I would not suggest trying to plant a clematis in the ground around where ivy has been, even if you have pulled out most of the ivy roots. Ivy, at least in my experience, is tenacious and will come back if not all the roots are removed. If you assume you have removed all the roots, plant the clematis in the ground, and the ivy returns, you will then have to try and kill the ivy with an herbicide and it might get on your clematis even being as careful as you might be. Herbicides on clematis will mean a certain death for the clematis. Chester, if you want to go ahead with your planting scheme, I would go the way of the barrels. Just make sure you keep the ivy from growing into the barrels and intertwining with your clematis vines.

I have been wanting to grow this myself for its beautiful color but been afraid of it being too aggressive. What I have found from research on it is that well it is aggressive just as much as the english ivy. To cover the fence with a good thick cover will probably take three summer seasons. Ivy rule goes something like this: first year it sits, second it creeps, third it leaps! Couldn't think of a rhyming word for the first year, ha, ha. I have found this to be true with my english ivy so should hold true for the boston. By the thrid year make sure you have a regular maintenance plan for keeping it contained to your desired area as it will keep climbing and reaching. Good luck!

I would also assume that it is legal in PA, but you can always check with your cooperative extension and see what they say. You mentioned that your slope is southfacing, so I assume that it gets sun. That should make it easier than a shady site to get something to grow. I am probably not the best expert, but what about forsythia bushes? The one I have in my backyard is prolific without being uncontrollable and I think would be deeply rooted enough to help stabilize the slope. Also I would think that they would be fairly inexpensive and common at the big box stores so that you could get a bunch in without breaking the bank. My other thought was some low growing evergreen bushes. I have some on my sunny slope and they are doing quite well. For the shaded portion of my slope I am looking at planting some hydrangeas (once I get the ivy out) and maybe some ferns. Litrope would also be a good choice and is sold at the box stores. I would consider digging it up now while it is not completely established and not out of control. Like a misplaced tree, it will just get to be a bigger proble to solve later if it is left to flourish now. Plus I would expect to find it establishing other places in your yard (far from where it is planted) and becoming a problem in many spots. Last, I know that in my english ivy is where the poison ivy starts. I only have poison ivy where the English Ivy is and that is a bigger problem! Here is a link that might be useful: MD Suggestions for Groundcovers

My feeling is the same as Smivies, though I have seen climbing PI here and I have seen vines other than PI with aerial roots. Though a bit out of focus, the smaller vines look more like Virginia creeper/Parthenocissus quinquefolia. Be aware that some folks react to Virginia creeper as well as to PI, so treat both with respect. Once leaves emerge, you can repost to get a firm ID.