Bulb · Asteraceae
Dahlia
Dahlia × hybrida
Give it full sun, rich soil, and steady water, and the dahlia powers out blooms from midsummer to frost.
Full sun
Every 2–4 days
50–84°F · 10–29°C
90–120 days
Overview
Few flowers light up a late-summer border like the dahlia. It joins the daisy family, springs from a fleshy tuber, and throws up fast, bushy growth that flowers from midsummer right up to the first frost. The blooms charge across a huge range, from tidy little pompons to plates as wide as a dinner plate.
Choosing your lineup is where the fun kicks in. Decorative, cactus, pompon, ball, and dwarf bedding forms unlock a massive spread of shape and size, with plants shooting anywhere from 1 to 6 feet (30 to 183 cm) tall. The tall kinds make standout cut flowers and want staking, while the dwarf seed-grown types pack the front of a bed with color.
Dahlias hand back serious rewards for very little skill. Give them sun, rich soil, and water that never quite runs dry, and they flower for months on end. Stay one step ahead of two things and you are golden: soggy soil, which rots the tubers, and winter cold, which just means digging and storing the tubers anywhere colder than zone 7.
Care
The short version: full sun, rich well-drained soil kept evenly moist, a low-nitrogen feed all summer, and a fall lift of the tubers anywhere colder than zone 7.
Light
Dahlias chase full sun, ideally 6 or more hours a day, and that is when they really crank out blooms. They take a little afternoon shade where summers run hot, but deep shade just hands you leggy plants and barely any flowers, so park them in the bright spot.
Water
Keep the soil evenly moist all season and never let it dry out, watering deeply every 2 to 4 days and laying down mulch to lock it in. Pull up short of soggy ground, though, because waterlogged soil rots the tubers fast.
Soil
Plant into rich, well-drained soil packed with organic matter. Drainage is the make-or-break part, since tubers stuck in wet ground rot, so dig in compost and raise the bed wherever your soil clings to water.
Temperature
Dahlias thrive from about 50°F to 84°F (10°C to 29°C) and flower hard through the warm months. They take zero frost, which wipes out the top growth and threatens the tubers below, so ride the warm season for all it is worth.
Humidity
Everyday outdoor humidity suits them fine, so skip the numbers. Airflow does the heavy lifting, since crowded, still air is exactly what lets powdery mildew grab hold of the leaves.
Feeding
Feed monthly through summer with a low-nitrogen fertilizer like 5-10-10 or 5-20-20. Pile on too much nitrogen and you push leafy growth over flowers and leave the tubers more likely to rot, so keep it lean.
Pruning
Pinch back the taller varieties while young and you force bushier, sturdier plants that really fill out. Then deadhead the spent blooms as they fade to keep fresh flowers firing until frost.
Repotting
Lift and divide the tubers every 2 to 3 years to keep them charging. Congested clumps flower less and throw smaller blooms, so dividing kicks their vigor right back up.
Size & spacing
Plants run from 1 to 6 feet (30 to 183 cm) tall and spread 1 to 3 feet (30 to 91 cm) across. Give each one room so air rips through freely and mildew gets nowhere to settle.
When to plant
Dahlias run on warmth and quit at the first frost, so timing pays off big. Kick them off indoors in late winter for a jump on the season, move them out once the soil warms past 60°F (16°C), and watch them flower from midsummer right up until frost calls it.
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 lands in one of two spots, the leaves up top or the tubers down below, and catching it early keeps a small snag from racing through the clump. Spot the signs, match them below, and jump on the fix while it still turns the season around.
Soft, mushy tubers that never sprout
This is tuber rot, and it moves in when the soil stays wet and drainage flops, while damaged or over-fed tubers cave faster in storage. The tell is a mushy, discolored tuber with a foul smell, or a plant that wilts at the base and never breaks ground. Pitch any soft tubers, work organic matter into the bed to drain it, and stash cured 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
Dusty white film racing across the leaves
That is powdery mildew, a fungus that grabs hold when plants crowd each other and the air stalls. It crops up late in the season and rubs straight off the leaf like flour. Open the planting up to get air moving, water at the base, and strip away the worst of the marked foliage to knock it back.
