San Diego Floral Association
See past editorial content: Growing Grounds | Favorite Tool | Friend or Foe | Floral Stories | Roots | Archives
Become an instant expert with hands-on growing advice and official dahlia basics
By Sharon Tooley
Photo: Barbara Swanson
Dahlias are not only some of the brightest flowers in modern gardens, they also have a long and fascinating historic association with man. Dahlias are native to Mesoamerica principally from the higher altitudes of Mexico. Pre-Columbian people of central Mexico, the Yucatan, and Guatemala cultivated them as a root crop. The genus includes about 30 species, all of which have edible tubers.
The dahlia tuber was long cultivated and hybridized as a nutritious food source that contains inulin (a fructose sugar) and antibiotic compounds concentrated in the tubers skins. A small Aztec herbal comprising wisdom of this rich heritage, was written in 1592 by two native students being taught by friars in Mexico. It includes dahlia tubers, acocohxibuitl, in a formulation of a cure for epilepsy. Currently, a sweet extraction of the roots called dacopa is said to combine the flavors of coffee, tea and chocolate. It has a variety of uses in beverages and flavorings.
Introduced into Europe in 1872, a box of tubers was sent from Mexico to the Netherlands. only one plant survived (Dahlia juarezii) and bloomed with brilliant red flowers with pointed petals. Abbe Cavanille from the botanical Gardens of Madrid named the dahlia in honor of Anders (Andreas) Dahl (March 17, 1751 - May 25, 1789), a Swedish botanist and student of Carolus linnaeus.
Dahlias were grown as a botanical curiosity until the mid-19th century when they took off as garden plants in major formal Victorian gardens where their diversity in size, color and shape was much appreciated in seasonal pattern gardens and borders. The principal decorative varieties grown in gardens today are thought to have been parented principally by the species Dahlia pinnata and D. coccinea. The species plants are rarely grown.
With over 50,000 thousand named cultivars dahlias are among the most beloved flowers in gardens around the world. Understand that not all 50,000 are still in existence, there are probably no more than 4,000-5,000 currently. The great variety results from dahlias being octoploids—they have eight sets of homologous chromosomes, whereas most plants have only two. For all their beautiful color and diversity of bloom, there is little to no fragrance. There are more than 60 dahlia societies in America and Canada under the umbrella of the American Dahlia Society (www.dahlia.org). One of them is the San Diego Dahlia Society, of which my husband and I are members.
Dahlias of all descriptions
Most people think of dahlias as either very large or quite small flowering plants, with “dinner plate” sized blooms or little border plants sold at their local nursery. Actually, dahlias have ten designated blossom sizes, indicating the diameter of the bloom, from under two inches to over ten inches. Bigger blooms demand proportionately more growing area. Stem/stalk lengths range from one foot and up to five feet. I generally grow the medium sizes because the plants are easier to control and there are many more blooms per season. However, if you have the space and love big flowers, there is nothing like a giant dahlia.
Dahlias are blessed with a great selection of colors; the varietal mix is intensely spectacular. There are no green or blue dahlias, and although black is listed as a color I do not know of a named, American Dahlia Society (ADS) recognized, “Black Dahlia”. However, very intense deep maroon can appear to be almost black. Blends are two or more evenly merging harmonious colors; bicolor are two or more distinct and sharply contrasting colors and variegated are two or more colors in dots, splashes or stripes which contrast with the basic color.
There are 19 forms of dahlias recognized by the American Dahlia Society. Although the names can be confusing, they are all dahlias. The FD, or formal decorative dahlia, has regularly arranged petals, whereas the ID, or informal decorative dahlia, has twisted, curled and irregular petals—what we call a bad hair day! The cactus dahlias have petals that curved either all the way or part way. The laciniated varieties have split petal ends that appear fringed or frilled. Balls are round and uniform, and the Pompon is just a very small ball. Waterlily types resemble an actual waterlily—a view from the side shows a bloom that is flat with slightly cupped petals. The other types listed are grown less frequently, but you can usually see all the varieties at the San Diego County Fair and at our Annual Dahlia Show the first weekend in August in Room 101 Casa Del Prado, Balboa Park, San Diego. To become an expert, familiarize yourself with the complete list:
Many people ask if dahlias are hard to grow. I maintain that they take about as much work as growing roses. If you just want something to toss into your yard and water weekly, dahlias are not the best choice. However, if you walk your gardens daily and enjoying watching each plant grow and bloom, then dahlias are well worth the time and energy.
Dahlias are like the chicken and the egg—which came first? If you wish to get the exact same bloom from year to year, you must harvest the tubers. If you are adventurous, harvest the seeds, which are not the same genetic “code” as the tuber. This is the method used to introduce new species into the dahlia genus.
Many dahlia growers ask if they should begin with digging tubers in the fall, or planting in the spring. As our climate in most of California is mild you should be able to leave your dahlia tubers in over winter and expect blooms to come up for several years as long as they are in well draining soil. However, there are at least four good reasons to dig tubers in the fall. They include: 1) if your area does experience winter freezes; 2) if you want to divide tubers and have lots of free plants; 3) if you want to share tubers; or 4) if you have winning varieties that you want to expand next year. If any or all of these circumstances are true, you should dig up your dahlias in the fall.
