Not a stupid question. The answer depends. Some of mine have grown up and over and are blooming more on the neighbor's side which irks me to no end but the neighbor's side is sunnier. They are planted on the east side and grow to the west.

Ivy is really tough. I sprayed Round up over our fence at the ivy that was growing behind the neighbor's shed up against our fence for years. Don't think the neighbor even knew it was there. Round up is not active in soil, just foliage so if you don't spray the clematis it should be fine. If there is ivy growing where you want to plant you are going to have to dig it out first. It will choke the clematis.

thanks for the tip about the ivy. I can't stand it.

I'm not a fan either. It smothers great swaths of woodlands here. Choking everything in it's path. It is listed as a noxious weed but people still have it. Some of the miniatures are pretty in pots in the house but the rest of it is a menace. It is almost as bad as kudzu here. I mean as bad as kudzu is in the South.