I have a Niobe which is constantly wilting. I just read that Clematis needs an alkaline soil and my Niobe is planted behind 2 Rhododendrons. Could that be the problem?
According to "Simply Clematis", clems prefer a slightly acidic soil of 6.5 (above 7.0 is alkaline). Rhodos do like acidic soil, so I guess it would be a matter of how acidic your soil is. You might want to check out a clematis website such as http://www.clematis.hull.ac.uk/index.cfm for more detailed info. Are you sure yours is getting enough water? If it is clematis wilt, some have expressed that only young clems get it- if that's so, maybe yours will outgrow the problem. Brandy
Clematis have the reputation of needing alkaline soils only because the original plants in England were found to be growing in chalky soil. That being said, if you are augmenting the soil with something to make the soil more acidic for the rhodos, then pH could perhaps be causing a problem. Brandy points out another good possibility--lack of water due to the root competition from the rhodos. If the plant recovers from the wilted state after giving it a good long drink of water, I would suspect root competition over a pH issue.
I doubt soil pH is causing much of a problem. PNW soils all tend towards acidic conditions, some more so than others, and clematis of all varieties thrive in this area. Plants tend to be much more tolerant of a range of soil conditions, including pH, than most people expect - I've got both lilacs and clematis (with reputations for a preference for more alkaline conditions) growing very well and very happily adjacent to rhodies and other acid lovers.
There is a concern that shrubs (or trees) with wide spreading root systems - like the rhodies - may be competing with the clematis for soil moisture and nutrients. Additional deep watering and a healthy layer of organic mulch plus some additional time may encourage the clem to outgrow the wilt.