That's a great question and I'm not sure there is a definitive answer. However, in my experience with barrier, the only thing I've seen penetrate it is bamboo rhizomes. I am inclined to think that tree roots would tend to be deflected when they encounter the barrier. Over quite a few years, the roots could get bigger and bigger and provide pressure that might deform the barrier a little, but I doubt this is going to be a major problem, and I'd be inclined to not worry much about this. However, if the barrier does get breached in 20 years or so, please repost!

Thanks Kudzu! Another question, say a rhizome penetrates the barrier say 20 inches down, will it have the tendency to climb strait up to the surface again? I am wondering if rhizome pruning on the other side of the barrier would be a good preventative measure. My fence line is 2 feet or so from my longest running barrier, and about 15 feet from the fence line on a couple of others, and I want to do the best I can to prevent the Boo from invading my neighbors property. The barriers in question are containing Bissetti, Rubro-Marginata, Vivax, Henon, Bory, and what I think is some Aurea. I know that pruning on the side with the Bamboo is a good idea, but it I think it may be too late for that in some of my contained areas.

My own experience with running rhizomes is that they tend to travel relatively close to the surface (2"-10"), and I think that any rhizome diving as deep as 20" will want to come back closer to the surface...but maybe not right at the barrier. One other thing you can do, if you are really concerned, is to dig a narrow ditch maybe 18" deep at the perimeter of your property, and fill it with sand or bark chips. This will be much easier to sink a shovel into when you want to check for escapees.