Many gardeners consider the dahlia to be the iconic flower for the late summer and fall garden. Hardy to USDA Zones 8 and above, dahlias do best in well-drained soil and excel in sunny locations.
With over 36 species and more than 57,000 cultivars, this genus has an incredible range of flower size and shape. Petal shapes range from soft and rounded, to spikey and pointed, rolled and ruffled, curved and curled. Whatever color, shape or size, the gardener can probably find that in a dahlia.
Botanically, dahlias are classified in the Asteraceae family, along with mums, daisies and zinnias. The blooms are composite flowers, with ray petals surrounding a disc of central florets. Dahlias have eight sets of chromosomes, so there are possibilities for many different genetic combinations.
Development of new varieties is constantly taking place, presenting the possibility for new flower forms, sizes and colors. At present, the American Dahlia Society (ADS) recognizes at least 21 different flower forms, six bloom size categories and 17 color categories.
Flower Forms
The easily identified, decorative flower forms include formal and informal types having uniform petal arrangement. The variety “Spartacus” is a large decorative type. Stellar type blooms are similar but have petal edges that are sharp and curve backward toward the stem.
Pompon, miniature ball and ball flower types are all globe-shaped flowers classified by size. These are considered the best cut flowers with fresh, tight petals and are more weather-resistant than other types. The ball type flower is represented by the variety “Jomanda.”
Less weather-proof are the single and mignon single types, which have a single row of decorative ray petals surrounding the disc petals in the open center. Despite being fragile, they can be a wonderful addition to the perennial garden or container planting.
Dahlias of the peony type have two to five rows of petals surrounding the open center. Similar is the collarette type, having a second ring of short, ruffled petals rimming the central disc. A striking example of this is the variety named “Pooh.” Anemone type blooms also have a unique center that resembles a pincushion surrounded by a single row of ray petals. “Totally Tangerine” represents this form.
The form of the petal is also a determining factor of flower type. Petals resembling quills or spikes are characteristic of the straight cactus, semi-cactus or incurved cactus types. The petals may be tightly rolled, partially rolled or incurved. Petals may also be notched, fringed or snipped at the end and would be typed as lacinated or fimbricated. These styles add creativity to the garden or arrangement.
The ADS has additional categories for forms considered to be novelties. An open or closed center as well as flower size are factors for classifying these. While the determinations may not be meaningful to the casual gardener, they are quite important to plant breeders and those interested in entering an exhibit at popular dahlia shows.
Flower size can vary between varieties and is another factor of classification. Mini-pompon blooms are less than 2 inches in diameter. At the other end of the scale, giant or dinnerplate blooms are 10 inches or larger in diameter.
While flower type and size are used in classification, plant height is a factor to consider as well. Many varieties will grow to 5 or 6 feet in height, needing support as the plant grows. Smaller types known as dwarf, mignon and dahlietta range from 2 feet to less than 8 inches in height.
Growing and Care
Dahlias can be purchased as tubers, rooted cuttings or potted plants. Tubers are available for purchase at garden centers in early spring or ordered from growers by midwinter. If purchasing in person, look for tubers that are undamaged and firm and contain at least one eye, the growing point where shoots will appear.
An increasingly popular way of propagating dahlias is from cuttings. Rooted cuttings may be available for purchase beginning in early spring but need protection from cool weather. Large potted dahlias with buds and blooms are available throughout the summer.
Plant dahlias in a location that receives six to seven hours of sunlight for the best result. The location should have good, well-draining soil with a pH of 6.5-7. A good amount of organic material is vital, as dahlias require good drainage, and applying compost will improve soil fertility. Consider applying a high-nitrogen fertilizer in late spring to promote leafy growth, then in early summer switch to a high potassium (K) fertilizer to promote flower growth.
Provide adequate space when planting, as crowded situations may create an environment for mildew. Commercial growers may plant densely, placing tubers 12-18 inches apart. More vigorous varieties may require up to 3 feet between plants. Wait until the threat of frost has passed and soil is not soggy, as tubers planted in cold, wet soil may rot. In West Sound, this is sometime between mid-April and early June.
Most dahlias benefit from support, so insert a stake or tomato cage when planting. Lay the tuber horizontally, 4-6 inches deep with the eye facing up. Cover with several inches of soil. As the shoots appear, continue to fill in the hole.
When actively growing, dahlias will need deep watering about once a week. Soaker hoses and drip lines are excellent watering methods, and mulching will help reduce evaporation and prevent weeds. Be diligent about pests such as slugs and snails.
When plants are 8-12 inches tall and have three or four sets of leaves, pinch out the growing tip. “Stopping” or “pinching out” encourages the production of multiple side branches, creating a bushier plant with more stems.
Dahlias have few pests, the major offenders being slugs, snails, aphids and earwigs. Use integrated pest- management methods beginning with the least invasive such as hand removal, hose spraying or insecticidal soaps and beneficial insects. Insecticides will affect the offending insects as well as pollinators and beneficial insects.
Cut dahlias in the cool of the day and place into clean water right away. Fully open dahlias should last up to five days in a vase. Deadheading — the removal of spent blooms — encourages the plant to put energy into the production of more flower buds.
Dahlias are perennials in USDA Plant Hardiness Zones 8 and above, but tubers must be protected from freezing winter weather. Tubers can remain in the ground, mulched with several inches of straw, leaves or compost, then covered with plastic. Leaving dahlias in place over winter allows more tubers to be produced the following season, but after several years, flower production may decline. The tubers cluster too tightly and become difficult to divide.
A widespread practice is to lift the tubers with a garden fork before the ground freezes or becomes too soggy, then store and replant in spring. When the plant is no longer actively growing or has been knocked down by light frost, cut off the top growth, leaving several inches above ground level. The tubers can be dug immediately or left for several weeks, but watch for freezing weather or heavy fall rains. Clumps can be stored whole or can be cleaned, divided and stored at this point. The goal is to keep tubers from wet and freezing conditions.
If dividing tuber clumps prior to winter storage, gently remove as much soil as possible without damaging the tubers. The neck of the tuber connects the carbohydrate-storing body to the crown on the stalk, which contains the eyes. Eyes may appear pink, white or red. Breaking or cracking the neck jeopardizes the viability of the tuber. Hose spray can be helpful in removing soil.
Tubers should be dry when stored. Dividing a clump may take practice, but a sharp knife and bypass pruners or scissors are helpful. The important thing is to have a body, neck and crown with eye, including part of the stalk. Trim away excess stalk and tail-like roots and discard damaged tubers.
Popular storage media include vermiculite, peat moss, sand and cedar shavings. Store dahlias in a dark, cool spot, with the optimal temperature of 40-50 F. Periodically check the moisture level, as excessive moisture will result in mold, but too little will result in shrunken tubers.
Dividing tubers in mid-to-late spring is also possible. The eyes should be evident and ready to sprout.
Dahlias do require a bit of work from the gardener. Spring planting and fall digging for dividing and storage require time-sensitive labor. The reward is a multitude of lovely, striking blooms for over three months of the year. The challenge is to find growing space for another “must-have” color, size or form.
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