It won't bloom! Why does my clematis not flower?

There are three main reasons why a clematis doesn't flower:
1) The plant is still immature and hasn't reached flowering size yet. Clematis will take up to three to five years to become a fully mature plant, but they should carry a small number of flowers even while still young.

2) The most common problem with vines that produce an over abundance 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 (see Pruning FAQ for a full explanation of prune groups), 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.

3) 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 years growth. If pruning removes this new growth, the plant will not flower. Please also see the Pruning FAQ to get a better understaning of pruning requirements and how this affects flowering.