A Year-Long Growing Plan
I begin to dig my tubers after the Thanksgiving in November. Before November I have usually cut my plants down leaving about a six-inch stalk. When the time comes to dig I carefully remove the entire tuber clump. I will either save the entire clump with dirt attached over the winter in a plastic tub—always keeping the nametag attached, or I will dig the clump and divide.
In preparation for dividing the tuber, take the clump out and wash with the hose; let dry several hours. After drying the tubers may be cut from the stalk after you have identified the “eyes.” The eyes look like small bumps on the end of the tuber that is attached to the stalk. Leave a bit of the stalk attached when cutting off the individual tubers. Dip the cut end of the tuber in sulfur dust to prevent fungus. Then put the tubers in a plastic bag with a few holes with the name of the flower on the outside. Cushion the tubers to prevent damage. I have tried shredded paper, coarse vermiculite and even sand; anything you can think of to cushion the tubers should work. Put the tubers in a brown grocery bag and store in the garage or outside in a ventilated tub that will not let rain in. Check your stored tubers a couple of times during the winter and throw away any that look rotten.
After successfully storing your tubers all winter bring them out in March to transfer to plastic trays. Fill the tray with two inches of potting mixture and place the tubers in a row about an inch apart with the eye or sprout facing up. The tuber end away from the eye should be buried in the potting mixture, but the eye end should be up. Keep the tubers in the flat until the eye starts growing into a sprout, usually two to four weeks. Gradually give the flat more exposure to the sun. Keep the potting mixture damp but not soaked.
During this time you should be preparing the soil in your garden. Till the soil and add compost or dehydrated cow manure. It’s a good plan to water this fertilizer for a good week or two before planting to insure it is not too “hot.”
It is very important to place stem/stalk support stakes where you will plant before putting the tuber in the ground, usually about 18 to 24 inches apart. It is very distressing to drive a support stake through a tuber, believe me, however, your average dahlia grows about four to five feet high so that support will be important. When the sprout is about two or three inches high, take it out of the growing mixture being careful to protect the delicate roots on the bottom end of the tuber. Place the tuber horizontally in a two to three inch hole with the shoot pointing up toward ground level. The edge of the plant should be two inches in front of the stake.
I prefer to plant on days when it’s cloudy, as well as to plant in the late afternoon. Shield small dahlia plants from too much sun when first planted. One of our growers used a wooden roof shingle to shade his new plants until they got used to the sun.
This is a good time to put out slug and snail bait—dahlias are snail candy. I usually start applying snail bait about a month before planting. You may be tempted to plant early, but dahlias need warm soil to grow and will just sit for weeks if the weather is hovering between 50 and 60 degrees Fahrenheit. Tie your plant label on the top of each stake so you can see it when your dahlia reaches maturity.
Once your dahlia plants have two to four sets of leaves you need to “top” it. Carefully pinch off the center growth. After topping the plant will send up lateral shoots that terminate in three flower buds. The two buds on each side should be pinched off forcing the growth into the remaining bud.
For those of us who show dahlias, we would like to have only four laterals with one bud on each end that blooms precisely at show time. This is how you get really good show flowers; however, if you are not interested in competition you can let your dahlias bloom without pinching back, but your blooms will be smaller. Dahlias should continue to bloom into September as long as you keep deadheading.
Dahlias are heavy feeders. Fertilizers list three numbers: the first number is for nitrogen, the second number is for phosphate and the third number is for potash. Nitrogen encourages foliage. Phosphate encourages roots and blooms. Potash encourages stems, roots and tubers. Growers who use a time released balanced fertilizer should need little additional fertilizer during most of the season. From mid summer on, whatever fertilizer you use should be low in nitrogen and higher in phosphate and potash. At the end of our growing season, usually October, sprinkle a teaspoon of potash around the base of your plants and water as usual. This will encourage good tuber growth.
Dahlias have a few more enemies beside snails and slugs. I frequently have ants that introduce aphids onto my tender buds. I generally use very few insecticides; however, for the ants I will spray a household ant killer lightly where needed and then just wash the aphids off. There is a dahlia virus that can be spread among your plants by sucking insects. If you have a dahlia plant that has misshapen or blotchy leaves, I suggest pulling it up and throwing it in the garbage—do not compost. The other problem almost all growers have with late season dahlia growth is powdery mildew here in coastal Southern California. I use sulfur powder around the base of my plants beginning in August since mildew comes from the soil. I also snap off the bottom leaves when they begin to deteriorate, as the plant grows taller. This will help keep the mildew at bay and help with air circulation. When your dahlias begin to look “ratty” just cut them back and let them rest until digging time in November.
Now we have gone full circle. Enjoy your blooms—share them with friends and neighbors, take a few into work or compete in your local dahlia show. In addition to the ADS website mentioned earlier, another easily navigable site for information is the Colorado Dahlia Society (www.dahlias.net). Their site shows examples of blossoms, gives growing tips and lists many national growers. While dahlias may take extra attention, the results are stunning and worth your time and effort.
Reprinted from July/August 2008, Volume 99, Number 4
© SAN DIEGO FLORAL ASSOCIATION. This story may not be published in any form or copied onto another website without written permission from San Diego Floral Association.
Mission: To promote the knowledge and appreciation of horticulture and floriculture in the San Diego region.