Clematis plants need just the right situation to thrive: rich, fertile, and fast-draining soil that is kept uniformly moist. They are not terribly resilient if their roots get dry, even occasionally, which could easily be the explanation for the lack of flowers in this very dry year. The roots of the plant need to be kept shaded and cool by other plants and the flowering vines should grow up into full sun. A thick mulch at the roots is appreciated.

Clematis are also some of those peculiar plants with complicated pruning instructions and cutting back at wrong time can limit or exclude flowering. (If you know what type yours is we can help with the right pruning advice. Email us at Plantinfo@NYBG.org.) But as the leaves of the plant turned yellow during the season, it seems more likely that something was happening with the growing situation rather than the pruning of your clematis.

One thing to investigate is whether any weed and feed products were used on a nearby lawn that might have drifted to the roots of the clematis or been carried there by water. Some grass seed and grass fertilizers also have weed suppressors in the mix and they can cause damage to nearby plants in a sneaky fashion. We see many garden plants, shrubs and trees that turn yellow or drop their leaves during the growing season for this reason and often the gardener was unaware that the toxic element was being applied in the turf treatment.

You should also check whether the garden area is getting too much or too little water. Both can cause leaves to turn yellow. If a drain spout empties nearby or water pools in the area during rain it will be too much for the plant. 

Courtesy of NYBG Plant Information Service.