You've got a couple of options, neither of which is easy to do if your pets have access to the garden. Snap mousetraps baited with peanut butter work on voles. It?s easy to get their population in check within a couple of days of setting 10 or 12 traps out. The other option is poison rodent pellets with zinc phosphide as the active ingredient. Use poison pellets with caution; they'll kill any mammal that ingests them.