I've knocked a few patches down with Roundup, but it never grows much over a foot or so up here in the north because of the winters. Not so in Florida however. We were out birding and the local curator at the park said to be careful of the poison ivy. He pointed out a tree just off the trail that had it growing up and around that tree at least 40' high!!! I had no idea that it would grow that large, but if it is a true ivy, it can climb. I was amazed.
