Dahlia

About Dahlia

Dahlias grow from tuberous roots and bloom from midsummer until frost, in every color but true blue and in shapes from small pompons to 12-inch dinner plates. Bees, butterflies and hummingbirds visit them (NC State). Where winters are cold, dig the tubers after frost and store them indoors until spring.

NC State rates tubers for zones 7-10: mulch them in place over winter in zones 8-10, but dig them in zone 7 and anywhere colder, since temperatures below about 25 F injure them.

When to plant

Timing is counted from your average last spring frost (and first fall frost). Sow & Tell works out the actual dates for your ZIP code.

• Start seeds indoors: 6 to 8 weeks before the last frost
• Set out plants: around the last frost

Plant tubers outdoors after frost danger is past and the soil is at least 55 F, 3-6 inches deep with the eye up; for earlier flowers, start them in pots indoors 6-8 weeks before (Clemson, NC State, Chicago Botanic Garden). In cold areas, after frost kills the tops cut stems to 6 inches, lift the clumps a few days later with a fork, let them dry 1-3 days, and store in peat, vermiculite, sand or wood shavings at about 35-50 F, checking monthly for rot (Penn State, UMN, NC State, Chicago Botanic Garden).

Soil

Rich, moist but well-drained soil high in organic matter; Chicago Botanic Garden adds compost and aims for a pH near 6.5. Mulch and water often (NC State).

Mix compost into rich, well-drained soil and aim for a pH near 6.5 (Chicago Botanic Garden).

Feeding

• Monthly through the summer, once plants are growing: water-soluble fertilizer, or a low-nitrogen fertilizer such as 5-10-10. Clemson suggests a monthly water-soluble feed, or 2-3 lb of 5-10-10 per 100 sq ft in July; Chicago Botanic Garden feeds regularly with 5-10-10 or 5-20-20. Follow a soil test.
• Mid-August: no more fertilizer. Chicago Botanic Garden stops feeding in mid-August so the tubers can mature before fall digging.

Keep water off the leaves to limit fungal disease (Chicago Botanic Garden), and set stakes before plants reach about 12 inches tall (Chicago Botanic Garden).

Harvest

Dahlias bloom from midsummer until frost. Pinch the main tip about 3 weeks after planting for a bushier plant, stake tall kinds, and deadhead to keep flowers coming (Clemson, NC State). For extra-large blooms, pinch off the side buds (Chicago Botanic Garden).

Varieties (10)

Extension and botanical-garden sources we could open document 10 dahlias; many popular named dahlias are described only in catalogs, which we don't use.

Pests to watch for

Aphids, Twospotted spider mite, Thrips, European corn borer, Four-lined plant bug, Foliar nematodes

Diseases and problems

Powdery mildew, Gray mold (Botrytis), Mosaic viruses, Crown gall, Tuber rot, Phytophthora root rot, Cucumber mosaic

In the kitchen

The Aztecs grew dahlias as a food crop, but the tubers are rarely eaten today (NC State).

Safety: Leaves can upset the stomach if eaten in large amounts, and the sap can irritate skin (NC State).

Pets: Toxic to dogs, cats and horses (ASPCA): usually mild stomach upset and mild skin irritation. NC State also rates it low severity.