How far apart to plant dahlias
Plant dahlia tubers 18 to 24 inches apart, laid flat on their side about 6 inches deep, as Colorado State University Extension's PlantTalk guide gives it. NC State Extension's plant toolbox allows a wider range of 12 inches to 3 feet, because dahlias run from 1 ft dwarfs to 6 ft giants, and covers tubers with 3 inches of soil. Use the close end for short cultivars and the wide end for tall ones.
The calculator opens at 21 inches each way, the middle of Colorado State's range.
How many dahlias fit a bed
| Space | At 21 in | At 18 in | At 24 in |
|---|---|---|---|
| 4 × 4 ft bed | 4 | 4 | 4 |
| 4 × 8 ft bed | 8 | 10 | 8 |
| 3 × 6 ft bed | 3 | 8 | 3 |
| 4 × 12 ft bed | 12 | 16 | 12 |
| 25 ft row | 14 | 16 | 12 |
Counts keep half a spacing clear at the bed edges and row ends, the way the calculator counts. In a 4 ft wide bed, 18 to 24 inches gives two rows of plants, which is why the counts barely change across that range; a 3 ft bed fits two rows only at 18 inches. In a single row, 25 ft at 21 inches takes 14 tubers.
Planting depth and timing
Colorado State lays each tuber flat on its side 6 inches deep, while NC State covers tubers to 3 inches, so 3 to 6 inches covers both. Do not water after planting: Colorado State waits until sprouts show above the soil.
Plant once the weather is warmer and the chance of a severe freeze has passed, since Colorado State finds that planting too early gives poorer blooms. NC State says the tuberous roots are injured below 25 °F.
Staking tall dahlias
Dahlias range from 12 inches to over 5 feet, and Colorado State says taller plants need staking. Give those tall cultivars the wider 24 inch spacing.
Dahlias in containers
Colorado State uses a container of about 12 by 12 inches per plant and plants at the same 6 inch depth as in the ground. The guide sizes the container to the plant's mature size, so give tall cultivars a larger pot.
Digging tubers in fall
Dahlias are not winter hardy in Colorado, so the tubers are dug in fall and stored. NC State digs them in zone 7, dries them without washing, and stores them in vermiculite or dry sand at 35 to 45 °F; in zone 8 a layer of mulch over the bed is enough.
To mix dahlias into a vegetable or cutting bed, use the raised bed planner or the general plant spacing calculator. For other cut flowers, see zinnia spacing.