How Big of a Container Do You Use for Small Dahlias?

Size

• Dwarf dahlias grow up to 12 inches tall, and even the smallest dahlia needs a square container of at least 12 inches by 12 inches or a round one that is 12 inches in diameter. Use larger containers for larger varieties of dahlias, depending how wide they spread when fully grown. Dahlias that grow to over 3 feet tall need a much larger container. The general rule is that while the branches of the plant can spread outside the container, the stem and root ball must fit comfortably in the container, with at least 3 inches of soil all around. Dahlias in containers need at least 12 to 14 inches of growing medium, so the container must be deep enough for this.

Container

• Dahlias cannot tolerate soil that dries out, so choose a container made of a nonporous material. Porous materials, such as terra cotta and clay, lose water faster than nonporous ones. Suitable materials include plastic and fired clay. Alternatively, line a porous pot with plastic to preserve the moisture. If you plan to move the container around the garden so the dahlia receives the most sun, choose a lightweight material. Dark-colored containers heat up in the sun and dry out the soil, so the best choice for dahlias is one that's a light color. If you choose to use a container made of wood, choose cedar or redwood as these are resistant to decay.

Growing Medium

• Dahlias need well-drained soil, so ensure that the drainage holes at the bottom of the container are open, and use filler -- such as stones, rocks or plastic bottles -- in the bottom of the container so the plant roots don't sit in wet soil. Cover this with about 14 inches of growing medium. Dahlias grow well in potting soil mixed with either peat moss or garden soil to better retain moisture. Use one-third potting soil and two-third garden soil, and mix a handful of bone meal for the plant's nutrition.

Planting and Care

• Plant each dahlia tuber about 6 inches deep, and water sparingly so the soil is slightly damp and not wet. When the plants have reached 12 inches high, water them thoroughly up to three times a week. The soil should remain damp yet not water-logged. Place the container in a sunny spot, though dahlias tolerate partial shade, as well. They can become overheated and wilt even though you're watering them regularly. Move the container to a more shaded spot with less afternoon sun. Dahlias are susceptible to poor air circulation, resulting in a white, powdery mildew on their leaves. Ensure that the dahlia is not crowded by plants in nearby containers and that its own container is large enough so the plant branches are not forced into close contact with each other.