Dahlias (Dahlia spp.) grow well in large pots with drainage holes. Hardy in U.S. Department of Agriculture plant hardiness zones 7 through 10, dahlias grow 1 to 6 feet tall and 1 to 3 feet wide, depending on the variety. The bulbs of dahlia plants grow larger over the growing season, and the containers must be large enough to fit the developing bulb. Dahlias in pots also need regular watering and fertilizer to produce their large, showy blooms in shades of white, pink, red, orange, yellow, lavender or purple.
Very small dahlia varieties can grow well in 1-gallon containers, but most containers for dahlias should be about 12 to 14 inches wide and deep. Dahlia varieties that grow more than 3 feet tall need larger containers. Grow one dahlia plant per container. Heavy pots, such as clay containers, are best for growing dahlias because they are more stable in strong winds than pots made of different, lighter materials.
A mixture of garden soil and potting soil provides the best growing medium for dahlias in pots. Dahlias in only potting soil may grow poorly, because potting soil is too light and porous to provide the rich, moist soil these plants need. Fill containers for dahlias with a mixture of 2 parts garden soil with 1 part potting soil.
Dahlias give their best flowering displays in sunny spots, such as light-filled patios or south-facing areas of the garden, but dahlias in pots can overheat at the hottest part of the day. Place dahlias in containers in a sunny spot, but if the plants regularly wilt even when the soil is moist, move them to a place where they receive shade during the afternoon.
Watering dahlias in pots two or three times per week when the weather is dry supplies the plants' water needs. Water dahlias in containers when the potting soil and garden soil mixture is dry to a depth of 1 inch. Pour water over the potting soil surface until it flows through the container drainage holes. Wait until the water has stopped dripping from the drainage holes before returning the dahlia containers to their drip trays. In very hot weather, dahlias may need water every day.
Fertilizer for Dahlias
When dahlias in pots are established, regular applications of low-nitrogen fertilizer boosts growth and flowering. Fertilize dahlias in containers with a 15-30-15 liquid fertilizer diluted at a rate of 1/2 teaspoon per 1 gallon of water. Pour the fertilizer solution over the potting soil surface every two weeks from midsummer until early fall. You can apply fertilizer solution in the place of water if the dahlias need water.
Dahlias in Pots in Winter
Dahlias can stay in their pots through winter. When flowering is over in fall, dahlia stems and leaves die back, and the bulbs store energy for sprouting the following spring.
Dahlia bulbs are damaged by frosts. To prepare dahlias in pots for winter in frost-prone areas, prune the stems to 4 to 6 inches tall and move the plants in their containers to a dry, cool, dark, and frost-free area, such as a shed or garage.