Bulb · Asteraceae
Dahlia
Dahlia × hybrida
With full sun, rich soil, and steady water, your dahlia rewards you with blooms from midsummer all the way 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 the way a dahlia does, and it is a forgiving plant to lean on. A member of the daisy family, it grows from a fleshy tuber and puts up fast, bushy growth that flowers from midsummer right up to that first frost. The blooms swing from tidy little pompons to plates as wide as a dinner plate, so there is a friendly face for every garden.
Picking your favorites is the joyful part. Between the decorative, cactus, pompon, ball, and dwarf bedding forms you get a wonderful range of shapes and sizes, with plants standing anywhere from 1 to 6 feet (30 to 183 cm) tall. The taller ones make lovely cut flowers and appreciate a stake, while the little dwarf seed-grown kinds settle happily at the front of a bed.
Dahlias give back far more than they ask, which makes them gentle on a nervous new grower. Offer sun, rich soil, and water that never quite runs dry, and they will flower for months on end. The two things worth keeping an eye on are soggy soil, which can rot the tubers, and winter cold, which simply means lifting and storing the tubers anywhere colder than zone 7.
Care
The short version: give your dahlia full sun and rich, well-drained soil kept evenly moist, feed it a low-nitrogen fertilizer through summer, and in any zone colder than zone 7 lift the tubers in fall to tuck them away for winter.
Light
Your dahlia is happiest in full sun, around 6 or more hours a day, and that is when it blooms its best. A touch of afternoon shade is fine where summers run hot, but a deeply shaded spot will leave you with leggy plants and not many flowers, so aim for the bright corner.
Water
Keep the soil evenly moist all season and try not to let it dry out, giving a deep drink every 2 to 4 days and laying down mulch to help it last. The one thing to avoid is leaving the ground soggy, since waterlogged soil is what rots the tubers, so steady and damp is the goal, not wet.
Soil
Plant into rich, well-drained soil full of organic matter, and you are off to a strong start. Drainage really is the part that matters most, because tubers left sitting in wet ground will rot, so mix in plenty of compost and raise the bed a little if your soil tends to hold water.
Temperature
Dahlias are content from about 50°F to 84°F (10°C to 29°C) and keep flowering right through the warm months. They simply cannot take frost, which kills the top growth and puts the tubers below at risk, so let that warmth guide your timing.
Humidity
Everyday outdoor humidity suits them just fine, so there is no target number to fret over. What helps far more is good airflow, because crowded, still air is exactly what lets powdery mildew settle onto the leaves.
Feeding
Feed once a month through summer with a low-nitrogen fertilizer, something like 5-10-10 or 5-20-20. Going heavy on nitrogen only pushes lots of leaves at the cost of flowers, and it leaves the tubers more prone to rot, so a light, balanced hand is kindest.
Pruning
Pinch back the taller varieties while they are still young, and you will be rewarded with bushier, sturdier plants. From there, deadhead the faded blooms as they go over, and the plant keeps sending up fresh flowers right until frost.
Repotting
Lift and divide the tubers every 2 to 3 years to keep them happy and full of life. Clumps that grow crowded tend to flower less and give you smaller blooms, so this gentle dividing is really just a way of renewing an old friend.
Size & spacing
Plants reach from 1 to 6 feet (30 to 183 cm) tall and spread 1 to 3 feet (30 to 91 cm) across. Give each plant a little elbow room so air can move freely around it, and mildew will have a much harder time getting started.
When to plant
Dahlias love warmth and have no patience for frost, so timing is on your side once the cold passes. You can start them indoors in late winter for an early lead, then set them out after the soil warms past 60°F (16°C), and from midsummer until frost they will keep flowering for you.
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 worries appear either on the leaves you can see or on the tubers tucked below, and the good news is that spotting them early usually means an easy fix. Take a calm look at the signs below, match what is happening, and step in while a small problem is still a small problem.
