Dahlia ‘Decorative Dark Red’. 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 +/- 80-90 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.