Early to Mid-Summer
This group of large flowered clematis are selected because they flower before June (many will flower again) and for the most part they require light pruning to achieve best flowering performance.
Tips for the Early to Mid-Summer Clematis.
These clematis prefer a good quality soil with high nutrient levels and a soil which is moist but free-draining. Improve planting holes with at least 2 forkfuls of well rotted manure or garden compost and if you don't have these use a 'Soil Improver' compost. The Improvement material added should be placed under the root of the plant not around it. This encourages the roots to grow downwards and remain deep and cool during summer.
If your clematis flowers in May-June(late spring), cut the dead flower heads when they have finished to encourage a second set of flowers to be produced in the late summer and autumn. Be prepared to wait for a couple of years before they will be at their best - Our saying is - 1st year = Roots, 2nd year = Shoots, 3rd year = Flowers!