How to Plant Dahlia Tubers
True stars of the cutting garden, Dahlias come in myriad shapes, colors, and sizes with more than 57,000 cultivars. Their incredible diversity is due, in part, to chromosomes: Dahlias are genetic octoploids, meaning they have eight sets of chromosomes (most plants just have two). Flowers range in size from petite "micro dahlias" to "dinner plates" (some reaching 14" wide!). The diverse flower shapes include collarettes, waterlilies, pompons, cactus, balls, and decorative formal types–among others.
Dahlias can be grown from seed or by tuber. Seeds from cross-pollinated Dahlias will produce offspring only partially resembling the parent plant, which can be exciting for gardeners who enjoy experimenting (for seed, take a look at Dandy Collarette). But many gardeners prefer the predictability of tubers, which reproduce the original traits of the parent plant.
Tubers are funny looking things; it's almost hard to believe a flower of such symmetry and elegance begins its season as a potato-like "ugly duckling." But take good care of these humble tubers at the start and you'll enjoy gorgeous cut flowers from summer to frost.
Once established, dahlias are relatively low-maintenance. Here are a few tips:
Where and When to Plant Dahlia Tubers
Native to Mexico and Central America, Dahlias are sensitive to frost. Plant them after all danger of frost has past, around the same time as tomatoes. Pick a sunny spot with well-draining, fertile, slightly acidic soil. Because dahlias are so diverse, spacing requirements will differ depending on type, but a good rule of thumb is to space tubers between 12-18" apart.
If your growing season is short, you can start dahlias indoors in large containers up to 4-6 weeks ahead of the final frost.
Will this thing grow?
The tuber itself stores all of the nutrients needed for the plant to send out its first set of leaves and fine roots. When planting outdoors, dig a hole approximately 6" deep by 6" wide. Larger clumps of tubers will need a wider hole.
Locate the crown of the tuber above the neck (see image left) and rest the tuber sideways into the hole with the crown toward the soil surface. Bury the tuber, lightly pressing the soil around it, avoiding damage to the eye(s).
Caring and Cultivation
There is no need to water in tubers that have not already formed the finer roots required to draw up water. Instead, wait until a few inches of green shoots emerge from the soil, then water only moderately. Planting in wet soil can lead to tuber rot.
Dahlias can grow top-heavy and fall over in the wind, so it's also a good idea to add stakes early on and tie your dahlia to the stake as it grows.
Before adding fertilizer, you may want to test the soil. Too much nitrogen can result in more leaves and less blooms. A side dressing of compost will do for most situations.
Harvesting Flowers and Storing Tubers
For more blooms, cut often! Don't just deadhead, harvest just-opening blooms for indoor vase arrangements and bouquets. Dahlias are generally very prolific and will bloom right up to the first frost.
Dahlias will not overwinter in zones below 8. After the first killing frost, cut back the plants, gently dig up tubers, shake or hose off the dirt, and cure tubers for a week or so in a warm, dry spot. Remove any withered or rotten bits and store in a cool–not freezing–area (40-50 degrees is ideal). A basement or attic is usually fine. Some people choose to store their tubers in sand or peat moss, but this is optional. Periodically check on your tubers to make sure they are not overly dry or moist, until it's planting time again.
To shop tubers in time to plant, visit our full collection here.