Dahlia tubers can be planted directly into the garden when the danger of frost has passed. Dahlia tubers consist of a stumpy stem or stems with several swollen parts attached to them. Where all of this meets is called the crown. The dahlia's stems will grow from buds, or eyes, on the crown.
Plant the tuber so that the eyes sit about four to six inches deep. (The larger the plant, the deeper it goes.) Lay the tuber horizontally in the hole with the eyes facing up. If you can't find the eyes, that's all right. Even if you set them facing down, the stems will work their way toward the soil surface. If your dahlias will be tall enough to require staking, set your stakes in the hole, too, about two inches from the crown of the tuber. Fill in the hole. Sprinkle the soil but wait to water thoroughly until after growth has appeared above ground.
After the shoots appear and have formed several sets of leaves, pinch them back by simply snipping or pinching the stem off just above a lower pair of leaves. This will encourage the plant to make more branches and therefore a bushy shape. If more than five shoots sprout from the tuber, remove them so that only five continue to grow. It may seem counterintuitive, but in the long run this will actually result in more flowers than if you had allowed additional stems to continue growing. It will also improve air circulation through the mature plant. (You can make these trimmings into new plants; see the info on taking cuttings, below.)
For earlier flowering in the colder regions, dahlia tubers can be started in pots several weeks before the expected last frost date, so that you'll have bushy young plants to gently transplant into the garden once the weather has warmed. Use a pot just big enough to fit the tubers, and keep it just barely moist until the plant has begun to grow. If you're planning to take stem cuttings (see below), you can set the tuber a bit high in the pot, so that the crown is just above the soil line, which will make it easier to see the stems that you're cutting.
Propagating Dahlias in Spring
You can propagate dahlias at planting time by dividing the tuber. Use a sharp knife to separate the tuber into pieces. Each piece must include at least one swollen, dangling part and a piece of the main stem with at least one eye. To be sure you include an eye in each division, you might wait until the eyes begin to sprout new growth before you divide the tuber. (They will often begin to sprout while still in storage as temperatures warm.) Plant the divisions in individual pots and keep them warm and slightly moist. They should root and send up stems, and then you can treat them as you would treat any dahlia.
You can also propagate dahlias in spring by taking cuttings. When your dahlia tuber has sent up a handful of stems and they're about three inches tall, cut some of these shoots off just above where they meet the tuber's crown. If you can get a little bit of tuber to come off with the cutting, that's great.
Then snip off the lowest set of leaves. Stick the cutting into a pot of light potting soil with good drainage, placing it at the edge of the pot so that the roots will grow touching the pot's sides. This will encourage budding at the roots. Keep the pot moist and warm. To conserve moisture, you can seal it in a plastic bag. Within a few weeks the cutting should take root; you will know it has when it resists a tug on its stem. Pinch the stem, then, to encourage a bushy habit.
Tuber image by F. D. Richards/CC BY-SA 2.0