Ornamental · Asteraceae
Cosmos
Cosmos bipinnatus
Give it full sun and poor soil, and cosmos blooms its heart out from summer to frost.
Full sun
USDA 2–11
2–4 ft · 0.6–1.2 m
Overview
Cosmos is the flower that thrives on neglect. A fast-growing annual from Mexico, it shoots up from seed to a haze of daisy-like blooms in a single season, in pinks, whites, and deep magentas waving on airy, fern-like foliage. Sow it once and it tends to come back on its own.
What sets cosmos apart is how little it asks. It actually flowers best in lean, dry soil, so the richer the bed and the more you feed, the more you trade flowers for leaves. Plants reach anywhere from 2 to 4 feet, and the tall types lean and sprawl unless you give them a little support.
Pollinators love it. Bees and butterflies work the open flowers all season, and a row of cosmos along a fence or at the back of a border keeps the cutting garden full. Snip the blooms for a vase or leave them to ripen into seed, and you'll have volunteers next spring.
Care
The short version: full sun and lean, well-drained soil, with medium water and almost no feeding. Deadhead the spent flowers to keep the blooms coming, and stake the tall types before they flop.
Light
Cosmos wants full sun, the more the better. Plants grown in shade stretch tall and weak, lean toward the light, and give you far fewer flowers.
Water
Give medium, even moisture while plants are getting going, then ease off, since established cosmos shrugs off a dry spell. The one thing to avoid is soggy ground, which rots the stems and roots from the base up.
Soil
Plant in lean, average, well-drained soil and resist the urge to enrich it. Cosmos tolerates poor and even occasionally dry ground, but rich, fertile beds and high nitrogen only grow leaves at the cost of flowers.
Humidity
Ordinary outdoor humidity suits cosmos fine, so there's no number to chase. What matters more is airflow, since crowded, still air invites powdery mildew and mold.
Feeding
Feed rarely, if at all, because cosmos blooms best a little hungry. Skip the rich soil and high-nitrogen feeds, or you'll grow tall, floppy plants with hardly a flower to show for it.
Pruning
Deadhead the spent flowers through the season to keep new buds coming, and snip out any damaged or diseased growth. Leaving a few late heads to ripen lets the plant reseed itself for next year.
Size & spacing
Plants run from 2 to 4 feet (61 to 122 cm) tall with a spread of 2 to 3 feet (61 to 91 cm). Give them that room so air moves freely, and stake the tall types before they topple.
When to plant
Cosmos is a warm-season annual that won't take frost. Start seeds indoors 6 to 8 weeks before your last frost or sow straight into the ground once it warms, then enjoy flowers from early summer right up to the first hard frost.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: 6 to 8 weeks before the last spring frost date
- Plant: After the last spring frost, once soil has warmed to at least 65°F (18°C)
- Harvest: Early summer to frost
Hardiness
USDA hardiness zones: 2–11
Temperature: down to -46°C
Troubleshooting
Cosmos is an easy plant, and most of its troubles trace back to soil that's too rich or air that's too still. Watch the new growth and the lower stems for the first signs. Match what you see below and step in before it spreads.
Yellowed, stunted plants with weirdly green or deformed flowers
Warped growth and odd greenish blooms point to aster yellows, a phytoplasma carried plant to plant by leafhoppers. There's no cure, so dig out and destroy any plant that shows it before the disease spreads. Keeping leafhoppers and weedy host plants down is the only real defense, and cool, wet seasons make it worse.
Fix:
- Remove and destroy infected plants promptly, as there is no cure
- Control leafhoppers that spread the phytoplasma
- Remove nearby weedy host plants
Rotting stem bases with gray or white mold, and plants caving in
Soft brown stem bases topped with fuzzy gray or cottony mold mean stem rot, Botrytis, or white mold has set in, all of them at home in cool, wet, badly drained ground. Pull and bin the affected plants along with any debris around them. Better drainage, wider spacing, and watering at the base rather than overhead keep these fungi from getting a foothold.
