In my opinion, there is no more striking addition to a garden than dahlias. With their vibrantly colored, large blooms and curled petals, they demand attention.
I have had dahlias before, but this is my first year growing them from tubers. They come in almost any color you could want and in sizes ranging from a few inches across to the diameter of a dinner plate (and actually called “dinner-plate dahlias”). I finally decided on a flashy cultivar with yellow and red stripes called ‘Sunrise Mango.'
What explains such diversity? Dahlias, as it turns out, have a more complex and variable genetic structure than most other flowers, leading to impressive variety. There are 42 different species and at least 57,000 different cultivars. The International Registry of Dahlia Names keeps track of them.
Before horticulturalists began breeding dahlias for ornamental purposes, the plants were grown for food by the Aztecs, who ate the tuberous roots. Today, the roots are still used in Oaxacan cooking, and an extract obtained from them is a common addition to Central American beverages. Don't go chopping up your dahlia tubers just yet, though. The ones you get from your local nursery have been cultivated for appearance, not flavor, and will likely not be to your taste.
The flowering portion of the dahlia is also unusual. Known as a "composite" bloom, it holds not one but many flowers. The multiple small flowers, called florets, are often mistakenly identified as petals. Lacking much fragrance, the plants use these showy blooms to attract pollinators.
When planting dahlias, choose a site with plenty of sun and good soil. Gentle morning sun is best. Warm, well-drained soil is another must. Make sure the planting area is not overly wet (light dampness is okay), and the soil temperature is at least 60°F. Cold, wet soil can cause tubers to rot.
Dig a planting hole four to six inches deep and slightly wider than the tuber. Do not add compost, fertilizer or potting soil containing fertilizer. Instead, sprinkle about a tablespoon of bone meal in the bottom of each planting hole. Thenplace the tuber sideways in the hole with the "eyes" facing up. Stems will grow from these eyes.
Do not water after planting. The natural moisture in the soil will suffice. When the sprouts are four to six inches tall, begin watering deeply once or twice a week.
Young dahlias are frost sensitive, so protect them if frost is predicted. Begin fertilizing when you begin watering, using a fertilizer with more phosphorus and potassium than nitrogen, such as one labeled 5-10-10. Too much nitrogen (the first number listed on the fertilizer package) can cause weak plants with few blooms, or even kill the tuber. Phosphorus and potassium, on the other hand, will help produce strong plants with plentiful flowers. Dahlias generally bloom about eight weeks after planting, or around mid-summer.
Bark is generally a good groundcover for gardens but not around dahlias. Mulching with bark might keep the ground too cool and damp and raise soil acidity, conditions dahlias don't like.
Avoid fungal diseases, common to dahlias, by planting in a location with well-drained soil and plenty of sun. Slugs and snails love dahlias. Handpick them at night when they are most active. If you're not up for a flashlight-enabled search and destroy mission, try placing a small piece of wood or an overturned flower pot in your garden. Snails and slugs will use it as a daytime hiding spot, and you can remove them then.
Aphids, thrips and spider mites can also badger dahlias. When encountering an infestation, I use my garden hose to gently spray off as many as possible. I follow this with an application of Neem oil, a natural treatment for many garden maladies. The oil suffocates pests and fungi but does not harm plants or people. Ladybugs, lacewings and predatory wasps also provide some control and are available at local nurseries. Children love releasing and watching ladybugs.
In Napa Valley, you can leave dahlias in the ground over winter. To give them some extra protection from cold, place a two-inch-thick mound of mulch over each tuber. Remove the mulch in the spring to allow the soil to warm again and encourage sprouting.
Over time dahlia tubers will divide and can become crowded. If you sense that this is happening, gently lift the tubers in fall, about two weeks after a frost. Then store them in paper bags in a cool, dry place until the following spring. Before replanting, divide the tubers, leaving at least one “eye” on each piece.
If you are ready to try your hand at dahlia growing, or just want to know more, please plant to attend the upcoming Master Gardener seminar "All About Dahlias." Details are below.
Master Gardeners are volunteers who help the University of California reach the gardening public with home gardening information. U. C. Master Gardeners of Napa County ( http://ucanr.edu/ucmgnapa/) are available to answer gardening questions in person or by phone, Monday, Wednesday and Friday, 9 a.m. to Noon, at the U. C. Cooperative Extension office, 1710 Soscol Avenue, Suite 4, Napa, 707-253-4143, or from outside City of Napa toll-free at 877-279-3065. Or e-mail your garden questions by following the guidelines on our web site. Click on Napa, then on Have Garden Questions? Find us on Facebook under UC Master Gardeners of Napa County.