That's looking really nice, I need to find one of these. I pulled some common ivy that I found growing in the yard, but the trunk at 1/4" caliper didn't grow at all even after putting out 4-6 feet of growth. So, my guess is that I'll need to dig something more established, that maybe has grown up a tall tree and has some caliper to it. Reminds me of porcelain berries - I dug some a couple of years ago, and even with extensions of a good 20-40 feet (these were growing across a fence, storage shed, other shrubs, etc.), the trunks were only about an inch and a half caliper.
This might be a little off topic, but over the weekend I was flipping through "Four Seasons of Bonsai" by Kyuzo Murata. It's a really great book with pictures of close to 200 trees/vines/grasses/ornamentals from his garden. In a day when collected Rocky Mountain trees that are hundreds of years old are all the rage, it was a good reminder that "bonsai" can mean lots of different things, and that one can have an equally nice collection using a variety of native plants of varying sizes and in different compositions.