A clematis won't climb an oak trunk without help from another support. Clematis climb by winding their leaf stems (petioles) around narrow support structures, so you would need wires or netting or a narrow-barred trellis for the clematis to climb. I don't know where you are, but the clematis I can grow won't get as tall as the lowest limbs on my oaks, so pruning wouldn't be a problem. I do grow clematis into shrubs like lilacs and Hydrangea paniculata which have narrow twigs and stiff branches that the clematis can use to climb. Some of the Clematis montana are tall enough to grow up into taller trees (20-30 ft.), but I am not sure it they will reach high enough to grow into a large oak's crown. They need little to no pruning, though, so it still wouldn't be an issue. They are only hardy down to zone 6 or so.

If you want a vine that will actually climb the oak's trunk, look into ones that grow with adventitious roots or little suction cup devices like Boston Ivy, Virginia Creeper, Schizophragma hydrangeoides, or Hydrangea petiolaris.

Thanks for the quick reply. I live in the mountains of Southern California, zone 7 or 8. I could easily put something up to help with support on the oak tree. It is a very tall tree, so maybe the montana would be a good idea. I'll check out some of the ivy, too.
I appreciate the help.

Not regular/English ivy if it is an invasive in your area, which it is in many warmer climates. I would be more likely to suggest the last two vines I listed since they bloom and the first two don't.

duplicate

This post was edited by nhbabs on Mon, May 19, 14 at 14:14

Just make sure it is a shade-loving clematis and you are good. I like 'Bees Jubilee' and it thrives here in Z5b in shade.

This post was edited by schiba on Sun, May 18, 14 at 20:45

nabhbs and schiba: Thanks for the advice. I ended up ordering a climbing hydrangea for the tree, and a clematis for somewhere else. I couldn't resist the clematis even though I haven't figured out where to put it.