Dahlia ‘Cactus Purple’. dahlias are rewarding summer bloomers that will give you months of pleasure. Dahlias bloom all summer from July until the first frost. These dahlias grow to +/- 90-100 cm. high. Sometimes it is necessary to support the flower stems. Dahlia tubers can be planted in the spring. Dahlias are sensitive to frost, so be careful with late night frost. Dig a spacious planting hole, loosen the soil and fill the planting hole with good potting soil. Carefully place the tubers in the planting hole and cover with 3-5 cm. soil. The planting distance of the tubers is 50 cm. Water the tubers immediately after planting. dahlias bloom most profusely in full sun. Store the tubers in a frost-free place after flowering and you will enjoy them for years to come. Dahlias combine well with perennials and other dahlia species. The flowers are real eye-catchers in the border. Dahlia is also an excellent cut flower and has a long vase life. The best time to cut dahlias is early in the day. Immediately place the stems in lukewarm water with cut flower food.