How-to Guide
How to Grow Dahlias
The short answer
Dahlias are tender tuberous perennials grown as summer annuals in cold climates. Plant the tubers in spring after all danger of frost has passed (Clemson Extension), in full sun and rich, well-drained soil — University of Maryland Extension experts suggest a planting depth in the 3-to-5-inch range and stress that well-drained soil is critical because wet, clay-heavy conditions rot tubers. Space large varieties 3 to 4 feet apart and smaller types about 2 feet apart.
In fall, University of New Hampshire Extension says to wait until top growth dies back or is killed by the first hard frost, then lift the clumps gently, cure them a few days at 60-70 F, and store them chilled — below 50 F but above freezing — packed in moistened vermiculite, wood shavings, or sand.
When and how deep to plant
- 1Wait for frost-free spring weather — Plant after the danger of frost has passed (Clemson). Emerged dahlia shoots are killed by freezes, so do not rush.
- 2Plant 3-5 inches deep — University of Maryland HGIC guidance puts tubers in the 3-to-5-inch depth range in well-drained soil.
- 3Give them room — Clemson spacing: 3 to 4 feet apart for large types, about 2 feet for smaller varieties. Set stakes for tall varieties at planting time so you do not spear the tubers later.
Sun, soil, and water
Full sun and rich, well-drained soil are the Clemson prescription. Drainage matters more than fertility: Maryland Extension flags wet or clay-heavy soil as the main killer of overwintering and newly planted tubers alike.
Digging tubers in fall
- 1Wait for the first hard frost — UNH: wait to dig until top growth dies back or is blackened by the first hard frost — the extra weeks let tubers mature.
- 2Lift gently — Loosen soil around the clump with a garden fork or spade and lift carefully. Tuber necks are delicate; cuts and skinned spots are entry points for disease.
- 3Cure a few days — Set clumps in a well-ventilated spot at a constant 60-70 F, out of direct sun, for a few days. Remove soil before storing — clinging soil carries decay organisms.
Winter storage
Store tubers chilled — below 50 F but above freezing — packed in moistened vermiculite, wood shavings, or sand inside plastic bags or cardboard boxes, with medium covering the tubers. Label each clump with a felt-tip pen, then check periodically through winter and pull any rotting tuber before decay spreads (UNH).
⚠ Do not store dahlias where they can freeze, and do not seal damp tubers in airtight containers — both are fatal. A spot that stays roughly 40s F, like many basements, is the target.
Dividing
UNH notes clumps can be divided in fall or spring; fall division makes for easier storage of the smaller pieces. Clemson describes spring division after shoots sprout and reach no more than an inch long, which makes the viable growing points easy to see. Each division needs a piece of crown with an eye — a tuber without an eye will never sprout. In zone 7 and warmer, Maryland Extension notes gardeners can instead leave tubers in the ground with mulch and frost protection.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- University of New Hampshire Extension — When should I dig dahlia tubers and how should I store them? — Dig after first hard frost kills tops; cure a few days at 60-70 F ventilated; store below 50 F above freezing in moistened vermiculite/wood shavings/sand; check in winter; divide fall or spring
- Clemson Cooperative Extension HGIC — Summer- and Fall-Flowering Bulbs — Plant in spring after frost danger; full sun, rich well-drained soil; spacing 3-4 ft large / 2 ft small; spring division when shoots are no more than an inch long; 3-4 inch mulch for winter protection in milder regions
- Ask Extension (University of Maryland HGIC) — How deep to plant dahlia tubers — 3-5 inch planting depth; well-drained soil critical, wet/clay soil causes rot; mulch and frost protection allow zone-7 overwintering