Bulb · Asteraceae
Dahlia
Dahlia × hybrida
With full sun, rich soil, and steady water, the dahlia carries a border in style from midsummer to frost.
Full sun
Every 2–4 days
50–84°F · 10–29°C
90–120 days
Overview
Few flowers set the tone of a late-summer border the way a dahlia does. A member of the daisy family that rises from a fleshy tuber, it puts on fast, bushy growth and flowers from midsummer right through to the first frost, holding the garden's attention through the season's long, golden stretch. The range runs from neat little pompons to blooms as broad as a dinner plate, so there is a register for every mood, from understated to frankly theatrical.
Choosing among the forms is where a planting takes on character. Decorative, cactus, pompon, ball, and dwarf bedding types give a generous span of shape and scale, with plants standing anywhere from 1 to 6 feet (30 to 183 cm) tall. The taller kinds earn their keep as cut flowers for the table and want staking, while the dwarf seed-grown forms settle in at the front of a bed and edge a path with ease.
What makes the dahlia such a willing partner is how much it returns for so little. Give it sun, rich soil, and water that never quite runs dry, and it flowers for months, slotting neatly into a weekly garden routine. Two things ask for attention: soggy soil, which rots the tubers, and the winter cold, which simply means lifting and storing the tubers anywhere colder than zone 7.
Care
The short version: a sunny bed of rich, well-drained soil kept evenly moist, a low-nitrogen feed through summer, and a fall lift of the tubers in any zone colder than zone 7.
Light
Give a dahlia full sun, ideally 6 or more hours a day, and it pays you back in the fullest bloom. A little afternoon shade is welcome where summers run hot, but a shaded corner leaves you with leggy plants and few flowers, which is no way to anchor a bed.
Water
Keep the soil evenly moist through the season and never let it dry out, watering deeply every 2 to 4 days and mulching to hold it in, an easy rhythm to fold into a weekly round. Just stop short of soggy ground, since waterlogged soil rots the tubers.
Soil
Plant into rich, well-drained soil generous with organic matter. Drainage is the part that decides everything, because tubers sitting in wet ground rot, so work in compost and lift the bed a little where the soil holds water.
Temperature
Dahlias are at ease from about 50°F to 84°F (10°C to 29°C) and flower right through the warm months. They have no patience for frost, which takes the top growth and threatens the tubers below, so the warm season is their stage.
Humidity
Ordinary outdoor humidity suits them, so there is no figure to chase. Air movement counts for more, since crowded, still air is what invites powdery mildew onto the leaves and dulls the display.
Feeding
Feed monthly through summer with a low-nitrogen fertilizer such as 5-10-10 or 5-20-20. Lean too hard on nitrogen and you trade flowers for foliage, while leaving the tubers more inclined to rot.
Pruning
Pinch back the taller varieties while young to coax bushier, sturdier plants that hold their shape in a border. Then deadhead the spent blooms as they fade, and fresh flowers keep arriving until frost, keeping the planting looking cared for.
Repotting
Lift and divide the tubers every 2 to 3 years to keep them performing. Clumps that grow congested flower less and offer smaller blooms, so dividing simply renews the show.
Size & spacing
Plants stand from 1 to 6 feet (30 to 183 cm) tall with a spread of 1 to 3 feet (30 to 91 cm), which is worth keeping in mind as you space a bed. Give each one room so air moves freely and mildew has less of a chance to spoil the look.
When to plant
Dahlias keep to a warm-weather calendar and bow out at the first touch of frost. For an earlier show, start them indoors in late winter, move them outside once the soil has warmed past 60°F (16°C), and let them carry the garden from midsummer until frost draws the season to a close.
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
The two places a dahlia display can falter are the leaves on show and the tubers working quietly below. Notice the early signs and a small fault stays a small fault rather than spreading through the planting. Find the match below, then act while the season is still yours to shape.
Soft, mushy tubers that never wake up
This is tuber rot, which takes hold where the soil stays wet and drains poorly, while damaged or over-fed tubers go faster in storage. The sign is a mushy, discolored tuber with a foul smell, or a plant that wilts at the base and never appears. Discard any soft tubers, work organic matter into the bed so it drains, and keep cured tubers dry and cool over winter so they return to the border.
