Ornamental · Asteraceae
Cosmos
Cosmos bipinnatus
Hand it full sun and poor soil, and cosmos floods the garden with blooms from summer to frost.
Full sun
USDA 2–11
2–4 ft · 0.6–1.2 m
Overview
Cosmos thrives on neglect, and that is exactly what makes it so much fun to grow. This fast-growing annual from Mexico rockets from seed to a haze of daisy-like flowers in a single season, throwing out pink, white, and deep magenta blooms that dance on airy, fern-like foliage. Sow it once and it usually comes roaring back the next year all on its own.
Here is the best part: cosmos wants almost nothing from you. It actually flowers hardest in lean, dry soil, so the more you enrich the bed and feed it, the more you trade flowers for leaves. Plants shoot up 2 to 4 feet, and the tall ones will lean and sprawl unless you give them a quick stake to push against.
Pollinators chase it down. Bees and butterflies work the open flowers all season, and a run of cosmos along a fence or behind a border keeps a cutting garden cranking out stems. Snip armfuls for the vase, or let a few heads ripen into seed, and volunteers will spring up to do it all again next year.
Care
The short version: give cosmos full sun and lean, well-drained soil, water it moderately, and barely feed it. Deadhead to keep the flowers firing, and stake the tall types before they flop.
Light
Cosmos craves full sun, and the more it soaks up, the harder it performs. Push it into shade and it stretches tall and weak, leans toward the light, and hands you far fewer flowers, so chase the brightest spot you have.
Water
Pour on medium, even moisture while plants get rolling, then back off, because established cosmos powers through a dry spell. Just dodge soggy ground, which rots the stems and roots right from the base up.
Soil
Drop it into lean, average, well-drained soil and leave the bed alone. Cosmos tackles poor and even occasionally dry ground with ease, while rich, fertile beds and high nitrogen pump out leaves instead of flowers.
Humidity
Ordinary outdoor air keeps cosmos happy, so forget chasing a number. Airflow is what counts, since crowded, still air opens the door to powdery mildew and mold.
Feeding
Feed it rarely, if at all, because cosmos blooms hardest a little hungry. Skip the rich soil and high-nitrogen feeds, or you grow tall, floppy plants with barely a flower to show off.
Pruning
Deadhead the spent flowers through the season to keep fresh buds firing, and snip out any damaged or diseased growth. Leave a few late heads to ripen and the plant reseeds itself, setting up next year for free.
Size & spacing
Plants surge to 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 rushes through, and stake the tall types before they topple.
When to plant
Cosmos is a warm-season annual that frost shuts down fast, so timing pays off. Kick seeds off indoors 6 to 8 weeks before your last frost, or sow straight into warm ground, and the plants reward you with 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 a breeze to grow, and nearly every hiccup traces back to soil that is too rich or air that sits too still. Scan the new growth and the lower stems often, since that is where trouble shows up first. Spot the match below and jump on it before it spreads.
Yellow, stunted plants with green or warped flowers
Twisted growth and odd greenish blooms flag aster yellows, a phytoplasma that leafhoppers ferry from plant to plant. There is no cure, so dig out and destroy any plant showing it before it tears through the bed. Knocking back leafhoppers and weedy host plants is the real defense, and cool, wet seasons crank it up.
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, plants caving in
Soft brown stem bases capped with fuzzy gray or cottony mold mean stem rot, Botrytis, or white mold has crashed in, all of them loving cool, wet, badly drained ground. Yank and bin the affected plants along with the debris around them. Sharper drainage, wider spacing, and watering at the base instead of overhead slam the door on these fungi.
Fix:
- Remove and destroy affected plants and debris
- Improve drainage and reduce moisture around stems
- Avoid overcrowding and overhead watering
Bud tips turning sticky and mobbed by little green or black bugs
Those soft clusters piling onto the bud tips are aphids, racing for the tenderest growth and leaving a sticky honeydew in their wake. Blast them off with a hard jet of water, and call in insecticidal soap if they bounce back. Dialing down nitrogen pays off too, since the soft growth it drives is exactly the buffet aphids love.
