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.