Ask Mr. Smarty Plants

Voles are mouse-like rodents that have a compact, heavy body, short legs, a short-furred tail, small eyes, and partially hidden ears. When fully grown they can measure 5 to 8 inches long, including the tail. They spend most of their time below ground in their burrow system but can be observed above ground. They are mostly herbivorous, feeding on a variety of grasses, herbaceous plants, bulbs, and tubers. They eat bark and roots of trees, usually in fall or winter. Vole populations are cyclic reaching peaks every 3-5 years. The clearest signs of their presence are well-traveled, aboveground runways that connect quarter-sized burrow openings. A protective layer of grass or other ground cover usually hides the runways. The maze of runways leads to multiple burrow openings. Fresh clippings of green grass and greenish-colored droppings about 3/16 of an inch long in the runways and near the burrows are further evidence of voles.

You have two species of vole in your area: the Meadow Vole (Microtus pennsylvanicus) and the Pine Vole (Microtus pinetorum). North Carolina State University has an excellent site that will help you identify which species you have and give you more information on their ecology and life history. The Missouri Botanical Garden has many excellent photos on their site to help you identify vole damage, runs, etc.

Protecting your plantings from voles will require some effort (as you have found) and, even if you were to use lethal control, will not be absolute. That being said, you are on the most productive track – using native plants that voles find unappetizing. You can take this further by adopting an Integrated Pest Management (IPM) approach to the problem. North Carolina State has an IPM center that will introduce you to the concept – an effective and environmentally sensitive approach to pest management that relies on a combination of common-sense practices. IPM programs use current, comprehensive information on the life cycles of pests and their interaction with the environment. This information, in combination with available pest control methods, is used to manage pest damage by the most economical means, and with the least possible hazard to people, property, and the environment. Another excellent source of information is at the Southern IPM Center’s website.

Minimizing damage due to voles involves prevention, avoidance, and monitoring. You will need to use a variety of strategies: cultural, mechanical, biological, and chemical. Without resorting to lethal control you still have several options to couple with selecting plants that are unpalatable to voles. You can enlist other aspects of cultural control (managing the physical features of your site to discourage voles); mechanical control (including exclusion); or non-lethal deterrence (but there really is no deterrent that has proven effective). 

Two excellent web site with extensive information on gardening with voles are: the North Carolina Master Gardeners and the Internet Center for Wildlife Damage Management.

Cultural and Mechanical Control

Ground vegetation, by providing food, concealment from predators, and protection from unfavorable weather, is the most important factor affecting meadow vole abundance. Thus practices that reduce cover in surrounding areas can help reduce vole density. These practices include controlling weeds and reducing ground cover.

GROWTH FORM is an important consideration in selecting ground cover plants that discourage voles. Dense covers that form a continuous canopy support high vole populations. Whereas plants with an erect bunch-type growth or covers that reach a short mature height provide voles with little cover. One way to effectively deter vole populations is to make the habitat less suitable to them. Weeds, heavy mulch, and dense vegetative cover encourage voles by providing food and protection from predators and environmental stresses. You can reduce the area from which voles can invade by regularly mowing grassy areas or field edges adjacent to gardens. The wider the cleared strip, the less apt voles will be to cross and become established in gardens. Mowing frequently to maintain short vegetation height also discourages voles.

FOOD PLANTS also influence whether voles will frequent an area.  Meadow voles forage almost exclusively above ground on fresh leaves and stems of a grasses and broadleaf plants. Seeds, woody materials, and bark are eaten when green foods are of low quality or in short supply. They prefer foods that have high water or calorie content. Select plants that are well-defended (waxy, woody, hairy, prickly, etc. or are highly aromatic) and plant sparsely.

Exclusion

Creating subterranean barriers or gravel barriers in lawns, vegetable gardens, or flowerbeds can reduce vole runway systems and aid in the dispersal of populations. Coarse particles like expanded slate have proven to be effective barriers. Some people have found it helpful to mix sharp-edged pea gravel throughout the soil as they prepare to put in new plants. Many sources recommend products like VoleBloc  to be used this way.

You can also resort to fencing. Wire fences at least 12 inches above the ground with a mesh size of 1/4 inch or smaller will help to exclude voles from the entire garden. These fences either can stand-alone or be attached to the bottom of an existing fence. Bury the bottom edge of the fence 6 to 10 inches to prevent voles from tunneling beneath it. A weed-free barrier on the outside of the fence will increase its effectiveness.

You will not be able to eliminate voles but using a combination of methods that reduce the cover and food choices for voles will reduce the damage that you experience.