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 across the leaves
That dusty white coating is powdery mildew, a fungus that storms in when plants crowd up and the air goes still. Thin or space the plants to open them up, and strip off the worst-coated leaves. It tends to crash the party late in the season, so a sunny, breezy spot shuts it down.
Fix:
- Improve air circulation by thinning or spacing plants
- Remove and dispose of severely affected foliage
- Apply a labeled fungicide if needed
Lanky, leaf-heavy cosmos sprawling sideways with hardly a flower
When cosmos balloons into lush foliage and weak, sprawling stems with few flowers, the soil is too rich and the nitrogen too high. Cut the feeding, stake whatever flopped, and let lean ground reel it back. Cosmos flowers hardest a little starved, so plain, average soil is exactly the ticket.
Fix:
- Stop fertilizing and avoid high-nitrogen feeds
- Stake or otherwise support tall plants
- Grow in lean, average soil
Toxicity
Great news for the pet lovers: cosmos is safe across the board. The ASPCA lists it as non-toxic to cats, dogs, and horses, and no part of the plant packs any poison for the other animals either. A stray nibble from a pet or a grazing animal will not cause a bit of harm.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats None
Cosmos is safe for cats, so a curious nibble will not cause any poisoning at all.
Dogs None
A dog that romps through the patch or mouths a stem comes away just fine, because nothing in the plant is poisonous to canines.
Horses None
Horses get the all-clear from the ASPCA, so a mouthful snatched while grazing carries zero risk.
Rabbits Safe
Rabbits can sample the foliage and flowers freely, with no risk of poisoning.
Livestock Safe
Goats, sheep, and cattle grazing beside a cosmos patch face no hazard whatsoever.
Birds Safe
Backyard and wild birds love to pick at the ripe seed heads, which are perfectly safe.
People Safe
Cosmos is harmless to people too, though it earns its spot for show rather than the table.
Varieties
Cosmos comes in a fun lineup of distinct strains, from towering border stars to tidy dwarfs and a few with show-stealing flower shapes. These are the ones worth chasing down.
Sensation
Sensation is the classic tall cosmos, rocketing to 3 to 4 feet with big 3 to 4 inch pink and white daisy flowers.
Sonata
Sonata is a compact dwarf series at roughly 18 to 24 inches that packs into containers and the front of a border.
Double Click
Double Click delivers plush double and semi-double blooms instead of the usual single daisy form.
Seashells
Seashells rolls its petals into fluted, tubular rays that look just like little shells.
Cosmos sulphureus
Cosmos sulphureus is a related sulphur cosmos in warm yellow, orange, and red that breaks away from C. bipinnatus.
How to propagate
Cosmos ranks among the most rewarding flowers to grow from seed, and it pretty much grows itself. Once it lands in the garden, it keeps the show running 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 is about the most rewarding flower going from seed. Sow it straight into the garden once the soil hits at least 65°F (18°C), or fire it off indoors 6 to 8 weeks before your last frost, barely covering the seed, since burying it deep stalls sprouting. It bursts up in 7 to 21 days and flowers within 7 to 10 weeks, and letting a few heads ripen means it seeds itself for next year.
Myths
Cosmos needs rich, fertile soil and steady feeding to bloom well.
It runs the other way entirely. Rich ground and extra nitrogen pump out tall, floppy leaves and barely any flowers, so cosmos blooms best in lean, average soil with little to no feeding.
Cosmos flowers are toxic to pets.
No worries on that front at all. The ASPCA lists cosmos as non-toxic to cats, dogs, and horses, so a curious nibble does zero harm.
Cosmos has to be replanted from new seed every year.
Leave it be and cosmos drops seed and storms back on its own. Let a few spent flowerheads ripen and the plants reseed themselves, refilling the same patch the next season.