The most common problem with vines that produce an overabundance of green growth and no flowers is caused by a nutrient imbalance, usually too much nitrogen. It is best to fertilize clematis with organic, slow release fertilizers when you prune group three clematis, usually in late February or early March. Fertilizers are characterized by 3 numbers representing the relative concentrations of nitrogen, phosphorus and potassium (N-P-K), in that order. Plants use nitrogen to grow foliage, phosphorus to develop root systems and form fruit, and potassium to promote flowers and develop resistance to disease. Choose a fertilizer with a low nitrogen number relative to the phosphorus number, such as 5-10-10, so that your plant will develop blossoms and roots, rather than lots of foliage. Some bulb or tomato fertilizers have concentrations similar to this. A second dose of fertilizer is recommended in June. Encapsulated three month slow release fertilizers are also a good choice.
Untimely pruning can also result in the loss of flowers, especially with the early flowered cultivars (these types include C. alpina, C. macropetela, and early large flowered hydrid types). These types of clematis form their flowers on the previous year’s growth. If pruning removes this new growth, the plant will not flower. -Birdy