Fix:
- Remove and destroy affected plants and debris
- Improve drainage and reduce moisture around stems
- Avoid overcrowding and overhead watering
Sticky new growth crawling with tiny green or black insects
Those soft clusters on the bud tips are aphids, drawn to the tenderest growth and leaving a sticky honeydew behind. Knock them off with a hard jet of water, and bring in insecticidal soap if they keep coming back. Going easy on nitrogen helps, since lush growth is exactly what they crave.
Fix:
- Dislodge aphids with a strong jet of water
- Apply insecticidal soap or horticultural oil if populations are high
- Encourage beneficial insects such as lady beetles and lacewings
A white, dusty film spreading over the leaves
This is powdery mildew, a fungus that settles in when plants sit crowded and air can't move. Thin or space the plants to open them up, and pull off the worst of the coated leaves. It tends to show up late in the season, so a sunny, breezy spot keeps it in check.
Fix:
- Improve air circulation by thinning or spacing plants
- Remove and dispose of severely affected foliage
- Apply a labeled fungicide if needed
Tall, leafy plants that flop over and barely bloom
When cosmos run to lush foliage and weak, sprawling stems with few flowers, the soil is too rich and the nitrogen too high. Stop feeding, stake the plants that have already flopped, and let lean ground do the rest. Cosmos flower best when they're a little starved, so plain, average soil is exactly what you want.
Fix:
- Stop fertilizing and avoid high-nitrogen feeds
- Stake or otherwise support tall plants
- Grow in lean, average soil
Toxicity
Good news for pet owners: cosmos is safe all around. The ASPCA lists it as non-toxic to cats, dogs, and horses, and there's no poison in any part of the plant for the other animals either. A stray nibble from a pet or a grazing animal won't cause harm.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats None
Cosmos is safe for cats, and a curious nibble won't cause any poisoning.
Dogs None
Dogs can brush against or chew the plant without harm, since none of it is toxic to them.
Horses None
The ASPCA clears cosmos for horses, so grazing near it poses no danger.
Rabbits Safe
Rabbits can sample the foliage and flowers without any risk of poisoning.
Livestock Safe
Cattle, goats, and sheep can graze around cosmos with nothing to worry about.
Birds Safe
Backyard and wild birds often pick at the ripe seed heads, which are perfectly safe.
People Safe
Cosmos is harmless to people, though it's grown for show rather than the table.
Varieties
Cosmos comes in a handful of distinct strains, from towering border plants to tidy dwarfs and a few with unusual flower shapes. These are the ones worth knowing.
Sensation
The classic tall cosmos, reaching 3 to 4 feet with large 3 to 4 inch pink and white daisy flowers.
Sonata
A compact dwarf series at roughly 18 to 24 inches that fits containers and the front of a border.
Double Click
Carries plush double and semi-double blooms instead of the usual single daisy form.
Seashells
Its petals roll into fluted, tubular rays that look like little shells.
Cosmos sulphureus
A related sulphur cosmos in warm yellow, orange, and red that sets it apart from C. bipinnatus.
How to propagate
Cosmos is one of the easiest flowers to grow from seed, and once it's in the garden it keeps itself going by reseeding year after year.
SeedEasy
Direct-sow outdoors after the last frost once soil reaches at least 65°F (18°C), or start indoors 6 to 8 weeks before the last frost · Germinates in about 7 to 21 days and typically blooms within roughly 7 to 10 weeks
Cosmos are about as simple as flowers get from seed. Sow them straight into the garden once the soil hits at least 65°F (18°C), or start them indoors 6 to 8 weeks before your last frost, barely covering the seed since burying it deep stops it sprouting. They come up in 7 to 21 days and flower within 7 to 10 weeks, and if you let a few heads ripen they'll seed themselves for next year.
Myths
Cosmos need rich, fertile soil and steady feeding to bloom well.
The opposite is true. Rich ground and extra nitrogen push out tall, floppy leaves and very few flowers, so cosmos bloom best in lean, average soil with little to no feeding.
Cosmos flowers are toxic to pets.
There's nothing to worry about here. The ASPCA lists cosmos as non-toxic to cats, dogs, and horses, so a curious nibble won't do any harm.
Cosmos have to be replanted from new seed every year.
Left to their own devices, cosmos drop seed and come back on their own. Leave a few spent flowerheads to ripen and the plants reseed themselves, filling the same patch the following season.