Bulb · Asteraceae
Dahlia
Dahlia × hybrida
On full sun, rich soil, and steady water, the dahlia opens bloom after bloom from midsummer to frost.
Full sun
Every 2–4 days
50–84°F · 10–29°C
90–120 days
Overview
The dahlia is one of the showiest faces of the late-summer border. A daisy-family plant that grows from a clump of fleshy, finger-like tubers, it sends up fast, bushy growth of dark green, toothed leaves and flowers from midsummer to the first frost. The heads span a wide range of size, from tight little pompons no bigger than a button to broad faces as wide as a dinner plate.
The flower heads themselves come in markedly different builds. Decorative, cactus, pompon, ball, and dwarf bedding forms set their florets in flat, rolled, or cupped arrangements, on plants that stand anywhere from 1 to 6 feet (30 to 183 cm) tall. The taller stems carry long-stemmed heads that cut well for the vase and lean enough to want staking, while the dwarf seed-grown plants stay low and dense.
Up close, the dahlia rewards a steady look for very little effort. Sun, rich soil, and water held just short of dry keep the leaves green and the heads opening for months. Two faults show plainly: soggy soil, which softens and rots the tubers, and winter cold, which means lifting those tubers anywhere colder than zone 7.
Care
The short version: full sun, rich and well-drained soil kept evenly moist, a low-nitrogen feed through summer, and a fall lift of the fleshy tubers in any zone colder than zone 7.
Light
Dahlias open their fullest, most tightly packed heads in full sun, ideally 6 or more hours a day. A little afternoon shade is fine where summers run hot. Deep shade stretches the stems thin and pale and leaves few flowers to show for it.
Water
The roots want the soil evenly moist all season and never bone-dry, with a deep soak every 2 to 4 days and a blanket of mulch over the top. Stop short of soggy ground, since waterlogged soil softens the fleshy tubers into rot.
Soil
Dahlias root best in rich, well-drained soil dark with organic matter. Drainage is the deciding thing, because tubers left in wet ground turn soft and rot, so compost worked in and a raised bed keep water from pooling around them.
Temperature
Dahlias hold their green from about 50°F to 84°F (10°C to 29°C) and flower through the warm months. Frost has no mercy on them, blackening the soft top growth and threatening the tubers packed below.
Humidity
Plain outdoor humidity suits them, so there is no number to chase. Moving air counts for more, since crowded, still air settles a powdery film across the leaf surfaces.
Feeding
A low-nitrogen fertilizer such as 5-10-10 or 5-20-20, fed monthly through summer, keeps the heads coming. Too much nitrogen swells the leafy growth at the cost of flowers and leaves the tubers softer and quicker to rot.
Pruning
Pinching back the taller varieties while young thickens the stems into bushier, sturdier plants. Deadheading the spent, browning heads as they fade keeps fresh buds rising until frost.
Repotting
Lifting and dividing the tubers every 2 to 3 years keeps the clump vigorous. Crowded clusters of tubers flower less and open smaller heads, so splitting them apart renews the show.
Size & spacing
Plants stand from 1 to 6 feet (30 to 183 cm) tall and spread 1 to 3 feet (30 to 91 cm) across in a leafy mound. Set them that far apart so air slips between the leaves and mildew finds no still, damp surface to settle on.
When to plant
Dahlias run on warmth, and a single frost blackens the soft stems and leaves. An indoor start in late winter wakes the pale eyes early, the plants set out once the soil warms past 60°F (16°C), and the flower heads open from midsummer until frost finally cuts them down.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: Start tubers or seed indoors 4 to 6 weeks before the last spring frost in colder zones.
- Plant: Plant tubers outdoors in spring after the last frost once soil has warmed to at least 60 degrees F.
- Harvest: Mid-summer through first fall frost (roughly July to first frost).
Hardiness
USDA hardiness zones: 7–10
Temperature: down to -18°C
Troubleshooting
Dahlia trouble shows on the green leaves above ground or on the fleshy tubers below, and the surfaces tell the story. A dusty leaf, a softened tuber, a mottled blade, each looks distinct up close. Match what your eye lands on below, then step in before it spreads through the clump.
Soft, mushy tubers that never sprout
A tuber gone soft and discolored is tuber rot, set off by wet, poorly drained soil, with bruised or over-fed tubers caving faster in storage. The clue is that mushy, off-color flesh and a foul smell, or a plant that wilts at the base and never breaks the surface. Throw out the soft tubers, work organic matter through the bed to drain it, and keep the cured, firm tubers dry and cool over winter.
Fix:
- Discard rotted tubers and avoid replanting in the same wet spot
- Improve soil drainage with organic matter and raised beds
- Store cured tubers dry at 35 to 45 degrees F in vermiculite or dry sand
A dusty white film coating the green leaves
That pale, floury coat is powdery mildew, a fungus that takes hold when leaves crowd together and the air sits still around them. It surfaces late in the season and smears off the leaf like a dusting of flour under a fingertip. Open the planting so air slips through, water at the base to keep the blades dry, and strip away the worst-coated leaves.
