Dahlia

Dahlias are most often grown from tubers, and planting tubers is the most common way to propagate them. Dahlias can also be grown from seed. Growing from seed is how new varieties are first created, and the plants that come from seed do not look the same as the plants that made the seed.

Indoor start. Sow dahlia seeds indoors 4 to 6 weeks before the last spring frost. Plant them about ¼ inch (6 mm) deep in seed-starting mix and keep the soil evenly moist until they sprout. At 65 to 70 °F (18 to 21 °C), seeds usually sprout in 3 to 5 days.

Outdoor sowing. You can also sow directly outdoors after the last frost, once the soil reaches 65 to 70 °F (18 to 21 °C). Sow thinly, about ¼ inch (6 mm) deep, and keep the surface moist until seedlings appear. Thin the seedlings to 9 to 12 inches (23 to 30 cm) apart after the first true leaves appear.

Transplanting and spacing. When seedlings have one or two sets of true leaves, move them outdoors to a well-drained spot in full sun. Give each plant room to grow, about 1 to 3 feet (30 to 91 cm) apart depending on the variety’s full size.

Soil. Dahlias prefer a soil pH of 6.5 to 7.0. Plants started from seed produce tubers that can be dug up in fall, stored, and planted again in spring.

Watering. Keep the soil consistently moist, never soggy and never dry. Avoid overhead watering when blooms are open, since it can cause bloom spot and bent stems.

Feeding. If fertilizer is needed, use a low-nitrogen type such as 5-10-10. Too much nitrogen leads to weak stems, few blooms, and tubers that rot or shrivel in storage.

Staking. Dahlias taller than 36 inches (91 cm) usually need stakes. Stakes should be about 4 to 5 feet (120 to 150 cm) tall.

Deadheading. Remove spent flowers to keep plants blooming.

Aphids, earwigs, slugs and snails can all attack dahlias, and mice, voles and gophers can eat the tubers. Powdery mildew shows up as a gray, powdery coating on leaves; watering at the base of the plant and keeping air moving around it help prevent fungal problems. Stunted growth and yellow streaks can signal a virus, which has no treatment, so infected plants should be removed.

Cut stems in the cool of the morning, when blooms are open or nearly open, because buds will not open after they are cut. Cut each stem on a slant, just above a set of leaves.

After a hard frost kills the stalks, cut them back to 2 to 4 inches (5 to 10 cm) above the ground. Leave the clumps in the ground for another six or seven days to cure, then dig them up carefully. Wash off the soil, let the tubers dry for 24 hours, and pack them upside down in boxes covered with vermiculite, peat moss or wood shavings. Store them in a cool, dry place at 40 to 50 °F (4 to 10 °C).

Dahlias are listed as toxic to both dogs and cats.