Clematis montana is the most vigorous of the deciduous Clematis species flowering in late spring on the previous year's ripened growth.
Pruning
of established plants is usually kept to a minimum. Winter-damaged
growth should be removed in spring. After flowering overlong
shoots can be cut back to healthy buds. The simplest way to
do this is to use shears to clip over the plant to keep it
tidy.
Due to their vigour older plants, especially if neglected, may become a mass of congested growth. It may be possible to thin out this growth, but often the only practical solution is to cut all the stems back to within a few inches of the base. If this is done in early summer and the plants watered and fed during the growing season they should flower the following spring but if the pruning is carried out in winter a whole flowering season will be lost.