Adapted from the CO-Horts blog originally authored by Alison O’Conner, Larimer County
Planting dahlias is no different than a daffodil or tulip. Yes, they do need to be dug from the ground in fall (they are not winter hardy in Colorado), but properly stored and cared for, dahlias (and other summer-blooming bulbs) can last for years.
Dahlias, if purchased via gardening catalogs, will generally be shipped once chilly weather passes for the local climate. At garden centers, Dahlia rhizomes are available earlier than the optimum planting time so that gardeners can choose early for best selection. Planting too early will result in poor blooms, so it’s important to wait until the weather is warmer and the chance of severe freeze has passed.
Upon buying your dahlias, read the instructions sent by the grower. One of the best things about planting dahlias is that they don’t need water following planting—only after sprouts appear above the soil surface.
Carefully inspect your tubers and make sure they are firm without any signs of rot. If they have sprouted, you can snip back the new growth to a length of one inch. This will not hurt the dahlias and will actually encourage better growth.
Plant dahlias in a sunny location in your landscape. If the area is mulched, rake it back to expose the soil and start digging. Dahlias need to be planted flat on their side 6″ deep and 18-24″ apart. The plants can get quite large at maturity (anywhere from 12″ to 5+ feet; taller plants will need staking) so proper spacing is necessary.
If you’re planting all your dahlias in one location, it may be easier to dig a trench and place the tubers, properly spaced, in the trench. A handy tip to remember where you planted your tubers is to mark each one with a flag or golf tee. Cover the tubers with soil, add mulch (in a thin layer) across the planting areas and, if desired, mark the location of the tubers. After the tubers sprout, you can irrigate as needed. Dahlias tend not to be xeric, so regular irrigation is necessary.
If you don’t have garden space, dahlias can easily be grown in containers. Just follow the same spacing and depth requirements mentioned above. Also consider the mature size of the plant and your container size–ensure it’s big enough to support the plant at maturity (generally a 12″ by 12″ container will be large enough). Cover the tuber with a few inches of soil and water it in. Be careful about watering until growth starts…but don’t let the potting media dry out. Also make sure your media is well drained so the tubers don’t rot. You’ll also need to fertilize throughout the summer.
For the official CSU Extension Fact Sheet on summer-blooming bulbs, click here.