I think I like the idea of pruning some roots from your lazy clematis. You can also try adding a thick mulch 4 to 6 inches on top of the roots of the clematis. A mulch is most commonly bark chips, compost or wood chips. This will help to keep the roots cool (clematis hate hot roots) and block out the weeds as well. My own clematis that refused to flower finally made buds after I root pruned it with a shovel, then watered the foliage with MiracleGro plant fertilizer. A dose of Osmocte sprinkled around that stubborn clematis will also help to wake up your vine and sing 'this bud's for you'......Keep growing, Marianne Binetti