When snow blankets your property, it can be hard to remember that there’s a whole world of activity underneath it. But if you notice damage to trees and shrubs in spring, especially at the base of their trunks, that’s a sign that critters have been busy beneath that snow. Most likely, rodents are to blame.
In this article, we share details about the most common rodents that harm trees in Colorado, plus our recommendations for how to keep your trees safe from rodent damage this winter.
Meet the Rodents Eating Your Trees
The most likely suspects for any damage higher up a tree are squirrels, who can climb and jump across tree canopies and drop to the ground in an instant. Squirrels damage bark, leaves, and fruit above ground level, and can seriously injure a tree. Their chewing can girdle twigs, branches, and the trunks of trees, with areas of removed bark becoming openings for pathogens and insects.
At or below ground level, burrowing rodent species, such as gophers and voles, are probably the culprits. Colorado has eight species of voles, each of which has evolved to its particular habitat. Meadow, stream bank, or woodland, there’s a vole for it.
These rodents are the ones that can do serious damage in winter under a concealing blanket of snow. You may not know the extent of their damage until the snow melts, and even then it can take some time for underground root system damage to show up in a tree’s overall vigor.
Rabbits are also responsible for winter damage to your garden, but they’re not rodents. Rabbits take advantage of hardened snowpack to move from tree to tree, chewing and gnawing on trunks not far above the snow level.
How to Identify Rodent Damage to Trees & Shrubs
Once you know what to look for (and the snow has melted), rodent damage to trees and shrubs is easy to see.
First, look at the base of your trees. Rodents scrape and gnaw bark at the base of trunks, especially on young trees whose bark is still thin and easy to break open. You may find the bark has been stripped all around the base of a tree’s trunk (this is called “girdling”).
If your tree has been girdled by rodents over the winter, you’ll need to monitor it closely over the months and years to come. The tree may not be able to distribute moisture and nutrients into the upper parts of the tree if the bark has been removed, leading to wilting, die-back and possibly death.
Plus, the base of a tree’s trunk is especially sensitive to pathogens. Bacteria and fungi are at home in the moist soil that surrounds trees, as are insect pests, and they’ll quickly invade the tree through open wounds left by chewing rodents.
Next, look at the ground around your trees. Look for lines of disturbed soil leading to trees or shrubs. These are probably caused by voles tunneling just below the soil surface. You may also see mounds of soil and openings where gophers have tunneled and made escape routes.
Voles and gophers not only gnaw through bark, they also eat both small, succulent roots and large, woody anchoring roots. Because this happens under the ground as they burrow and expand their network of tunnels, it’s difficult to spot the damage. These unseen injuries can be severe but you likely won’t notice it until the damage affects a tree’s growth above ground.
And don’t be surprised if you find damage in unlikely places or on unlikely species. As winter drags on and food sources become scarce, hungry animals expand their palates.
How to Prevent Winter Tree Damage by Rodents
The best rodent damage prevention starts before winter arrives. It’s also easier to get your tree and shrub protection in place before the snow flies.
Keep Rodents Away From Your Trees
The standard prevention method is to surround tree trunks with an exclusion zone made of ¼-inch metal mesh or hardware cloth. Here’s what we recommend:
- To prevent vole damage, wrap the trunk of your trees or shrubs with 24-inch wide mesh. It will be most effective if you bury six inches below the soil surface, with 18 inches above ground.
- To prevent damage by pocket gophers, the hardware cloth should stand free from a tree’s root ball and be buried a foot or more into the ground. Create a 90-degree outward bend at the base to prevent gophers from tunneling under it.
- To stop rabbits from gnawing on bark, be sure to wrap the metal mesh higher than the snowpack surface around your trees. You may need to adjust the height of the mesh throughout the winter as more snow falls.
With young trees this is easy, but with older trees that have a pronounced trunk flare and anchoring roots, you’ll need to take more time and effort to secure the metal mesh. Good leather gloves and sharp snips will make the job easier
Trap Rodents to Reduce the Population
Baited traps are commonly used to catch and kill mice, gophers, and voles.
In the fall, set and place traps strategically in tunnels or burrows and check them daily. If gophers have made a large network of tunnels, find the areas where the gophers are active and put traps in the most recently dug areas.
We do not recommend using poison. Not only does it have unintended effects on non-targeted animals, pets, and children, the use of fatal poisons for rodents also has repercussions up the food chain. Predators can be sickened or killed after eating poisoned prey, reducing the number of important predators that help keep rodent populations in check.
Repel Tree-Damaging Rodents
If you’re uncomfortable with using traps, you may want to try one of the repellents that are recommended to keep rodents away. These include real or synthetic animal urine, as well as sprays made of castor oil and capsicum (the active ingredient in hot peppers). You can spray pre-mixed repellents directly, following label instructions.
Thiram is the brand name of a fungicide that is also used as a surface repellent to keep rodents away from trees during the dormant season. It contains a sulfide, with the sulfur odor and taste making it unappealing to rodent pests. Be careful though – it can cause irritation to humans and injury to pets, although it’s not considered fatally toxic.
NOTE: Unlike physical barriers, repellents need to be applied regularly and repeatedly, and they’re more likely to reduce rodent damage, rather than eliminate it.
Create an Inhospitable Environment for Rodents
Another approach to preventing rodent damage is changing the environment around your valuable trees and shrubs. This means that you make the areas surrounding your plants less inviting or comfortable for the burrowing furballs of destruction.
Among the strategies suggested for gophers and voles are:
- Cutting down or mowing tall grasses and weeds around trees
- Growing low, non-matting grasses instead
- Tilling soil to disrupt their tunnels (although we don’t recommend this option as it can damage important tree roots)
- Planting vetch around trees, which voles don’t like
Note that many of these recommendations are consistent with fire mitigation best practices.
Once winter snow starts, shovel snow away from trees and shrubs to prevent covered trails and to allow clear views for predators.
You may have heard that mothballs can repel rabbits. However, mothballs are made from a toxic pesticide and are labeled for indoor use only. Please don’t scatter mothballs around your garden.
Squirrels are too numerous and too crafty to let most protection methods prevent them from getting what they want. You may trap or repel them, but more always appear. For squirrels, predation is the best way to reduce populations.
Invite Natural Predators to Your Property
Predators and prey are in permanent states of balance and imbalance as their populations fluctuate. You may not want to encourage large predators (you probably don’t want threatening animals around your house!) and all the recent development in our area is shrinking natural habitat ranges and driving larger species away. However, there are a number of smaller predators that will be happy to visit your property and contribute to rodent-control efforts.
The most common predators in Colorado that can help control rodent populations include:
These species will be appreciative of your mown grasses and cleared weeds, as open areas expose scurrying, hopping, and digging prey. Maintaining a property that allows these predators to do their work may be all you need to do during the rest of the year to reduce winter rodent populations.
Winter damage to trees from rodents can be severe and can cripple or kill trees. If you want to protect your trees and other plants but don’t know where to start, call your local arborists at LAM Tree Service. We can explain the benefits and drawbacks of different control methods, and advise you on what’s best for your particular property. Plus, we can examine any rodent damage to your trees and recommend the best ways to protect them over the coming seasons.