Fix:
- Improve air circulation by proper spacing and thinning
- Avoid overhead watering and water at the base
- Remove and dispose of severely affected foliage
Foliage puckering and mottling as the plant stalls out
Flip the leaves and hunt down aphids and leafhoppers, the small soft-bodied insects that suck sap and ferry mosaic virus from plant to plant. Mottling that will not wash off flags virus, not just feeding damage. Blast light infestations off with a strong jet of water, yank and destroy any plant with stubborn viral symptoms, and keep the insect carriers in check.
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
Good news for the keen grower: dahlias rate only mildly toxic, nowhere near the danger some garden plants pack. Chew the foliage and a pet might land a mild stomach upset, and the sap can sting skin on contact, but people come off essentially safe and the tubers even eat well once cooked. Just keep curious pets from grazing the leaves and tubers, and grow on with confidence.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds
Safe for
People
Cats Mild
A cat that nibbles the foliage may drool or vomit and go briefly off its food, and a brush against the plant can leave a patch of mild skin irritation.
Dogs Mild
A dog that chews the leaves or stems usually lands a mild upset stomach and a little drooling, and the sap can redden the skin on contact.
Horses Mild
A horse grazing the foliage tends to pick up no more than a passing stomach upset, with a chance of mild skin irritation where the plant touches.
Rabbits Mild
The same irritant compounds upset a rabbit's gut, so keep the plant clear of the hutch and run rather than letting it browse.
Livestock Mild
Cattle, goats, and sheep usually leave dahlias alone, but a hungry animal that eats a quantity of foliage can wind up with a mild digestive upset.
Birds Caution
Solid research on birds is scarce, so play it safe: count the plant as a caution and steer pet birds away from nibbling the foliage and stems.
People Low risk
People come off essentially safe, and the tubers even eat well once cooked, so the main nuisance is contact dermatitis. Pull on gloves if your skin reacts to the sap.
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 storm in with thousands of named cultivars, and growers wrangle them into a few broad groups by flower shape. Here are the ones every grower should have on their radar.
Decorative dahlias
Fully double blooms with broad, flat-tipped ray florets make this the most common type you will meet in gardens and as cut flowers.
Cactus dahlias
Double flowers whose narrow, rolled, pointed florets fire off a spiky, star-like look.
Pompon and ball dahlias
Each bloom packs its cupped petals into a tight, rounded little globe, small and satisfyingly neat.
Dwarf bedding dahlias
Compact seed-grown plants around a foot tall that pack a bed, though they cope poorly with real heat.
How to propagate
Dahlias multiply fast, and three routes open up the moment you want more: split an existing clump, root spring cuttings, or sow seed for a fresh, lively mix.
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 is the go-to move for multiplying a favorite dahlia. Split the clump in fall after you lift it, or hold off for spring when the eyes start to swell, and make sure every piece carries part of the crown with at least one live eye. Drop the divisions into warm soil and they take several weeks to push up.
Stem cuttingsModerate
Take basal cuttings in spring from sprouted tubers. · 2 to 4 weeks to root
Cuttings let you clone a variety you are itching to grow more of. In spring, grab the basal shoots rising off a sprouting tuber and root them, which runs about 2 to 4 weeks, to bulk up more plants of the kind you want.
SeedEasy
Start seed indoors 4 to 6 weeks before last frost. · 1 to 2 weeks to germinate
Seed is the easy, fast route, sown indoors 4 to 6 weeks before your last frost, with sprouts breaking through in a week or two. The twist is that seedlings don't come true to the parent and mostly turn out as dwarf bedding plants.
Myths
Dahlias grow in so many gardens that they must be harmless to cats and dogs.
A spot in the average flower bed tells you nothing about whether it is safe to chew. 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.
There is no need to lift dahlia tubers in fall; they ride out winter in the bed anywhere.
They run hardy only to about zone 7, so in colder regions the ground freeze kills the tubers outright. In those zones you dig the tubers in fall and stash them cool and dry, or just start over with new ones each spring.