Our small and young (40 tree) Heirloom Organic Apple Orchard (9 years old) suffered girdling from voles because the organic landscape covering (used to suppress weeds the past 3 years) served as a vole magnet over the winter! The landscape covering will be removed before autumn. The covering is set in a square formation with the tree in the middle. It stretches out to the drip line. The orchard is a desert with islands of trees amongst a meticulously mowed carpet orchard grass, dandelions and some clover. Our grower / orchardist suggested that once we remove the coverings we plant comfrey because it could serve as a fine mulch and fertilizer encouraging a finer ecosystem. We don't want to spend another dime on certified organic fertilizers if we don't have to. However, my concern is that the comfrey will spread too close to the tree and serve brilliantly as a harbor to host insects right next to the Apple Trees! :-0 The orchard is surrounded by a neighboring hayfield on one side and a wooded area (poplars) on the other side. We would like to bring in more permaculture methods into our orchard, however, if comfrey is loved by the eastern Japanese Beetle, I've just created a bigger problem as Japanese Beetles adore apple trees! If voles use comfrey to hide from our great horned owls in the same way they used the organic landscape coverings, then I'm really in for it. If deer, snowshoe rabbit and other wild animals use comfrey as a major food source, then we're doomed!

I watched the video on comfrey, but am not certain which varietal grows in the Northeast U.S. Thanks so much for your advice... Susie

Plant the sterile variety far enough from the truck to ease your concerns. Then bring in more variety of plants. Some to repel bugs, some to encourage good bugs, etc.

I've given up the vole fight and planted potatoes EVERYWHERE. I'm hoping the potatoes keep them occupied enough that they don't go for my trees again.

once established , the comfrey roots are too burly and thick for the voles to be among them....at least this is my observation.
they will seek out the easiest places to tunnel, nice fluffy soil, wood chip mulch or other loose mulch, and places with shallow rooted or not very established plantings. you tend to find their holes will go around any established big roots, or anywhere the ground starts getting harder.
comfrey roots become massive very quickly, and i am not 100 % sure but i dont think they are palatable, may even be a bit toxic, being a medicinal usually goes along with some intense qualities. i am also pretty sure that deer dont eat comfrey, or much else, for that same reason.

i've even gotten them to move into a new spot, by setting up piles of loose mulch nearby, then torturing them by filling their holes with water and messing up their tunnels..they also like to be in spots that stay pretty dry. possibly the landscape cloth was keeping the area underneath them dry...

hau Susie Brown, voles are indeed a problem critter. For an organic orchard your choices are limited but also they are the best methods.

Hardware cloth used as a trunk fence works to keep these meadow mice from being able to chew the bark and so girdle the tree.
Place it far enough from the trunk that you won't have to make it larger for several years.
Putting it so at least 6 inches is below soil level is also a good idea since that will keep them from burrowing under the hardware cloth fencing.

Next is predator urine, this is great to deter these little fellows from wanting to come near where ever you place it.
coyote, wolf, fox are all natural predators of voles so these are the best urines to purchase and use. Get them at trapper supply houses for the best prices.
You can even work your way out from each orchard tree and move the voles far away from their targets.
use of these two methods of control keeps you from having to use poisons, traps or other means to get rid of them.

Cats are also a natural hunter of voles, but you would need several of them to work well.

Voles like cover, they may or may not travel in tunnels, I've seen them make above ground trails.
I would not mulch near any orchard tree that was not protected by a hardware cloth fence buried 6 or more inches deep.
While many think of comfrey as a good crop to prevent them, it is more a sacrificial lamb than true obstacle to a determined vole.
That isn't to say that it won't help, but it is just one cog in the wheel of prevention.
Combination attacks are usually the best method to get them to move on and leave your orchards or gardens alone.

Hope this helps you with your problem.

There's always the bone sauce; supposed to work for voles. I've never tried it, although making some is on my list. I did run across this, though, and might give it a shot, since Bird Cherry (Prunus padus) grows all over the place around here. From http://www.mustila.fi/en/taxonomy/term/82?page=1 :

"The bird cherry has long clusters of small flowers, which later develop into black berries. Their bitter taste comes from the seed, which contains very small amounts of amygdalin, which also occurs in higher concentrations in the bark and the wood, which smell unpleasant. Outside the growing season, branches cut from the tree can be used to keep hares from eating the bark of other trees, such as apples."

I've never heard of this, but I gather the method involves using branches as repellents, strategically located near vulnerable trees. Now that I think about it, I don't recall ever seeing one touched by a vole. They definitely go for other Prunus species I've got. We're in the middle of a vole explosion right now, and I'm going to have to protect my trees for winter or damage may be severe.