Every flower has it's following of fans, and the dahlia certainly has it's share. The plant is native to Mexico and South America, and named for Anders Dahl, the Swedish botanist. Dahlias were first introduced to Spain in the 1780s. They are now available in so many colors, sizes and forms that they have some appeal for practically any gardener. I've seen them growing in gardens fabulous and famous, and in gardens beside humble dwellings in mountain hollows. No matter the setting, they lend a grand elegance. They are not only beautiful in the perennial border, they make excellent cut flowers and are often grown for show.
Dahlias are tuberous-root plants that are grown from seed, cuttings or tubers. Most gardeners start with tubers because they are easily obtained and predictable.
Dahlias require exposure to full sun for at least 6 hours per day, and well-drained sandy loam with pH between 6.0 and 7.5. To determine nutrient needs, take a soil sample to your nearby Cooperative Extension Office for testing. Call first for details. Prepare the soil by cultivating deeply and amending the soil according to soil test recommendations. Amendments should be incorporated into the soil.
Large-flowering dahlias require higher levels of soil nutrients. Fertilizers high in potash such as 5-10-10 or 4-8-12 are often recommended. Applications of composted manure, bone meal and blood meal also improve the soil. Medium-flowering plants require half as much fertilizer as the large ones. Small-flowering dahlias need very little fertilizer since smaller blooms are considered to be more desirable.
Large dahlias will need staking to prevent them from bending or breaking. Begin by driving 6' stakes into the ground at the places where you intend to plant. The stakes should be driven 18" deep and 3' to 4' apart. Then prepare planting holes next to the stakes. If the stakes are spaced 4' apart, you should be able to plant two dahlias per stake. Planting holes should should be about 4" deep unless the soil tends to be dry. If it tends to be dry, the holes should be 6" deep. Place the tubers 4" or more away from the stake. The upper end should be slightly elevated in the bottom of the hole. Sometimes it's difficult to determine which end of the tuber is up. Look for growing points, or "eyes". The end with the "eyes" should be the upper end. Most of the eyes should be facing upward. Cover with soil and water well. Take care that bits of synthetic fertilizer do not come into contact with the tubers.
Watering is necessary if rainfall is not adequate. Dahlias require at least 1" of water per week, perhaps more if weather is hot. Irrigate weekly so that the soil can drain between watering.
Cultivate frequently to prevent the soil from compacting and to remove weeds, being careful to avoid disturbing the plants and stakes. If you prefer not to cultivate, a layer of mulch will help to retain moisture and suppress weeds.
Additional fertilizer may be applied around the large-flowering dahlias during mid-summer. Apply 5-10-10, 4-8-12 or another recommended fertilizer at a rate of 1 or 2 tablespoons per plant. Do not allow fertilizer to come into contact with the plants.
As the plants grow, selected branches and buds should be removed to encourage better blooming. Pinching to remove is usually sufficient, provided that pinching is done at the right time. The first pinching should occur when 4 pairs of leaves have developed. At that time, pinch out the very top of the plant to encourage branching along the side. As side branches develop, select the best 4 to 6 branches to remain and remove the others. As those remaining branches develop, remove buds that appear along the sides of them to encourage development of best quality blooms.
Gardeners who grow dahlias for exhibition do even more pinching and disbudding to produce the best flowers possible. They also apply more fertilizers and insecticides, and take steps to protect the flowers from sun exposure to enhance bloom color. It is not within the scope of this article to discuss the various techniques here.
When cutting flowers, use a sharp knife or pruners and cut at a slant. Immediately place the cut ends in a container of cold water. If the flowers begin to wilt, cut the stem once again a couple of inches above the last cut, and place in the water.
The most common pests include aphids, thrips, leaf hoppers and spider mites. Various insecticidal soaps and chemicals can be used to good advantage. Always carefully follow label instructions.
After the first hard frost, cut off the plant stalks close to the ground. After a couple of weeks, the tuberous clumps can be dug and stored over winter. Store them in a very cool and dark place. Care must be taken to prevent them from freezing and drying. Baskets make excellent storage containers because they allow ventilation along the sides. Without adequate ventilation the tubers will rot. Sprinkle with water every week or so to replace lost moisture.
In spring, the tuberous clusters can be divided. Each division may consist of one or two roots with "eyes" at the top. The division can then be planted as before, or shared with others.
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