I'm sorry to come back to this so late -- non-gardening life intervened and I forgot about the post. I have containers of all sizes. The squirrels don't care. They dig out everything. I've recently tried a novel approach that seems to be working (keeping my fingers crossed) -- I smeared Vaseline on a few containers' rims, both thin and wide-lipped. So far the squirrels have avoided those planters... (or if they have approached, I missed the fun :)

Hmm...I haven't heard of using vaseline. I feel you though. The squirrels are relentless here.

I hadn't heard of it either :) I was using the Vaseline for something else and regretting the huge holes dug out around my plants... and I was picturing the squirrels perched on the planter rims (since they don't just jump directly onto the soil) and I thought "Why not?" ... Since the Vaseline is invisible I can enjoy my containers with unrestricted view -- and so far it seems to be working... Who knows? :)

Top dress with lava rock or similar rough stone. They don't like digging in the sharp stones.

Good idea. I think gravel deters them a bit too (if it's a thick enough layer).

In my yard, they don't just dig plants. They yank them. They have taken to yanking out hens 'n chicks from planters amongst other things. Today I saw one nibbling on the tender young buds of my lilac. They are evil.

The only thing that stopped them at my last place was when new neighbors moved in with two outdoor cats.

Not a PC answer, but a little sprinkle of Sevin powder might help. I'm considering a pellet gun also. They are stripping my apples and believe it or not, they love tender hibiscus plants. My neighbor has a loquat tree, and the squirrels are lovin' that.