University of Vermont Extension 
Department of Plant and Soil Science

Spring News Article

DELIGHTFUL DAHLIAS

 
Dr. Leonard Perry, Extension Professor
University of Vermont
 
 
We have the Aztecs to thank for the dahlia, a tender bulb (actually a tuber) planted in late spring for gorgeous blooms in late summer.  A long-lasting cut flower, the dahlia makes a great addition to any garden.

Dahlias range in color from white and yellow to orange, pale pink, lavender, and red.   Bloom size ranges from half an inch to a foot or more across.  Flowers may be in tight balls to very open, from single to double, with petals that are flat, curved, or rolled into tubes.  Based on flower type, the American Dahlia Society (www.dahlia.org) lists 20 classes.
 
The dahlia, a relative of the daisy, was first cultivated by Aztec botanists in Mexico. In the early 1500s it was discovered by Spanish explorers who brought this tuberous plant back to Europe. Interestingly, they had the same problem with storage of the tubers as do many modern-day gardeners. The genus Dahlia gets its name from an 18th-century Swedish botanist, Andreas Dahl.

The dahlia became a favorite in the gardens of working class Europeans after being distained by the upper class as being too flamboyant for their carefully manicured gardens. However, it gained prominence in the mid-1800s after a devastating blight wiped out the potato crop in France as it was thought to be a good substitute for this starchy vegetable. Unfortunately, it was not, but it soon became popular in gardens both for its flowers and interesting foliage.

Although a perennial plant, in northern climates dahlias don’t survive winter so are treated as annuals. They are planted in the spring as soon as the soil has warmed up and after the last chance of frosts—about when you plant tomatoes outside.   They can be grown one year as annuals, or tubers lifted and stored in the fall after the first hard frost and the foliage is killed.

Dahlias do best in a sunny spot with light, fertile, well-drained soil. If you have heavy clay soil, work in a two- to four-inch layer of well-rotted manure or compost a few weeks before planting. Add fertilizer--about a quarter pound of a balanced fertilizer like 5-10-5 per ten square feet of garden—once a month after plants start growth, or lightly sprinkle fertilizer around plants. Avoid high nitrogen fertilizers (those with a higher first number) as they will result in leaves at the expense of flowers.  Use a general garden or tomato fertilizer.

Wait until the danger of frost has past before planting the tubers, unless you planted them in pots in early spring. Plant at least four to six inches deep, laying them horizontally on their sides, with roots down and buds facing upward. Cover with two inches of soil, adding more as shoots appear.

Space smaller varieties two to three feet apart, larger ones three to four feet. Larger plants also will require support as they grow. To avoid damage to the roots later on, drive a stake into the ground now, a few inches from where you plan to plant each tuber. As the plants grow, tie the stalks to the stakes with double strands of garden twine. Pinching back the center shoot once three sets of leaves appear will keep plants more bushy. 

Most dahlias will bloom in late summer and early fall. Some of the newer and bedding varieties are relatively short, a foot or so high, and generally bloom repeatedly through the season. Pinch off spent blooms from these to encourage continual flowering. Once frost has blackened the foliage, and after the tubers have hardened in the soil for a week, it's time to dig them up and store until the following spring.

With a sharp knife, cut the stalks at a height of about a foot. Then carefully dig up the clumps, taking care not to injure or spear the tubers. Trim the stalks to a few inches. Shake off the loose dirt and separate the tubers, allowing them to dry for only a couple days (or they will start to shrivel and dry too much).

Brush off the remaining dirt, then place in a plastic bag in a box, or plastic box (to keep them from drying out) containing peat moss, wood shavings, coarse vermiculite, fairly dry compost, or similar. Cover, label and store in a dry, cool (non freezing) place. The ideal storage temperature is 40 degrees F. Check every few weeks to make sure tubers aren’t shriveling (add a little moisture) or staying too wet which will cause tubers to rot. If too wet, leave uncovered until the storage medium dries out, or replace it with drier material.

If you have large clumps of tubers, individual ones can be separated off with a knife.  Just make sure to keep a piece of the crown (the thickened stem where the tubers join together), which has the future growing points or eyes.  Then wrap each tuber with plastic wrap.  Easiest, especially if you are already growing tubers in large pots, is to just bring the pots into a non-freezing area for winter, keeping them dry.

More dahlia tips, cultivar (cultivated variety) listings, resources and sources can be found from the American Dahlia Society.   More information, as well as heirloom cultivars, can be found at Old House Gardens (www.oldhousegardens.com).

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