I've been battling voles for years, ever since we moved into our home in 2001. When I am vigilant about controlling them, I can usually keep the vole damage to a minimum, but every few years it seems like their population explodes and I need to rethink my tactics. Following are tips that have worked for me.
If you don't know what a vole is, it's a mouse-like creature that likes to burrow just under the soil to make its home. They aren't good climbers, so you don't usually have to worry about them getting in the house, but they love to nest in areas with lots of leaf litter or in tall weeds and grasses. They use the vegetation to help cover the entrances to their runs which are about 1-1/2" to 2" in diameter. In the spring, you can usually see lots of evidence of voles as their runs are near the top of the soil, especially if you have a lot of snow cover in the winter.
There are about 70 species of voles, and the majority eat plants, especially the roots and tender shoots of smaller plants. That is where most of my damage occurs. I buy a lot of small plants and grow them in pots for a couple of years before planting them in my garden.
The easiest way for me to avoid vole damage is to repel them. They don't like the scent of castor oil, so I have great luck with a product called Molemax that repels not only voles, but gophers, skunks, squirrels and other burrowing animals. I sprinkle it all along the area I want to protect, water it in, and watch it work. This works very well starting in spring, right thru the autumn months. Once it begins to snow, I can't reapply, but by then, the voles have generally left the area, providing I have been diligent about putting down the MoleMax all season long.
Another great repellent is powdered fox or bobcat urine. Fox and bobcat are predators to voles, so the voles will stay clear of an area that appears to have a fox or bobcat in residence. I find using the powdered urine around the perimeter of our property and using the MoleMax in the garden areas is pretty affective.
To protect any bulbs I plant, I soak them in Ropel Animal Repellent. This has worked amazingly well, and not even a squirrel wants to dig up my bulbs. Ropel is a very nasty tasting liquid, so make sure you wear rubber gloves when using it and handling any treated bulbs as it takes a while to get the taste off your skin. Just soak the bulbs for one minute, let them dry, and then plant as usual. It's not safe for food crops, so keep that in mind.
When populations of voles are high, you may have no choice but to protect your prized plants with hardware cloth. Some people like to create a box out of this mesh and plant the root ball of the plant inside the box. This can work well for small bulbs also. The squares in the mesh would need to be a ¼", or voles can still get inside the box you've made. I find that this method can hinder the growth of certain kinds of plants, so you need to be careful what type of plant you put inside the mesh. Make sure the stems of the plant can grow out of the holes on the mesh and if the root ball gets large, make the box larger than the root ball you are planting. An alternate solution would be to fence the area you want to protect with ¼" mesh making sure the mesh goes down below the ground at least 6", and remains above ground 6"-12" based on how much snowfall you get in the winter.