In the Pacific Northwest, where I garden, the clematis Jackmanii, Nelly Moser, and Niobe are the longest-blooming and easiest to grow. These three are more resistant to disease and bloom later in the summer. Most important is to know that clematis must have moist, cool soil. To achieve this, shade the roots with a low-growing plant and add a layer of compost. Clematis also need fertilizer every spring to bloom best. Pruning these three clematis is as easy as pruning back the long vines to a healthy bud in early spring. You can also choose not to prune and let clematis climb up and over trees and shrubs, but without pruning, the flowers will only develop on the tips of the vines where the new growth forms.   Pruning creates shorter vines with flowers from the top to the bottom. Enjoy this queen of the vines and Keep Growing, Marianne Binetti