Dahlias are a late-season flower with bright, colorful blooms. They are related to daisies, and bloom from late summer into autumn. Dahlias come in sizes ranging from dwarf to giant, and the smaller varieties are well suited for pots and window boxes. Cutting dahlias encourages further blooming, so take advantage of the flowers inside and out. While lumped with summer bulbs at garden centers and seed catalogs, dahlias actually grow from tubers, which are pieces of root specifically formed to propagate the plant.
Fill containers and pots with two parts compost to one part peat moss. Choose containers that allow plenty of drainage. Clay and terracotta pots wick away excess moisture from the soil and work well for dahlias.
Plant tubers outside in pots after the last frost for your area, or encourage earlier blooming by planting them inside six weeks before the last frost. Choose a sunny location for your pot placement.
Dig a 6 inch deep hole for each bulb. Space holes 18 inches apart. Place the bulb inside the hole so it sits horizontally. Fill the hole with soil.
Plant dahlia bulbs in moist soil and avoid watering until the shoots of the plant show above the soil's surface. Water often enough to keep the soil moist to the touch, but not wet. Containers dry out quicker than garden beds, so check the soil moisture with your fingertips one or two times a week.
Fertilize with low-nitrogen fertilizer four weeks after planting, and again one month later. Fertilize once in the spring thereafter.
Leave containers outside until the first frost forces the bulbs into winter dormancy. Move containers into a a cool place, such as a garage, enclosed porch or basement to overwinter.