Dahlias were named after Andreas Dahl, who considered the dahlia a vegetable more so than a garden plant because of its edible tuber. The plants have circular blooms that are made from many tiny petals of orange, red, yellow or white. The dahlia will bloom from mid-summer until the first good frost in fall, in USDA hardiness zones 3 to 10.
When to Plant
Plant the tubers in spring when the ground when the soil is warmed. The time to plant the tubers is when you plant root vegetables such as radishes or carrots when the days are longer and warmer and there are no chances of a frost. If you want an early bloom, you can start the tubers in the house with good sunlight one month before you will plant the plant outdoors. This will allow the plant to start spouting and bloom earlier than if you plant it outside as a tuber.
Where to Plant
Plant the dahlia in soil that has good drainage. You can grow dahlias in pots or in the ground, but the plant must be in a sunny location where it will receive full to partial sun each day. The plants or tubers should be planted at least 2 feet apart to allow for spreading during growth.
Japanese beetles and slugs can destroy the blooms if not removed. Slugs should be removed in the mornings and Japanese beetles must generally be removed to a bucket of soap water to kill them. You can use a insecticide for other pests such as earwigs, which will leave bit marks in the leaves, but you must choose a insecticide that is safe for the dahlia and the blooms such as an insecticide that contains chlorpyrifos or carbaryl.
If the area where the dahlia is planted receives at least 1 inch of rain every week, you do not have to water the plants. If you don't receive this much water, you can water the soil so that the soil receives at least 1 inch of water a week. If the dahlia is planted in a pot, it may require more water. When planting in a pot, add enough water to soak the soil and allow to dry until just damp before watering again.
Feed the plant with a high nitrogen fertilizer such as 10-10-10 (nitrogen, phosphate, potassium) once a month through the middle of summer and reduce the nitrogen content to 5-5-5, once a month for the remainder of the growing season.
Remove the tubers from the ground before the cold sets in. You will see many new tubers that have developed from the mother tuber. These tubers can be divided and planted the next spring as single plants. Keep the tubers in the house in a cool, dry place such as a basement or in a plastic bag.