I have a Pink Champagne and a Nelly Moser clematis, planted 7 years ago. They have a trellis supported by a fence. For several years they flowered beautifully, but the last two years they exploded with vine growth, so heavy it grew in mats and this strangled out most of the flower buds. The only thing I did different was give them a large topping of homemade compost in April. No chemical fertilizer. What can I do to calm down their growth? Thank you.
Bone meal added to the soil should help with the blooms. As for keeping the vine in check, it may be necessary to prune. This article should help: https://www.gardeningknowhow.com/ornamental/vines/clematis/growing-clematis.htm