It actually sounds very much like my main nemesis....
Short tailed voles.
They look like mice, but live underground, and eat plant roots...
They took out my iris one year.... I can't even grow jerusalem artichokes....
or taters... Believe it or not... They even ate the root off of a pokeweed!

iris
pokeweed
They're even eating the camellias!
Cats are of some help, but voles must taste nasty...
You can try dumping the cat litter box in the bed, that gives a little relief... And... youtube is full of videos describing how to trap these pests...