I have been having the same problem and it is extremely bad this year. I am betting on either voles or mice (most likely voles). It is able to squeeze through a 1 inch chicken wire with the plant, so it is not squirrels or rabbits, and birds would not drag through a hole this size. Whatever is doing the damage is also chewing off all my beans and vining flowers at the base, too. Very frustrating. Could a chipmunk fit through this size hole?

I would also say VOLES, they are mouse size moles, but no tails. So, I had something like this happen to a plant and I asked my friend who did landscaping she immediately said Voles, and all you need to do is set out a mouse trap with peanut butter and keep doing it till you don't catch anymore. Sure thing, next day, and a few days after had one in trap, got them all and no issues since. They just trim off the stems so neatly, and mine went from outside to the inside in rows, they then suck the moisture out so it looks like someone trimed them off at the bottom with scissors and just let them lay there. Good Luck!

FYI, thread from 2017.

Chip33 said: and all you need to do is set out a mouse trap with peanut butter and keep doing it till you don't catch anymore. Sure thing, next day, and a few days after had one in trap, got them all and no issues since.

Obsolete thread, but I didn't want to leave that terrible advice unanswered.

It is hard to imagine someplace with just a few voles and so little other wildlife that such a method would work. I don't believe that got them all. Maybe they just avoided the trap for a while and were never the actual problem.

Here, voles outside are too numerous to do anything about (and the stem cutting is very clearly done by rabbits, not voles). Every fall some voles move into the attic. Then mouse traps with peanut butter are needed to get rid of them before they colonize the rest of the house. More often than not, they trip the trap and escape, so it takes many traps reset many times each, even in an attic that has far few voles than outdoors.

Some year, squirrels manage to move in when the voles do. They get to peanut butter baited mousetraps much quicker than the voles do and trip them so the traps just grab onto hair. The squirrel then drags the trap away and shakes it off in the eaves. Anyway, the squirrels must be removed with a different trap before even trying to get rid of voles. Outdoors (at least here) squirrels would destroy any attempt to get voles with peanut butter baited traps. Here, the chipmunks would also get to the traps before voles do.