Who Ate My plants?! Thoughts on Rodent Damage
Rodents are an order of mammals with a monstrous pair of perpetually growing incisors.
This order includes mice, squirrels, voles, gophers, beavers, capybaras, and many others. Some of whom have a voracious appetite for everything beautifully growing in your garden. Rabbits have these same characteristics, continuously growing incisors and desire for garden plants, but they are not taxonomically considered rodents since they possess four incisors rather than two. However, for the purposes of this discussion we will consider rodents and rabbits together and classify the plant injury each of these mammals cause as rodent damage. Rodent damage is the injury inflicted when mice, voles, squirrels, gophers, or rabbits gnaw on or consume parts or the whole of plants. These pests go after the plants for food and to wear down their teeth preventing them from growing too long. Rodents and rabbits target a wide range of species including woody plants, turf, and herbaceous plants (perennials); the damage caused can range from fatal to insignificant but with careful planning and maintenance the issue of rodent damage can be managed.
Rodent damage on woody plants happens mostly in the winter, particularly during times of heavy snow cover when some rodents have difficulty accessing plants on the ground. The young wood just under the bark affords sufficient nutrients to the animals who gnaw through the outer bark layer and eat the young wood. This type of damage is immediately apparent in the winter and in the spring, one often finds the bite marks start at the point up to which snow had covered the bark and reach as high as a rabbit could have nibbled while standing on the snow. In the area that this chewing occurs, the plant’s ability to transport nutrients and water between the leaves and roots is disrupted. If the damage circles the trunk or stem it will typically kill the portion of the plant above the wound. In some cases, if the chewing was not deep into the young wood and didn’t encircle the stem, the wounds will heal and the plant will continue to grow. If the damage is severe, there is little that can be done to cure a plant with this type of damage. With shrubs, if the plant does not appear to leaf out well in the spring the best option is to remove the damaged stems and the plant will produce new ones to fill the gap. If the damage was severe on a tree it will likely never recover and should be removed before infection moves into the wounds and causes the tree to structurally fail and fall unexpectedly. Prevention of this damage is possible through the use of trunk protectors; these are plastic or thick paper barriers placed around the base of the plants. As this type of rodent damage just occurs in the winter it is only necessary to have the protection around the plant at that time.
Another type of woody plant damage caused by rodents which can happen anytime of the year is the destruction of plant roots. If a small tree or shrub suddenly appears tipped, loose at the base or has completely fallen over rodents may have eaten the roots. If only a few of the main structural roots have been damaged, the plant may recover if replanted and watered regularly. However, if most of the roots were damaged and there is only a nub left on the base of the plant it will not live. This damage is usually only found on small trees and shrubs, once the tree trunk is more than a few inches in diameter it is no longer a target.
Above: Euonymous shrub damaged December 2017, photo taken May 2018. This amount of damage is considered minimal for most shrubs. The shrub shows no foliar signs of damage as seen below (Click images for a closer look):
Rodent damage on turf also occurs over winter. This crime is committed by the rodents that prefer to tunnel under the snow, generally voles. Voles eat the crown of the grass plants right at the ground, leaving tunnels through the thatch and killing the plants they have eaten. These little tunnels are quite visible after snow melt. If the tunneling is not extensive the neighboring grass will easily fill in the dead spots. If there is little neighboring turf, these dead areas should be reseeded as they will not regrow with the crown of the plant missing. The dead grass leaves which the voles left can be raked up to improve appearances of the area.
Damage inflicted on herbaceous plants occurs primarily during the growing season. It is not uncommon, just when you are beginning to enjoy your perennials, to step out the door one morning and find them completely mowed down, all the tops having been chomped by rabbits and rodents. Perennials that have undergone this type of damage have a better outlook than woody plants and grass. If the perennials were established for at least one year they generally will come back from the base underground if only the tops were eaten. When this sort of incident occurs, it is best to be patient and wait to see if the plants will regrow. Occasionally, the damage is not limited to the tops of the plants. If the entire plant appears tipped, loose at the base, or suddenly wilts down the roots may have been eaten (see image below). Again, be patient, if replanted and water regularly these plants often revive. Damage caused by squirrels, however, has a less positive outlook. These pests have been known to dig up and eat flower bulbs. These plants will not regrow. One method to prevent this is dig up the entire area where you will install bulbs, place the bulbs in and lay coarse, mesh screen over the bulbs, then replace the soil. When the squirrels dig they will encounter the screen and stop before reaching the bulbs.
What, then, can you do to ensure your yard, lawn, or garden won’t be demolished by these toothsome critters? There are a few planning and maintenance options which can help: variety selection, repellents and eradication. The first option is careful selection of plant varieties that are “rodent resistant.” One word of caution, rodent resistant does not mean that they will never be eaten or tasted, simply that they are not preferred provided other options are available. Perennial varieties that are less desirable to rodents and rabbits include coarse-textured plants and those in the mint family, characterized by square stems, and others. Better Homes and Gardens publishes a suitable list here. Rodent resistant trees and shrubs include varieties that thick-barked, as rodents prefer thin bark and soft wood. Another option to protect your landscape is repellents. There are several effective rodent repellents which can be applied to the plant material; these must be applied repeatedly as the plants grow out of them or they wear off. A third option for protecting your yard is eradication. There are also several poison and trapping methods to help remove the critters from your vicinity; this method works only until new visitors arrive and the system must be repeated. When used in combination, these three basic methods, variety selection, repellent, and eradication can yield a landscape less frustrated by the many rodents and rabbits that dwell in our urban landscapes.
By: Cathy Nickelson, B.S. Horticulture, University of Minnesota, ISA Certified Arborist