Named varieties of hybrid clematis bloom in two different ways: Some bloom on wood grown the previous year and some on wood grown in the same year. Clematis in the Jackman group of hybrids (such as jackmanii) bloom on new growth (wood grown in the same year). You’ll have to watch where the blossoms occur on the other clematis to determine if they are on new or old wood. If you wish to prune the Jackman clematis, do so in spring as the buds swell or slightly before. You can prune to within 4-6 inches of the base if you wish.
If the other clematis flowers on old wood, prune dead and broken growth in the spring. After flowering, a portion of old shoots should be cut back severely. Clematis enjoy a rich, humusy soil. Give them lots of organic matter such as compost or composted manure. Clematis prefer having their heads in the sun and their feet in the shade, so mulch is helpful to keep roots cool and to keep competing weeds at bay.