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Dahlias in the Garden

It’s the beginning of a new year. What this signifies to many gardeners is that it’s time to start planning for the upcoming spring and summer gardening season. Like clockwork, plant and seed catalogs are arriving in the mail, tempting us with pictures and descriptions of plants they want us to think we can’t live without.

One catalog I’ve been waiting for in particular, and just received, is a dahlia catalog from a large grower in the Pacific Northwest which lists over 350 varieties. The range of color, flower size and flower form is astounding, with flowers ranging in size from less than two inches to nearly a foot across, and in every imaginable color except true blue. Flower forms range from single flowers to fully double ones; cactus types with narrow, quill-like petals; anemone form with tufted centers; ball and pompon types with fully double, nearly spherical flower heads; and several others.

Over the last 100 years, roughly 50,000 named dahlia varieties have been registered! Surprisingly, the great amount of variation we see in today’s dahlias originates from, most likely, only two species: Dahlia coccinea, with single, scarlet flowers (although there is a yellow form), and Dahlia sorensenii (pinnata), with single, mauve-colored flowers.

Both of these are native to higher elevations in Mexico and Guatemala. It wasn’t until the early 1800s that these two species were sent to Europe from Mexico, at which time the first crosses between them were made. By selection of naturally occurring mutations and continued breeding, we have ended up with the amazing variety of dahlias available to the gardener today.

Dahlias are warm-season plants, meaning they grow and flower during the late spring, summer and early fall months; and are herbaceous perennials, meaning they die back to the ground in the fall and put on new growth in spring. The tuberous roots, commonly referred to as just tubers, are planted in the spring. Tubers are usually available from garden centers and catalogs starting in mid-winter. I just checked the list of dahlia tubers that Roger’s Gardens will receive in January and counted over 60 varieties. Although it is too early to plant at this time, I like to buy them then while the selection is good. I keep them in a cool location until ready to plant.

A good guideline to use for planting time is to plant when the soil temperature is over 50ᵒ F. If you are gardening in a climate where the soil never freezes, tubers may be left in the ground all year, as long as the soil is well-drained. Living in southern California, I have had a plant of the variety ‘Bride to Be’ in the ground for six years now. It gets bigger and better with each passing year. Starting to bloom last year in mid-May, at one point, in June, there were over thirty, 3½ inch, pristine white flowers open and twice that number of buds at different stages of maturity. It was a real “WOW” factor in my garden. With judicious deadheading (removal of dead flowers) my plant kept blooming until late October. Let’s see—that’s five months of continuous bloom! This illustrates the long flowering season that dahlias have. This year, before new growth begins, I plan to dig and divide the massive clump before it starts crowding itself out.

Let me give a few tips to help you grow fantastic dahlias. Before planting, I always mix a good garden soil amendment into the ground. Harvest Supreme is my preferred one, but there are many others that will fit the bill. I also add an organic, all-purpose fertilizer at this time. Be sure to select a location which receives, at least, a half-day’s sun. A full-sun location is best, though. Tubers should have 2-3 inches of soil covering them. If planted in moist soil, don’t water until you see new shoots peek above the soil surface, unless the weather is unusually hot and dry. Large-flowered varieties should be staked to keep the large, heavy flowers from bending or breaking the stems. I give my plants another shot of fertilizer in mid-summer to keep new growth and flowers coming.

An alternative to growing dahlias in the ground is to grow them in containers. I plant many of my standard-sized dahlias, which can reach heights of 3 to 5 feet, in large containers that are 18 or 20 inches in diameter. Dwarf, bedding-type varieties, which grow to 12 to 18 inches tall, are happy growing in a 10 or 12 inch diameter container. I’ve had good results using Roger’s Premium Potting Soil, straight out of the bag, but any good quality potting soil will give good results. As I do when planting in the ground, I mix in an organic, all-purpose fertilizer before planting. For dahlias growing in containers, closer attention must be paid to watering and fertilizing throughout the growing season. A month after growth starts, I start fertilizing with a water-soluble fertilizer every two to three weeks. After trying several different fertilizers, my favorite is Sea Grow All Purpose (16-16-16) fertilizer which has several trace elements included. My superstar last year was ‘Pinelands Princess’, a pink and white lacinated variety with split petal tips, giving the flowers a lacy appearance, and 7 inch flowers. Other favorites were ‘Maarn’ with 4 inch orange flowers, ‘Uptown Girl’ with flowers having an unusual blend of rust orange and pink tones, and ‘Sisa’ with bright yellow flowers on a low growing plant.

So that’s dahlia-growing in a nutshell. Now it’s time for me to get back to my catalog and try to decide which new dahlias I can’t live without.

By Steve Hampson