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 dulling the leaves
This is powdery mildew, a fungus that settles in when plants crowd one another and the air goes still. It tends to arrive late in the season and wipes off the leaf like flour, leaving the planting looking tired. Open things up so air can move, water at the base rather than overhead, and clear away the worst of the marked foliage to restore the look.
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 stalled plant
Turn the leaves over and look for aphids and leafhoppers, the small soft-bodied insects that sip sap and pass mosaic virus from plant to plant. A mottling that will not wash off points to virus rather than feeding alone. Wash light infestations off with a strong jet of water, lift and destroy any plant carrying stubborn viral symptoms, and keep the insect carriers in check to protect the rest of the planting.
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
As garden flowers go, dahlias sit easily in a planted home, since they are only mildly toxic. A pet that samples the foliage may feel briefly off, and the sap can leave skin a touch irritated, yet people are essentially safe and the tubers are even edible once cooked. The sensible house rule is simply to keep curious pets from grazing the leaves and tubers.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds
Safe for
People
Cats Mild
A cat that nibbles the foliage may drool or be sick and lose interest in food for a short while, and a brush against the plant can leave a patch of mild skin irritation.
Dogs Mild
A dog that chews the leaves or stems tends to get a mild upset stomach and a little drooling, and the sap can redden the skin on contact.
Horses Mild
A horse grazing the foliage usually comes away with 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 out of the hutch and run rather than letting it browse.
Livestock Mild
Cattle, goats, and sheep generally leave dahlias alone, but a hungry animal that eats a quantity of foliage can end up with a mild digestive upset.
Birds Caution
There is little firm data on birds, so treat the plant as a caution and keep pet birds from pecking at the leaves and stems.
People Low risk
People are essentially safe, and the tubers are even edible once cooked, so the main nuisance is contact dermatitis. 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
Thousands of named dahlias exist, but for planning a planting it helps to think in a few broad groups sorted by flower shape. These are the ones to know as you compose a scheme.
Decorative dahlias
Fully double blooms with broad, flat-tipped ray florets make this the most familiar type in gardens and the one most often cut for the vase.
Cactus dahlias
Double flowers whose narrow, rolled, pointed florets give the bloom a spiky, star-like look that reads beautifully in an arrangement.
Pompon and ball dahlias
Small, rounded, tightly packed globes built from neatly cupped petals, with a tailored, formal finish.
Dwarf bedding dahlias
Compact seed-grown plants around a foot tall that suit bedding and edging, though they cope poorly with real heat.
How to propagate
Building up a favorite dahlia is part of the pleasure, and there are three graceful routes: divide a clump you already grow, root spring cuttings, or sow seed for a fresh, varied 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 everyday way to multiply a favorite. Split the clump in fall after you lift it, or wait until spring when the eyes begin to swell, making sure every piece carries part of the crown with at least one live eye. Planted into warm soil, the divisions take several weeks to push up and join the display.
Stem cuttingsModerate
Take basal cuttings in spring from sprouted tubers. · 2 to 4 weeks to root
Cuttings let you clone a variety you have grown to love. In spring, take the basal shoots rising off a sprouting tuber and root them, which takes about 2 to 4 weeks, to build up more plants of the kind that suits your scheme.
SeedEasy
Start seed indoors 4 to 6 weeks before last frost. · 1 to 2 weeks to germinate
Seed is the easy, open-ended route, sown indoors 4 to 6 weeks before your last frost, with sprouts up in a week or two. The catch is that seedlings don't come true to the parent and mostly turn out as dwarf bedding plants, a happy surprise for a relaxed planting.
Myths
Dahlias are perfectly safe around pets because they are such common garden flowers.
A flower's place in the average garden says 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, yet the plant still belongs out of a pet's reach.
Dahlia tubers can be left in the ground over winter anywhere.
They are hardy only to about zone 7, so in colder regions the ground freeze kills the tubers outright. In those zones you lift the tubers in fall and store them cool and dry, or begin again with new ones each spring.