I have a beautiful clematis Montana which stretches across my 10 feet wide trellis but over the last few years, on one side the flowers have turned yellow while the rest is still pink (the original colour ).I have had the plant for about 25 years, is this normal?
Regards
Evelyn

Could it possibly be a self-sown seedling you didn't notice until it bloomed? That's one possible reason for a change in flower color.

I think Kyle must be right. I've had it happen with a Jackmanii but not the montana.

My C. viorna reseeds everywhere. It is weedy but I love it. Less so my C. crispa but quite a bit. I even have seedlings from some named cultivars.

The only other explanation would be if it was grafted. If purchased from a nursery as a blooming sized one it could have been grafted. With age the other one surfaced, which happens with older grafts. I have no clue as to why some clematis would be grafted. They are roots. But if it did not come from a nursery potted I would agree with a seedling getting in there. A seedling can also take years to surface.

May joy find you every day!