Some people plant their hostas
1 still in pots in the ground.
2 wire baskets in the ground
3 spinout bags
4 sharp gravel in planting hole
5 NH Hosta Nursery has You Tube video of castor oil application
The key is supposed to be that the rim of any of these methods stays an inch or so above ground. Seems voles don't like to be above soil.
Best of luck
You can google hosta vole control.
