Bulb · Asteraceae
Dahlia
Dahlia × hybrida
Hand it full sun, rich soil, and steady water, and the dahlia blooms hard from midsummer to frost.
Full sun
Every 2–4 days
50–84°F · 10–29°C
90–120 days
Overview
Not many flowers carry a late-summer border like the dahlia does. It's a member of the daisy family, it grows from a fleshy tuber, and it throws up fast, bushy growth that flowers from midsummer right up until the first frost. And the blooms? They run from tidy little pompons all the way to plates the size of a dinner plate.
Picking your favorites is honestly the fun part. Between the decorative, cactus, pompon, ball, and dwarf bedding types, you've got a huge range of shapes and sizes, with plants standing anywhere from 1 to 6 feet (30 to 183 cm) tall. The tall ones make great cut flowers but want staking, while the little seed-grown dwarfs are perfect up front in a bed.
Dahlias give back way more than they ask for. Give them sun, rich soil, and water that never quite dries out, and they'll flower for months. The two things to stay ahead of are 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 want full sun, ideally 6 or more hours a day, and that's when they really bloom. A bit of afternoon shade is fine where summers get hot, but deep shade? That'll just give you leggy plants and not many flowers.
Water
Keep the soil evenly moist all season and don't let it dry out, watering deeply every 2 to 4 days and mulching to hold it in. Just don't let it go soggy, because waterlogged soil is what rots the tubers.
Soil
Go with rich, well-drained soil that's full of organic matter. Drainage is really the big one here, since tubers sitting in wet ground will rot, so mix in compost and raise the bed if your soil tends to stay wet.
Temperature
Dahlias are happy from about 50°F to 84°F (10°C to 29°C) and bloom right through the warm months. They can't take frost, though, which kills the top growth and puts the tubers below at risk.
Humidity
Regular outdoor humidity is plenty, so there's no number you need to hit. What matters more is good airflow, because crowded, still air is exactly what lets powdery mildew settle in on the leaves.
Feeding
Feed once a month through summer with a low-nitrogen fertilizer, something like 5-10-10 or 5-20-20. Too much nitrogen just pushes leafy growth instead of flowers, and it makes the tubers more likely to rot.
Pruning
Pinch back the taller varieties while they're young, and you'll get bushier, sturdier plants. Then deadhead the spent blooms as they fade, and fresh flowers keep coming right until frost.
Repotting
Lift and divide the tubers every 2 to 3 years to keep them going strong. Crowded clumps flower less and give you smaller blooms, so dividing is just a way to freshen them 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) wide. Give each one some room so air can move, and mildew won't get such an easy start.
When to plant
Here's the thing about dahlias: they love warmth and can't handle frost at all. Want a head start? Start them indoors in late winter, then set them out once the soil's warmed past 60°F (16°C). After that they'll flower from midsummer right up until frost.
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
Most dahlia trouble turns up in one of two spots, the leaves up top or the tubers down below. Catch it early and you can usually step in before it spreads through the clump. So take a look at what's happening, find the match below, and deal with it while it still helps.
Soft, mushy tubers that never sprout
This one's tuber rot, and it sets in when the soil stays wet and won't drain, while tubers that got damaged or overfed rot faster in storage. The giveaway is a mushy, off-color tuber that smells foul, or a plant that wilts at the base and never comes up. Toss any soft tubers, work organic matter into the bed so it drains, and store 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 creeping across the leaves
That's powdery mildew, a fungus that moves in when plants are crowded and the air sits still. It usually shows up late in the season and rubs right off the leaf like flour. Open the planting up so air can move, water down at the base instead of overhead, and clear away the worst of the marked 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 plant that's stalled
Flip a leaf over and look for aphids and leafhoppers, those little soft-bodied bugs that suck sap and carry mosaic virus from plant to plant. If the mottling won't wash off, that's virus talking, not just feeding damage. Blast light infestations off with a strong jet of water, pull and destroy any plant with stubborn viral signs, and keep those bugs 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, dahlias are only mildly toxic, so they don't carry the danger some garden plants do. Chew the leaves and a pet might get a bit of a stomach ache, and the sap can leave skin a little irritated, but people are basically fine and the tubers are even edible once they're cooked. Still, it's smart to keep nosy pets from grazing on the leaves and tubers.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds
Safe for
People
Cats Mild
A cat that nibbles the leaves might drool or throw up and go off its food for a bit, and brushing against the plant can leave a little patch of mild skin irritation.
Dogs Mild
If a dog chews the leaves or stems, you'll probably see a mild upset stomach and a bit of drooling, and the sap can redden the skin where it touches.
Horses Mild
A horse grazing on the foliage usually gets nothing worse than a passing tummy upset, with maybe a touch of mild skin irritation where the plant brushes it.
Rabbits Mild
Those same irritant compounds bother a rabbit's gut too, so keep the plant out of the hutch and run instead of letting it browse.
Livestock Mild
Cattle, goats, and sheep mostly leave dahlias alone, but a hungry one that eats a bunch of foliage can wind up with a mild digestive upset.
Birds Caution
There isn't much solid data on birds, so it's safest to treat the plant as a caution and keep pet birds from pecking at the leaves and stems.
People Low risk
People are basically safe here, and the tubers are even edible once they're cooked, so the main nuisance is contact dermatitis. Just wear 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
There are thousands of named dahlias out there, which sounds like a lot, right? Growers make it simple by sorting them into a few groups based on flower shape. Here are the ones worth knowing.
Decorative dahlias
These have fully double blooms with broad, flat-tipped ray florets, and they're the type you'll run into most in gardens and as cut flowers.
Cactus dahlias
These are double flowers whose narrow, rolled, pointed florets give the bloom a spiky, star-like look.
Pompon and ball dahlias
These are small, rounded globes packed tight from neatly cupped petals.
Dwarf bedding dahlias
These are compact seed-grown plants around a foot tall that work great for bedding, though they don't handle real heat too well.
How to propagate
Want more dahlias? You've got three easy options: split a clump you already have, root some spring cuttings, or sow seed for 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 is the usual way to make more of a dahlia you love. Split the clump in fall after you've dug it up, or wait until spring when the eyes start to swell, and just make sure every piece carries part of the crown with at least one live eye. Pop them into warm soil and the divisions take several weeks to push up.
Stem cuttingsModerate
Take basal cuttings in spring from sprouted tubers. · 2 to 4 weeks to root
You can also clone a variety from cuttings. In spring, grab the basal shoots coming off a sprouting tuber and root them, which takes about 2 to 4 weeks, and you'll have more plants of exactly the kind you wanted.
SeedEasy
Start seed indoors 4 to 6 weeks before last frost. · 1 to 2 weeks to germinate
Seed's the easy route, sown indoors 4 to 6 weeks before your last frost, with sprouts up in a week or two. The catch? The seedlings don't come true to the parent and mostly end up as dwarf bedding plants.
Myths
Dahlias must be totally safe around pets since they're such common garden flowers.
How common a flower is doesn't tell you a thing about whether it's safe to chew. The ASPCA lists dahlia as toxic to cats, dogs, and horses, with mild stomach upset and maybe some skin irritation. The signs stay mild, sure, but the plant still belongs out of a pet's reach.
You can leave dahlia tubers in the ground over winter no matter where you live.
They're only hardy to about zone 7, so where it's colder, the ground freeze just kills the tubers outright. In those zones you dig the tubers in fall and store them cool and dry, or you start fresh with new ones each spring.