Jennifer Schultz Nelson
Extension Educator, Horticulture
Honestly, I think if I would have read more about dahlias before I planted one last summer, I might have decided against choosing this plant for my garden. Fortunately, ignorance was bliss and I bought the plant with the giant orange blooms for my Illini garden without a second thought. I also bought some dahlias with smaller dark pink blooms that were spectacular in their own right.
I treated the dahlias I bought last year just as I would any other flower in my garden, and I knew enough that they needed to be dug and stored in the garage if they were to survive the winter. By sheer beginners luck alone, the small dark pink dahlias survived, but unfortunately the giant orange ones didn't make it.
In reading more about dahlias for this article, I was amazed to learn that while there are only about 30 species known, there are over 20,000 cultivars developed from these species. If you are a serious dahlia grower, there are nineteen specific bloom forms recognized by the American Dahlia Society (www.dahlia.org), ten different color categories, and 225 recognized shades among these color categories.
Much like roses and orchids, growing dahlias can be an extremely complex pursuit and a competitive pursuit if you choose that route. Personally, I'll be happy if I can just overwinter my dahlias successfully. I'll leave the serious competition to others.
Dahlias are native to Mexico, and are the native flower of Mexico. Europeans encountered dahlias in their exploration of Mexico, and were included in several collections of New World plant life.
Today we think of spectacular flowers when dahlias are mentioned, but initially European plant breeders were interested in the potential of dahlias as a food crop, since they have very starchy tuberous roots, and the first specimens collected didn't have particularly striking flowers.
Some breeding experiments to improve flowers began as well, but didn't last very long. They were able to breed double flowers and expand on the available colors somewhat, but that seemed to be the limit of their success.
It looked like dahlias were somewhat limited in form and color until a particular shipment of dahlia roots arrived in Holland in 1872. As the story goes, only one dahlia root survived the long journey, and this root grew and produced a spectacular large red bloom with unique curved petals never seen before.
Breeders almost immediately began to cross this new dahlia with the ones they had already been working with. These new crosses ultimately led to the dahlia cultivars we see today.
Dahlias grow from a tuberous root that is typically planted after all danger of frost is past. It is possible to start them indoors to get a jump on the season. Like most plants, they do best in well-drained soil. Water-logged soil encourages disease problems.
They prefer a pH of 6.0 to 6.5, with lots of organic matter incorporated into the soil prior to planting. Fertilizers used should be at least balanced, or preferably lower in nitrogen than phosphorus and potassium. Too much nitrogen will encourage green growth rather than blooms.
Some dahlia cultivars grow quite large, to a height of three feet or more. Dahlia stems tend to be brittle.When shoots have first emerged and have three or four pairs of leaves, experts advise home gardeners to pinch out the topmost pair of leaves to encourage branching. This will produce a sturdier plant.
Flower buds are borne in sets of three. To produce larger blooms, pinch out the two smallest flower buds among each set of three. If you don't pinch the extra flower buds, they will still bloom, but all the flowers will be smaller.
It also helps to grow dahlias with staking, or cages placed around the plant. I wish I had done this this summer! I grew a gorgeous blue-purple large flowered cultivar, that was just getting ready to bloom when a storm blew up. The high winds broke the plant off at the base. Next year I'll be using stakes or cages right from the start.
There seem to be a million variations out there on how to overwinter dahlias. In general, dahlias can be left in the ground after the first frost, but must be dug before the ground freezes. Remove the foliage and leave a three to four inch stem. The root system is shallow and quite extensive, so begin your digging at least a foot away from the main stem to avoid slicing into the tuberous roots.
Gently remove all soil from the roots, and cure them in a shady area with good air circulation for a few days. Dust with sulfur to inhibit fungal diseases. Store the tubers upside down in damp vermiculite or peat moss at temperatures between 35 and 50°F. If you notice the tubers have shriveled over the winter, water lightly.
The American Dahlia Society published a method using plastic wrap–wrap individual sulfur-dusted tubers in at least one layer of the plastic wrap, up to five tubers to a bundle. There is no peat moss or vermiculite needed with this method. Members have reported more success in overwintering dahlias with this method than with the traditional peat moss/vermiculite method.
In the spring, tubers that are wrinkled and mushy are likely dead from rot. Discard these, but don't feel bad–even the experts expect some mortality in their dahlia collection over the winter. Separate the remaining tubers so that each one has one "eye" of new sprouts on it. Plant shallowly with the eye just below the soil surface to begin another year of spectacular blooms.