Soft, mushy tubers that just never wake up
This is tuber rot, and it creeps in where the soil stays wet and drains poorly, while tubers that were damaged or overfed give way faster in storage. The telltale sign is a mushy, off-color tuber with a sour smell, or a plant that wilts at the base and never pushes up. Toss any soft tubers without guilt, work organic matter into the bed so it drains, and store your 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
A dusty white film creeping across the leaves
What you are seeing is powdery mildew, a fungus that moves in when plants crowd one another and the air goes still. It tends to show up later in the season and wipes off the leaf like a smear of flour, so do not be alarmed. Open the planting up so air can move, water down at the base rather than over the top, and gently clear away the most 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 has stalled
Turn a few leaves over and look for aphids and leafhoppers, the small soft-bodied insects that sip sap and carry mosaic virus from plant to plant. If the mottling will not wash away, that points to virus rather than feeding alone, which is worth knowing early. Blast light infestations off with a strong jet of water, pull and destroy any plant with stubborn viral signs, and keep those little 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
There is no need to panic about dahlias, since they are only mildly toxic and far gentler than many garden plants. A pet that nibbles the leaves might get a brief tummy upset, and the sap can leave skin a little irritated, but people are essentially safe and the tubers are even good to eat once cooked. Still, it helps to keep curious pets from grazing on the foliage and tubers.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds
Safe for
People
Cats Mild
A cat that nibbles the foliage may drool a little or be sick and go off its food for a short while, and a brush against the plant can leave a small patch of mild skin irritation.
Dogs Mild
If a dog chews the leaves or stems, look for a mild upset stomach and a bit of drooling, and the sap can redden the skin where it touches.
Horses Mild
A horse grazing the foliage usually gets nothing worse than a passing stomach upset, with a small chance of mild skin irritation where the plant brushes against it.
Rabbits Mild
Those same irritant compounds unsettle a rabbit's gut, so the kind move is to keep the plant well away from the hutch and run rather than letting it browse.
Livestock Mild
Cattle, goats, and sheep mostly leave dahlias be, but a hungry animal that eats a good amount of foliage can come away with a mild digestive upset.
Birds Caution
There is not much solid data on birds yet, so it is safest to treat the plant as a caution and keep pet birds from pecking at the leaves and stems.
People Low risk
People are essentially safe here, and the tubers are even edible once cooked, so the only real nuisance is contact dermatitis. Pop on gloves if your skin tends to react 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 arrive in thousands of named cultivars, which sounds overwhelming until you learn that growers tuck them into a handful of groups by flower shape. These few are the friendly ones to start with.
Decorative dahlias
These carry fully double blooms with broad, flat-tipped ray florets, and they are the friendly, familiar type you will run into most in gardens and as cut flowers.
Cactus dahlias
Their double flowers are made of narrow, rolled, pointed florets, which give each bloom a lovely spiky, star-like look.
Pompon and ball dahlias
These form small, rounded globes packed tight from neatly cupped petals, neat as a button.
Dwarf bedding dahlias
These are compact, seed-grown plants of around a foot tall that are perfect for bedding, though they do struggle when real heat sets in.
How to propagate
Making more dahlias is genuinely satisfying, and you have three easy doors in: split a clump you already love, root spring cuttings, or sow seed for a cheerful mix of new 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 everyday way to make more of a dahlia you love. Split the clump in fall once you have lifted it, or hold off until spring when the eyes begin to swell, and just be sure every piece carries part of the crown with at least one live eye. Set into warm soil, the divisions take several weeks to push up, so be patient with them.
Stem cuttingsModerate
Take basal cuttings in spring from sprouted tubers. · 2 to 4 weeks to root
You can also clone a favorite from cuttings, which is easier than it sounds. In spring, take the basal shoots rising off a sprouting tuber and root them, a step that takes about 2 to 4 weeks, and you will 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 is the relaxed, low-stakes route, sown indoors 4 to 6 weeks before your last frost, with little sprouts up in a week or two. Just know going in that seedlings don't come true to the parent and tend to turn out as dwarf bedding plants, which can be a happy surprise of its own.
Myths
Dahlias must be perfectly safe around pets since they are such common garden flowers.
How common a flower is 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 a chance of skin irritation. The signs stay gentle, but the plant is still best kept out of a pet's reach.
You can leave dahlia tubers in the ground over winter no matter where you garden.
They are only hardy to about zone 7, so in colder places the ground freeze finishes the tubers off completely. In those zones the kind thing is to dig the tubers in fall and store them cool and dry, or simply begin again with fresh ones each spring.