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How to Grow Dahlias

Dahlia

Dahlias are among the showiest of annuals. With a little effort, you can save the tubers for another year.

There are shrinking violets and then there are dahlias, which are nothing if not flamboyant. Their flowers can range from dinnerplate size to petite pompoms. Native to the Andes of South America, this daisy-family plant is a tender perennial in most areas of the country, but is hardy outdoors in USDA zones 8 to 10. In zone 7, they can survive winters with a thick layer of protective mulch.

Choosing Dahlias

Horticulturists have divided dahlias into 10 classifications, with numerous subcategories and fine distinctions. Because there are so many varieties, choosing dahlias is much more difficult than growing them. However, you can put dahlias into three broad groups: large, medium and small:

Large dahlias range from about 3 to 4 feet tall. The most well-known dahlias in this size range are dinnerplate dahlias, known for their 10- to 12-inch-diameter double flowers. These are best grown in the back of perennial borders with other large perennials. They put on a stunning late-summer and early-fall show, but will require some kind of support to withstand summer winds and rain. Try thick bamboo stakes or whimsical spiral supports.

Medium dahlias, also known as border dahlias, are compact 1- to 2-foot-tall plants. These dahlias look great in the front of a perennial bed mixed with other late-blooming perennials, such as phlox and asters. In addition, all border dahlias can be grown in containers. For something different, consider cactus-flowered dahlias, which have distinctive rolled petals.

Small dahlias are perfect for windowboxes. These 10- to 20-inch plants bloom for a long time and remain fairly compact. You can also use them at the edge of a border or in pots and planters.

Planting

Dahlia tubers

Dahlia tubers don't look like much when you plant them. They should be firm and plump. In some cases, you'll see signs of new growth — usually at the point where the tuber joins last year's stem.

Dahlias grow and flower best in a well-drained, sunny site. They like warm weather and will not tolerate frost, so don't rush to plant them outdoors. Plant your dahlias in beds about the same time as you'd plant corn—when the soil has warmed. Most dahlias will begin flowering by midsummer.

If you live in a cold climate and want to get an early start, you can start your dahlia tubers in 1-gallon pots about two to three weeks before your last expected frost date. Once they've sprouted, place the pots under grow lights or in front of a sunny window (a south-facing window is best). After danger of frost has passed, transplant them gently into their garden location.

To plant all but the diminutive-sized dahlia types, dig a 6- to 12-inch-deep hole and amend the soil with compost. If the soil drains poorly, mix in some fine gravel. Backfill the hole with native soil and plant the tubers 2 to 4 inches deep, and about 1 to 3 feet apart, depending on the variety. Growth begins in 14 to 20 days.

Tall varieties, such as dinnerplate dahlias, will need support or staking to keep the heavy blooms from falling over. A simple wood stake is fine, but you can also use round tomato cages or Rainbow Spiral Supports, which are quickly covered by lush foliage. Just make sure to put the stakes or cages in place before the plants get large. To create a bushier plant, pinch off the top growth of the dahlia shoots above the third set of leaves while plants are still young. Shorter varieties are naturally bushier and won't need pinching.

Summer and Fall Care

Fertilize dahlias monthly with a water-soluble, organic fertilizer that's formulated for flowers. Avoid using a high-nitrogen fertilizer, or you'll get lots of green growth with little flowering. Mulch with straw or shredded bark to keep weeds to a minimum and retain moisture. Water regularly, especially during dry periods. Cut blossoms for indoor flower arrangements and remove old blossoms to encourage more branching and flower production. Despite a relatively short vase life, dahlias make gorgeous bouquets.

Overwintering

Dahlias are tender annuals, but you can overwinter them pretty easily. In fall, after the first frost has blackened the foliage, cut off all but 2 to 4 inches of top growth, and carefully dig tubers without damaging them. Allow tubers to dry for a few days in a frost-free location, out of direct sunlight. Once dried, remove any excess soil, leaving 1 to 2 inches of stem. Store each clump of tubers in a ventilated box or basket. Fill the box with slightly moistened sand, peat moss or vermiculite and place it in a cool, dry location with temperatures that remain between 45 and 55 degrees F. Check tubers periodically through winter for rotting and drying out. If the tubers appear shriveled, mist them lightly with water. If any start to rot, trim the rotted portion of the clump so it won't spread. The tubers are fragile, so be careful when handling them.

When warm weather arrives, you can plant the overwintered tubers and begin the cycle again.

Slideshow: Overwintering

How to Save Dahlia Tubers for Another Year

Photos: Kathy LaLiberte

This is a dahlia called Who Dun It. I never thought I could like a flower that looked like this. Part of what I enjoy is the effect it has on people — the abundant blooms and their extravagant size is irresistible. The dahlia row in my cutting garden. I had several more colors last year. This year I limited it to Who Dun It and Black Narcissus. In early October the plants stood 6 feet tall and were still pumping out flowers. But a heavy frost was coming and they were doomed. Cut the dahlia stalks to the ground with pruning shears. The cut stalks. Some of the old stalks from last year are still in evidence. Removing one of the root balls, which averaged about 2 feet across and 15 inches deep. Dahlia tubers are brittle. Just the weight of this root ball resting on a tuber will snap it off. The soil was pretty wet, so it clung tightly to the tubers. If the soil is drier or a sandier texture, it will probably come away from the tubers more readily. Here we've gently dislodged some of the soil from the root ball (and broken a tuber in the process — see the white inside). It's not necessary to remove the soil, but it makes the root ball a bit lighter. In the spring, this clump should be teased apart to create multiple plants. I prefer to do that in the spring because it's almost impossible to separate the tubers without breaking them. Those breaks invite rot during the winter. The clumps get put into black plastic trash bags, tub trugs or other plastic containers. I keep the same varieties together in a bag — marking the clumps of tubers is difficult. The bags get gathered loosely at the top. I leave them partially open so there's some fresh air and moisture transfer. I store the bags on the floor, under the stairs in my unheated basement where it stays between 45 and 50 degree during the winter. Check on them a couple times during the winter. The tubers should stay firm and hydrated — like a potato. Mist them with a little water if they start to get too dry, though being on the dry side is better than too wet. Haul them out in late May and separate into smaller clumps for replanting. You can usually give away at least as many as you keep.

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