From the time you plant your dahlia tubers to the time they start to bloom can be more than three months. But they are worth the wait. Be sure to follow these tips and tricks to ensure your dahlias put on their best show.
1. There’s an exception to the full-sun rule
In areas that have summer temperatures over 100°F for much of the growing season, dahlias should be planted in partial shade (preferably morning sun and afternoon shade).
2. You may have to prune before you plant
If your tuber has a sprout of more than 2 inches before planting, be sure to clip it back to about 1 inch to avoid a weak stalk as it grows. Dig a hole 4 to 6 inches deep. Lay the tuber on its side, as if you were putting it down for a nap, then cover it with soil.
3. Stake them early
For varieties that get more than 3 feet tall, put in a garden stake when you place the tubers in the soil.
4. Low-nitrogen fertilizer is your friend
Dahlias are heavy feeders. Fertilizer generally should be first applied 30 days after planting and then repeated every three to four weeks throughout the season. You can use either a liquid or a granular fertilizer, but make sure that it is low in nitrogen. Too much nitrogen results in a lot of foliage and not as many flowers.
5. Do NOT water after planting
You don’t have to water until the sprouts appear above the soil. Up to this point the plant has been living off the tuber while it develops roots and cannot process water or fertilizer until roots have begun to form. Once sprouts are up, water if needed. The exception is in hot, dry climates where limited watering at tuber stage may be necessary.
6. Summer moisture is essential
Once the drier part of summer begins, you may need to water two to three times a week—deeply. Water must penetrate the soil past the dahlia roots, at least 8 inches into the soil. Soaker hoses or drip irrigation are ideal.
7. Hot water makes the blooms last longer
Choose blooms that are about two-thirds open. Cut the stems at least 12 inches long. Place the cut stems in 2 to 3 inches of very hot water (160° to 180°F—not boiling) in a plastic bucket. (Glass cools too quickly.) Leave them there until the water has cooled, usually at least an hour. You will see a darker water mark on each stem. This sets the blooms and helps them last.
8. Store them after the frost
When the dahlias have turned brown or have stopped producing and are no longer actively growing in late fall, it’s time to harvest the tubers for winter (in Zones 2–7). Dahlia tubers can be stored in cardboard boxes lined with newspaper and filled with peat moss or sawdust that is slightly dampened. Ideal storage temperatures are about 40° to 50°F.
To learn more about dahlias, check out the full Out-of-the-Ordinary Dahlias article.
Nicholas Gitts is a second-generation farmer and owner of Swan Island Dahlias in Canby, Oregon.