes two, before it goes into visible decline and dies. This delay is exactly why so many gardeners never connect a dead tree to strimmer work done a year or more earlier.
Safety note (for the tree, not the operator). Damage doesn’t need to be a clean ring in one session to be fatal. A strip removed this year, another overlapping strip removed next year, and the tree can be fully girdled over 2-3 seasons without a single obviously severe incident.
How much damage is actually fatal
| Bark damage extent | Likely outcome |
|---|---|
| Light scuffing, bark intact | Tree recovers, no real impact |
| Bark stripped on less than ~1/3 of circumference | Usually survives; may callus over slowly |
| Bark stripped on 1/3 to 2/3 of circumference | Reduced vigour, dieback likely on one side, weakened long-term |
| Bark stripped all the way round (full girdling) | Usually fatal within 1-2 seasons, sometimes faster on young/thin-barked trees |
Young trees (under 5-8 cm trunk diameter) and thin-barked species — birch, maple, magnolia, many fruit trees — are hit hardest because their bark is thinner and their trunk area smaller, so the same strip of line damage covers a much bigger share of the trunk’s circumference.
Keep a mulch ring — the single best fix
The most effective thing you can do is stop grass growing right up against the trunk in the first place, so there’s simply nothing to strim there.
- Clear a circle of grass and weeds around the trunk, working outward from the base with a spade or hand tool — don’t use the strimmer for this first cut.
- Aim for a ring at least 60-90 cm (2-3 ft) across for an established tree, smaller (30-45 cm) for a young whip or sapling.
- Lay 5-8 cm of mulch (bark chip, wood chip, or well-rotted compost) over the cleared circle, keeping it pulled back 5-10 cm from the trunk itself — mulch piled directly against bark holds moisture against it and encourages rot and pests, which is its own way of harming the tree.
- Top up the mulch once or twice a season as it breaks down or gets kicked around.
With a mulch ring in place, the strimmer never needs to go anywhere near the trunk — you cut the grass up to the edge of the ring and stop.
If there’s no mulch ring: technique that actually protects the bark
Not every tree has a ring, especially in a field, orchard, or paddock setting where mulching every trunk isn’t practical. In that case, technique is what stands between the tree and girdling.
- Approach at low throttle. Slow the head down well before the line gets near the trunk — you don’t need full cutting speed to trim thin grass right at the base, and a slower line is far more forgiving of a momentary misjudgement of distance.
- Use the tip of the line only, never the full arc. Angle the head so only the last centimetre or two of line makes contact, rather than sweeping the full spinning circle past the bark. The tip does the cutting; the rest of the arc should be cutting air.
- Work around the tree, not across it. Circle the trunk with short, controlled passes rather than one continuous sweep that carries the line past the bark at full radius.
- Stop short and finish by hand. For the last few centimetres right against the bark, it’s genuinely faster in the long run to snip the final grass with shears or pull it by hand than to risk the line touching wood.
- Never let the guard or head brush the trunk for balance. Leaning the machine against the tree to steady yourself transfers the same abrasive risk through the housing and guard edge, not just the line.
Safety note. If you can’t get a clear, unhurried look at the base of the trunk — long grass, poor light, awkward footing — stop and clear by hand first. Rushing the approach on a tree you can’t see clearly is how most girdling damage happens.
Guards and tree protectors
Physical protection removes the risk of a lapse in concentration turning into permanent damage, and it’s worth fitting on any tree you’ll be strimming near repeatedly:
- Spiral or slotted plastic tree guards wrap around the lower trunk and take the abrasion instead of the bark. Fit loosely — a guard cinched tight against the trunk can itself rub and girdle as the tree grows, which defeats the purpose.
- Rigid mesh or corrugated pipe guards work well on young whips and are easy to slide up as the trunk thickens.
- A simple stake-and-string boundary around the mulch ring, or a length of hose slit and clipped around the base, is a low-cost stopgap if you don’t have proper guards to hand.
- Check under any guard once or twice a season — trapped moisture, bark beetles, and slugs can do their own damage if a guard is left on too long without inspection, and a guard left on a fast-growing young tree for several years can itself constrict the trunk.
Machine-side guards matter too: keep the strimmer’s own debris guard fitted and correctly positioned, since it’s designed in part to deflect the line’s outer edge away from solid obstacles, trunks included.
Alternatives to strimming right at the base
You don’t have to strim all the way to the trunk on every visit. Options that avoid the risk entirely:
- Hand tools for the last ring. Shears, a sickle, or a hand hook for the final 10-15 cm around the trunk takes a couple of minutes per tree and removes all girdling risk.
- Mulching instead of cutting. Once a mulch ring is established (see above), there’s usually nothing to cut there at all — the mulch itself suppresses regrowth.
- A mower with a raised deck, kept back from the trunk, for the wider grass area, leaving only a narrow band around each tree for hand-finishing.
- Herbicide spot-treatment on the ring itself (where appropriate and permitted) as a one-off way to knock back grass before mulching, keeping any chemical well clear of the bark and root flare.
Quick checklist
- Mulch ring in place around every tree you regularly strim near, mulch pulled back from the bark itself
- Where no ring exists: approach at low throttle, tip-only contact, work around the trunk
- Guard or protector fitted on young and thin-barked trees especially
- Final centimetres finished by hand rather than with the line
- Machine’s own debris guard kept fitted and in good condition
- Trunks checked once a season for any sign of scuffing, frayed bark, or a developing ring — catching light damage early, before it becomes a full girdle, is what saves the tree
Established trees can sometimes shrug off a single bad pass, but the real damage comes from doing the same thing every cut, every season. Fix the habit once — mulch ring, guard, or hand-finishing — and the risk disappears for good rather than needing constant vigilance on every pass.