Juglone, the bad stuff is secreted by the roots of the walnut. The roots will give off the chemical as far out as the dripline, or further. How you would get the clematis growing on the branches (without being in the ground) is something I cannot fathom. Give your clematis a nice trellis and just accept that walnuts don't generally play well with other plants.

~Chills

Chills-
Thanks for the word juglone. I may have to give up, but I wondered if I could grow the clematis in a planter box rather than in the ground. How deep are clematis roots? Would a planter box not provide enough protection or insulation for the roots during winter?

Happy clematis in my experience are clematis with their roots plunged further down than I would want to dig. They root very deeply. Also, even though many clematis are pretty hardy, the problem with planters is one of lack of moisture or flooding and freezing in the winter.

If you're really stuck on the idea of having a clematis growing into your walnut tree, the obvious candidate would be Sweet Autumn Clematis. It could be planted outside of the dripline of the tree and if trellised up until it could reach the branches of the walnut, then it would likely spread itself through the tree pretty effectively.

The biggest problems you would face then is the clematis would compete with the tree for available light and each winter you would want to remove the previous year's growth (or your flowers will all end up at the very top of the walnut in just a couple years).

~Chills

Well, it was just a thought- probably won't work out. But knowing they have deep roots is helpful. I'll find a site for it yet. I like clematis on trellises, but I am also entranced by pictures of clematis growing up/on/through shrubs.

There are lots of plants that tolerate juglone. Call your extension agent and ask about clematis. I think they would look nice growing up a black walnut.

My neighbor has black walnut trees which are planted very close to the propertly line. I feel for you.

I had spent hours looking for a modern list of plants that are ok, but there are always new cultivars and you never know how one will react to the juglone. BTW, the juglone is mostly in the roots.

Here is a pretty decent list of plants that are ok and ones that aren't. The plants on the list came from field observations, not from testing in a controlled environment.

It's not much help, but that's all we have.

Here is a link that might be useful: Planting near black walnut trees

Janet, I had a list at one point but lost it in a major computer malfunction so THANK YOU for that link!

I plant sensitive plants in big cardbox (I think this is what it is)containers in the soil. I reckon that I give them an edge for about a year or two before they get established and then both parties can break the container and fight. This way I have had success with plants under a huge white pine tree, maple tree and close to the hedge cedars. Almost everything I planted there before died on me. I also made raised beds lined with the gardening fabric at the edge of the drip line and under a big maple tree and very close to mature, overgrown cedars. So far (four years) it has worked fine.
Clematis seems to be very resilient, by the way. I have it everywhere, and most of my places are around or under very possessive trees.

I don't know about cultivated varieties of clematis, but a native, (I think it's Clematis virginiana), has self-seeded and grows all too well immediately below our large black walnut & will go up the trunk somewhat. However, it mostly heads for the anchor wire fence & spills out over the ground when we don't get it trimmed back or pulled out. It's pretty, but very vigorous.

I left a Blue Light (double flowering) Clematis in a one gallon container all winter long this past season under my neighbors Black Walnut tree (on my own property). This last season was one of the coldest and snowiest we have had in years here in ST. Louis MO. Mind you it was touching the ground surface and insulated by dense died off vegetation from the previous season. I set the pot in the middle of the vegitation at the end of the season to see if it would survive. It has came back growing better than when I bought it last year at the end of july.