The only way to tell if the plant needs a larger pot is to see if it is rootbound by gently taking it out of its pot or seeing if you can see roots coming out of the drainage holes in the bottom of the pot. It could be that it is rootbound and needs replanting. As for changing the soil, I have had clematis in pots for several years and not yet needed to change the soil in the pots.

The one other question I have for you is what did you do with the pot over the winter? If you left it outside in an unprotected pot, it could be that did the plant in if the pot froze solid and the plant could not take up water during the winter months. Could be that the crown rotted if it stayed too wet. There are a couple of things that could be going on with your clematis in my opinion since it sounds as if the ones in the ground did okay.

Miquel hit the nail on the head..My first thought was oh no..potted plants in your cold zone are in more danger than those planted in the soil for if left outside to fend for themselves as they have no insulation like mothernatures soil...I would have put my pots either in a warmer garage or stored somewhere in my house till Spring...Jeanne

I haven't brought any of my clems in for the winter and haven't had any problems yet. We got a lot of snow this winter, and I always hunker them down into a snowbank near the house so they aren't exposed to the really cold temps. We don't have a garage and our house is too small to bring pots into, or I definately would!

So, I guess my next question is how in the world do I get this plant out of the pot to check the roots without severely damaging it? It's still alive, it just isn't being "vigorous" at all.

Being alive and showing growth is ok and wonderful..I wouldn't bother it to be honest..some are slower to get going than others and isn't it just beginning Spring there??...Jeanne

Ok. Bad news. I took it out of the pot and it was rotten. *sigh* Guess I get to buy another one. Maybe I should just wait until I have a real place to put one before I buy anymore.

ahhh..so sowwy to hear that....Jeanne