The best fertilizer for clematis is a complete fertilizer that contains equal amounts of nitrogen, phosphorus and potassium. This kind of fertilizer is sold as 10-10-10 fertilizer.
The best time to apply fertilizer to clematis is in the spring. Gardeners work the fertilizer into the soil around the plant when the buds are about 2-inches long. Depending on the quality of the soil, additional applications are valuable approximately every six weeks throughout the summer.
In addition to fertilizer, clematis benefits from frequent watering. The roots need additional deep watering whenever the soil is dry. A thick layer of mulch helps keep in moisture and protects the roots from the sun.