To grow dahlias, get them started indoors six weeks before the last expected frost, then transplant them outside after the frost has passed. Water the plants consistently to keep the soil moist, fertilize them weekly and stake tall plants.Continue Reading
Six weeks before the last expected frost, fill a 5-inch pot with all-purpose potting mix. Plant each tuber, and place the pot on a sunny window sill. Keep the soil consistently moist, without saturating it. Transplant young seedlings into individual pots.
After all frost has passed, find an outdoor area that gets full sunlight, and put down rich soil with plenty of drainage. Space the tall plants 20 inches from each other, 3 to 4 inches deep in the soil. Space the short plants 10 inches apart, and 2 to 3 inches deep. Plant tall dahlias in the back of a border, and average sized dahlias grouped together, or with other flowers.
Keep the soil damp, and conserve moisture by mulching regularly. Fertilize the plants with a balanced liquid fertilizer on a weekly basis.
To prevent tall dahlias from toppling, use soft twine to tie them to stakes. For additional support, add two stakes in a triangle shape when necessary.