Dahlia Care: From Planting Tubers to Big Blooms

Few flowers reward a little effort quite like dahlias. From a single knobby tuber planted in spring, you can grow a plant that pumps out armfuls of blooms in colors and forms that range from dinner-plate spheres to spiky cactus stars. This guide walks you through the whole journey, from waking up tubers after the last frost to digging and storing them so they bloom again next year.

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Planting dahlia tubers

Dahlias are tender plants that originate from the highlands of Mexico, which means they have zero tolerance for frost. Getting the planting right sets the tone for the entire season, so it pays to slow down and pay attention to timing, depth, and spacing.

Timing: wait for warm soil

Plant tubers outdoors only after your last expected frost date, when the soil has warmed to at least 60°F (15°C). Cold, wet ground is the number-one killer of dahlia tubers, which will simply rot before they sprout. In most of the US this means mid-spring to early summer. To get a head start, you can pot tubers up indoors about four weeks before your last frost and transplant the sprouted plants once the danger has passed. Quality stock matters, so consider browsing a range of dahlia tubers to find varieties suited to your garden.

Depth and spacing

• Dig a hole about 4 to 6 inches deep and lay the tuber on its side with the “eye” (the growing point near the old stem) facing up.
• Cover with 2 to 3 inches of soil at first, then fill in the rest as the shoot grows taller.
• Space dwarf bedding types 9 to 12 inches apart, and large border or dinner-plate varieties 18 to 24 inches apart for good airflow.
• Drive a stake into the ground at planting time for tall varieties so you do not spear the tuber later.

Hold off on watering until you see green growth above the soil. A freshly planted tuber has no roots to take up moisture, so watering early just invites rot.

Light, soil, and watering

Give them sun and good drainage

Dahlias are sun worshippers. Choose a spot that gets at least 6 to 8 hours of direct sunlight a day; more sun generally means more flowers and sturdier stems. In very hot southern climates, a little afternoon shade can keep blooms from scorching, but everywhere else, full sun is the goal.

The ideal soil is rich, fertile, and above all well-drained. Heavy clay that stays soggy will rot tubers, so work in plenty of compost or aged manure before planting to improve both fertility and drainage. Dahlias prefer a slightly acidic to neutral pH of around 6.5 to 7.0. If your soil is heavy, consider planting in raised beds or large containers instead.

Watering rhythm

Once plants are up and growing, dahlias are thirsty. Water deeply two or three times a week, aiming for the equivalent of about an inch of water, more during heat waves. Deep, infrequent watering encourages strong roots far better than frequent light sprinkles. Water at the base of the plant rather than overhead to keep foliage dry and reduce the risk of powdery mildew. A layer of mulch helps conserve moisture and keeps the soil cool, but keep it a couple of inches away from the stems.

Feeding for healthy plants and blooms

Dahlias are hungry plants, but how and when you feed them makes a real difference. The most common mistake is using a high-nitrogen fertilizer, which produces lush leafy growth at the expense of flowers and can leave you with soft, rot-prone tubers at season’s end.

When to start feeding

Wait until plants are established and about 12 inches tall before you begin feeding, roughly 30 days after sprouting. Starting too early just fuels foliage. From that point, switch to a fertilizer that is lower in nitrogen and higher in phosphorus and potassium, the nutrients that drive flowering and tuber development. A formula such as 5-10-10 or 0-20-20 works well. Feed every three to four weeks through the growing season, then stop feeding by late summer so the plant can focus on maturing its tubers. Look for a balanced bloom-boosting low-nitrogen fertilizer to keep blooms coming.

Staking, pinching, and disbudding

This trio of techniques separates good dahlia displays from spectacular ones. None are complicated, and each pays off in stronger plants and better flowers.

Staking tall varieties

Large dahlias can reach 4 to 5 feet tall and grow top-heavy with blooms, so they need support or a single gust of wind will snap them. Set a sturdy stake at planting time and tie the main stem loosely as it grows, adding ties every foot or so. For a row of dahlias, running twine between stakes on either side creates a corral that holds everything upright. A set of strong plant stakes makes this job quick and tidy.

Pinching for bushier plants

When your dahlia has three or four sets of leaves and is roughly 12 to 16 inches tall, pinch out the central growing tip just above a leaf pair. It feels counterintuitive to remove the top of a healthy plant, but this single act forces the plant to branch out from the side shoots, creating a bushier plant with many more stems and therefore far more flowers over the season.

Disbudding for bigger blooms

Each dahlia stem typically forms a central bud flanked by two smaller side buds. If you want one large, show-quality flower, pinch off the two side buds so the plant pours all its energy into the central bloom. This is how growers produce those impressive dinner-plate flowers. If you prefer more flowers for cutting and a longer overall display, simply leave all the buds in place. The choice is yours.

Deadheading and pest control

Keep deadheading for nonstop blooms

Dahlias bloom from midsummer right up until the first frost, but only if you keep removing spent flowers. Deadheading tricks the plant into producing more blooms rather than setting seed. The tricky part is telling a spent flower from an unopened bud, since both can look rounded. The trick: opened-then-faded blooms are pointed and conical once the petals drop, while fresh buds are round and firm. Snip spent flowers off down to the next leaf or side bud.

Slugs, earwigs, and other pests

• Slugs and snails love tender young dahlia shoots and can devour seedlings overnight. Protect new growth with copper tape, organic slug pellets, or beer traps, and clear away hiding spots like debris and dense mulch near the stems.
• Earwigs chew ragged holes in petals and leaves at night. Trap them by placing rolled-up damp newspaper or upturned flowerpots stuffed with straw nearby, then empty the traps each morning.
• Aphids, spider mites, and thrips can cluster on buds and new growth. A blast of water or insecticidal soap usually keeps them in check.
• Powdery mildew appears late in the season as a white coating on leaves. Good spacing, airflow, and base watering go a long way toward preventing it.

Digging, storing, and dividing tubers

In USDA zones 8 to 11, dahlias can usually stay in the ground year-round with a protective layer of mulch. In colder zones, where the ground freezes, you will need to lift the tubers and store them indoors to grow them again next year.

Digging tubers in fall

After the first hard frost blackens the foliage, cut the stems down to about 4 to 6 inches. Wait a week or two to let the tubers mature, then carefully dig a wide circle around each clump with a fork to avoid spearing them. Gently lift, shake off loose soil, and rinse the clumps clean. Let them dry in a cool, frost-free spot for a few days to cure the skins.

Storing over winter

Pack the cured tuber clumps in boxes or crates filled with a dry medium such as peat-free compost, vermiculite, sawdust, or wood shavings so they do not touch. Store them somewhere cool, dark, and frost-free, ideally between 40 and 50°F (4 to 10°C). Check them every few weeks through winter, discarding any that turn soft or moldy and lightly misting any that look shriveled.

Dividing for more plants

Over time a single tuber multiplies into a large clump, which you can divide to make more plants and keep them vigorous. You can divide in fall before storing or in spring before planting. Each viable division must include a piece of the central crown with at least one visible eye, since a tuber without an eye will never sprout. Use a clean, sharp knife and dust any cuts. If you enjoy multiplying your favorite plants, you may also like our guides on how to grow tulips and the easygoing tiger lily.

For more seasonal flower growing inspiration, explore our full plant & flower care guide.

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