Gardening : Dahlias’ Splendid Summer Show : The many varieties available offer wide selection in colors and plant sizes.
From the enormous exhibition varieties with flowers as big as dinner plates to the less dramatic but lovely bedding types, dahlias are one of the most versatile and spectacular of all summer flowers.
Dahlias make beautiful cut flowers and are easily grown from bulb-like tuberous roots. In our mild Southern California climate, the larger varieties can be treated as perennials, left in the ground to bloom year after year. Now is the time to buy dahlia tubers while selection and availability are good. Tubers should be planted in April or early May and will flower July through September.
Dahlias are native to the mountains of Mexico, Central America and Colombia. Most dahlias grow to about three to five feet tall, occasionally to eight feet. The tallest dahlia is Dahlia imperialis , the tree dahlia, which grows to 20 feet, with six- to eight-inch pink, lavender or white flowers. Dahlias are found in every color except blue.
The American Dahlia Society categorizes all dahlias into 12 different groups, based on the size and form of the flowers. Dahlias are often categorized in more general groups according to flower size, such as miniature varieties such as pompon; bedding dahlias; medium varieties, including cactus dahlias and the giant dinnerplate varieties with flowers up to 15 inches in diameter.
Exhibition dahlias include all the larger, showy varieties seen at shows, but seldom seen at most nurseries. Bedding dahlias are sold in bloom at nurseries during spring and summer--these are usually seed-grown flowers, more tender than tuber-grown varieties and are best treated as annuals.
The huge exhibition dahlias are fun to grow. Because the flowers are so big, you only need to plant a few tubers to get a stunning display of summer color. Plant the taller varieties at the back of your garden as background color or as a screen. Dahlias make great fillers between perennials and shrubs. Miniature varieties are used as border plants and in containers.
Plant tubers horizontally, three to five inches deep and at least a foot part. You will need to stake the medium and tall varieties. At planting time place a five-foot stake two inches from the tuber. If you wait to stake the plant later you may accidentally spear the tuber.
Along the coast, dahlias need full sun to do well. In hot inland areas they like bright light with shade from hot afternoon sun. Dahlias thrive in light, well-drained soil. If your soil is hard or clay-like, amend with plenty of redwood compost or planter mix to a depth of about 10 inches. After planting the tubers, water deeply.
Once foliage starts to appear, water thoroughly once or twice a week. Dahlias like to be kept moist but not soggy. Once the plants are up and showing lots of foliage, apply a liquid fertilizer with a formulation of about 10-10-5. If you are using a stronger fertilizer, such as a 20-20-20 formulation, apply at half-strength. Fertilize once a month during the growing season. Do not over-fertilize, as too much nitrogen reduces flowering.
When there are three sets of leaves on the plant, pinch out the center tip. Pinch again after three new sets of leaves appear. This will create a stronger, bushier plant. Remove spent flowers frequently to force the plant to direct its energy into making new flowers rather than making seeds from the old flowers. Cut flowers for the house are best collected in the morning and immediately placed in warm water.
In Southern California, dahlias can be left in the ground year round, and only need to be dug up if you want to divide them. In colder climates they need to be dug up and stored every year. When digging up the tubers, start digging about a foot away from the plant and carefully lift the tubers. Shake off loose soil, and put tubers in the sun to dry for a few hours.
You can divide the clumps at this time, or you can wait until planting time next spring when buds are easier to see. When separating, make sure that each new tuber has a part of the stem with a bud. If a tuber does not have any buds, leave it attached to the original clump. Label and store in a cool, dry place, in peat moss or sand.
For tubers left in the ground year round, cut the foliage back in the fall to within about six inches of the ground. Whether you are leaving the tubers in the ground or digging them up, wait to cut the foliage back until after the leaves have turned yellow or have been frosted. As long as the leaves are green they are still taking in nourishment from the sun to store in the tuber for next year’s flowers.
Dahlias are sometimes susceptible to mildew, snails, caterpillars, leaf miner and mites. Spray with Fundinex for mildew; Mavrik for insects, mites and caterpillars. Bait regularly for snails.