Fix:
- Improve air circulation by proper spacing and thinning
- Avoid overhead watering and water at the base
- Remove and dispose of severely affected foliage
Curled, mottled leaves on a stunted plant
Turn the leaf and the undersides crawl with aphids and leafhoppers, small soft-bodied insects that sip sap and ferry mosaic virus between plants. A yellow mottling pressed into the blade that will not wash off marks virus rather than feeding alone. Blast the light clusters off with a hard jet of water, pull and destroy any plant patterned with stubborn viral mottling, and keep the insect carriers down.
Fix:
- Spray off light aphid infestations with a strong stream of water
- Remove and destroy plants showing persistent viral symptoms
- Control insect vectors to limit virus spread
Toxicity
Dahlias carry only mild toxicity, so they pose far less danger than some garden plants. A pet that bites into the leaves or stems may get a mild stomach upset, and the watery sap can leave a reddened patch on skin. People stay essentially safe, and the knobby tubers are edible once cooked. Even so, keep curious pets from chewing the leaves and tubers.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds
Safe for
People
Cats Mild
A cat that bites the leaves may drool or vomit and turn briefly off its food, and a brush of the foliage can leave a small reddened patch of irritated skin.
Dogs Mild
A dog that chews the leaves or stems tends to get a mild upset stomach and a little drooling, and the watery sap can redden the skin on contact.
Horses Mild
A horse cropping the foliage usually shows no more than a passing stomach upset, with a chance of mild skin irritation where the plant brushes against it.
Rabbits Mild
The same irritant compounds turn a rabbit's gut, so the plant stays clear of the hutch and run rather than within reach of its teeth.
Livestock Mild
Cattle, goats, and sheep mostly pass dahlias by, but a hungry animal that crops a quantity of foliage can end up with a mild digestive upset.
Birds Caution
Firm data on birds is thin, so the plant rates a caution, and pet birds are kept from pecking at the leaves and stems.
People Low risk
People stay essentially safe, and the tubers eat well once cooked, leaving contact dermatitis as the only real nuisance. Gloves help where the sap reddens the skin.
Wear gloves when handling foliage if you are prone to contact dermatitis, and keep tubers and cuttings out of reach of pets.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
United Kingdom
Animal PoisonLine: 01202 509000 (fee applies)
Australia
Animal Poisons Helpline: 1300 869 738
Varieties
Dahlias number in the thousands of named cultivars, sorted by the shape and set of their florets. The groups below are the ones whose flower heads are easiest to tell apart by eye.
Decorative dahlias
Broad, flat-tipped ray florets pack into a fully double head, the most common dahlia face in gardens and the vase.
Cactus dahlias
Narrow florets rolled to a fine point quill outward from a double head, giving it a spiky, star-like outline.
Pompon and ball dahlias
Tiny cupped petals curl tightly into a small, rounded globe of a head.
Dwarf bedding dahlias
Compact seed-grown plants stand around a foot tall with low, dense heads, though real heat sets them back.
How to propagate
Dahlias multiply readily from the plant's own parts: a split tuber clump, a rooted spring shoot, or a sown seed that opens into a fresh mix of plants.
DivisionModerate
Divide tuber clumps in fall after digging or in spring once eyes begin to sprout. · Several weeks to emerge after planting in warm soil
Division splits the fleshy tuber clump into more of the same plant. Pull the clump apart in fall after lifting, or in spring once the pale eyes start to swell, leaving every piece with part of the crown and at least one live eye. Set into warm soil, each division takes several weeks to break the surface.
Stem cuttingsModerate
Take basal cuttings in spring from sprouted tubers. · 2 to 4 weeks to root
Cuttings clone a variety from its own young growth. In spring, snap off the basal shoots pushing from a sprouting tuber and root them, which takes about 2 to 4 weeks, to raise more plants of the same flower.
SeedEasy
Start seed indoors 4 to 6 weeks before last frost. · 1 to 2 weeks to germinate
Seed is the easy route, sown indoors 4 to 6 weeks before the last frost, the small sprouts breaking through in a week or two. The catch is that the seedlings don't come true to the parent and mostly open as dwarf bedding plants.
Myths
Dahlias are completely safe around pets because they are common garden flowers.
A familiar face in the flower bed says nothing about how the plant sits in a pet's stomach. The ASPCA lists dahlia as toxic to cats, dogs, and horses, with mild stomach upset and possible skin irritation. The signs stay mild, but the plant still belongs out of a pet's reach.
Dahlia tubers can be left in the ground over winter anywhere.
The tubers are hardy only to about zone 7, so in colder ground a hard freeze turns them to soft mush. In those zones the tubers come up in fall for cool, dry storage, or new ones go in each spring.