Most gardeners underestimate the damage grass trimmers can do to trees or shrubs, overestimating the resilience of bark against them. These devices (also known as weed wackers, string trimmers, weed eaters and strimmers) are invaluable to keen gardeners like me who are obsessive about their border edges, but in the wrong hands are tree killers! It only takes a second. And yes, as a younger, very inexperienced gardener, man I killed a young birch that I had lovingly planted myself, in this way.
Tree trunk bark damage of greater than 25% of the tree’s circumference has a high chance of killing the tree within months. Removing excess loose bark and dressing the wound can improve the chances of a full recovery. Grafting techniques can be used to manage a large wound.
Tree damage from trimmers is particularly common in trees that are already not very well planted: if the tree is surrounded by grass right up to the tree trunk (instead of mulch – scroll down to see what I mean), it’s having to compete with grass for nutrients and water – a battle that it usually loses.
I love to help the average gardener to get hands-on and fix their own problems! In this post I’m going to give you all I’ve learned about this surprisingly common issue. What can be more satisfying for a gardener than saving the life of a tree? But first – unpacking the big question.
Will a tree survive the damage to its bark?
Yes, probably. But here’s the situation. When a trimmer damages a tree, its devastating because that outer later of the bark is the living part of the trunk – where cell division and growth, water movement and nutrient transmission all take place.
Clearly, the tree won’t make it if this has happened around the whole bark. A tree that’s had its bark cut around the circumference of the trunk has been girdled. Girdling with a sharp cut is a method of intentionally killing trees that’s often used in forestry and to control overgrowth.
With trimmers, usually just one side of the trunk has been damaged. According to New Mexico State University (and my own experience) a cut into the bark of less than a quarter of the circumference, the tree will probably survive without too much trouble. If it’s 50% or more, it’s likely to suffer quite noticeably.
There are numerous other ways in which tree bark can be wounded – mammals may have gnawed at your tree bark; a vehicle or your mower may have scraped against the trunk. The advice outlined in this article will apply to all these types of injuries.
What’s going to happen to the tree now?
Generally, only if the tree is completely girdled will you see immediate signs straight away. Because there’s grass growing around the tree, a wound often goes unnoticed for some time.
It might take 3 or 4 months for signs of tree decline to actually show up – particularly if the injury happened when the tree is dormant in winter.
Signs of stress caused by bark damage include:
- wilting branches
- dry, curled leaves, or leaf drop
- yellowing of leaves, or prematurely turning orange/red in late summer
Signs of bark damage can, in some cases, become evident years after the trimmer injury. This is often seen in a hedgerow, where the damaged tree grows at a slower rate than the others or leans to one side, or it may have poor foliage or branch structure.
Just as human wounds can become infected, all open bark wounds carry a risk of fungal infection. This is particularly common in trimmer wounds as they occur close to the ground where there’s a high degree is moisture, which the fungi flourish in. Fungal infection causes poor growth and blotches or dead areas on the leaves.
However, trees are remarkable – just like humans, they release protective substances into the wound and commence a healing process. A wound callus will form, similar to that seen after a tree is pruned. In the case of a trimmer wound, this starts at the edge of the wound and gradually moves inward. In this way the tree attempts to heal and to mitigate the risks of infection. We want to support this natural process in every way possible.
What to do when your tree has sustained an injury to its bark
Almost all arborists agree on two things.
- You should carefully pare away any loose pieces of bark that are still attached. Do this as soon as possible. These extra bits will gradually die off and increase the risk of tree infection if they aren’t cut off. Use a sharp knife to do this and aim to leave the bark which is tightly adhered to the trunk. You’ll usually end up with an oval shaped wound in the trunk, without any ragged edges.
- Avoid using any commercial tree sealant – research has indicated that these may delay recovery and reduce airflow to the wound, which is important in triggering the tree’s natural healing process and callus formation. It’s also thought they could potentially lock in infection from fungal spores.
After this, there are three options, and there’s a lack of clear consensus on which is best:
- Cover simply the wound with thin plastic, such as polythene, for 3-4 months and then remove it- this keeps the remaining tissues (which aren’t completely severed) from drying out, which may help to preserve the flow of nutrients and water thought the phloem and xylem. This could be loosely tied with twine over the wound.
- If you have any moss nearby, you can make a moss poultice and apply that for a similar period of time. Apply a one-inch thick sod of moss to the wound and then wrap a layer of polythene around that, and tie it loosely with twine. This is a bit like a dressing for a human wound, and ensures a more consistent moist wound-healing environment for the tree.
- Simply let it be. Some, including academics at Purdue, believe this is the best option because exposure to oxygen is so important to trigger the tree’s natural healing process and callus formation.
If your tree has sustained a very large wound, some grafting techniques can be considered. This means cutting a part off the upper part (scion) of the tree (or indeed, another tree) and actually inserting it into the wound.
If you’re planning to try this, you’ve probably lost a large proportion of the circumference of the bark. This can even work for fully circumferential, ‘girdling’ cuts.
Here’s an excellent video that demonstrates the process.
When a tree’s been injured by a trimmer, having had damage to its phloem and xylem tissues, it’s more susceptible to drought. It’s particularly important that it’s kept well watered during dry spells, so consider giving the roots a weekly soak for 2-3 months after the injury if this is the case.
Some would advise applying fertilizer or manure to the roots to give the tree the best possible environment for recovery. I’d suggest a slow-release tree fertilizer – but be sure you don’t apply it any more often than the manufacturer recommends.
Prevent bark damage from happening again
With proper string trimmer technique, the guard should always rest against the tree trunk. The problem is that this means there’ll still be a tuft of grass around the tree. The simplest way is to hand-trim around the tree carefully.
But remember that trees aren’t supposed to have grass growing right up to the trunk anyway. Trees are far better grown when surrounded by mulch, such as wood chippings, in a 2-3 foot radius This simulates the forest floor, keeping the roots cool and providing them with a constantly decaying source of organic matter.
If your tree trunk is surrounded by grass, this might be a good opportunity to remove the upper grass layer surrounding the tree (carefully with a spade, starting near the trunk and working outwards), and then replacing it with a 3-4 inch mulch layer. It’s best not to pile the mulch up against the trunk – leave a divot around the tree itself so the trunk doesn’t become susceptible to rot.
If someone else is doing your trimming for you, such as a local gardening company – be sure to tell them about the damage they’ve inflicted to your tree, so it doesn’t happen again! But you could consider applying a tree guard or protector. These are available online and have the added bonus of protection from small mammals – some of which like to gnaw at tree bark.
So make sure your weed wacker wacks weeds, not trees! I hope this article has been helpful. Check out some of my most popular posts below.