Ornamental · Asteraceae
Cosmos
Cosmos bipinnatus
Hand it full sun and poor soil, and cosmos just keeps blooming from summer to frost.
Full sun
USDA 2–11
2–4 ft · 0.6–1.2 m
Overview
Cosmos is that rare flower that does better when you leave it alone. It's a fast-growing annual from Mexico, and it'll go from a seed to a soft cloud of daisy-like blooms in one season, in pink, white, and deep magenta sitting on feathery, fern-like leaves. Sow it once, and odds are it'll show up again next year all by itself.
Here's the thing about cosmos: it barely asks for anything. It actually blooms best in lean, dry soil, so the richer you make the bed and the more you feed, the more leaves you get and the fewer flowers. Plants land somewhere between 2 to 4 feet, and the tall ones tend to lean over unless you give them a hand with a stake.
Want a garden full of pollinators? Cosmos delivers. Bees and butterflies work the open flowers all season, and a row along a fence or at the back of a border keeps you swimming in cut flowers. Snip some for a vase, or leave a few to ripen into seed, and you'll get volunteers popping up come spring.
Care
The short version: give cosmos full sun and lean, well-drained soil, water it moderately, and don't bother feeding it much. Deadhead the old flowers to keep more coming, and stake the tall ones before they flop.
Light
Cosmos wants full sun, and the more it gets, the better. Stick it in shade and it'll stretch up tall and weak, lean toward the light, and give you way fewer flowers, so a bright spot really is the way to go.
Water
Give it medium, even moisture while the plants are getting established, then ease up, because settled cosmos handles a dry spell just fine. The one thing to steer clear of? Soggy ground, since that rots the stems and roots from the bottom up.
Soil
Plant it in lean, average, well-drained soil, and try not to fuss over enriching it. Cosmos is happy in poor and even occasionally dry ground, but rich, fertile beds and high nitrogen just grow leaves where you wanted flowers.
Humidity
Regular outdoor air suits cosmos fine, so there's no number to worry about. What does matter is airflow, because crowded, still air is what brings on powdery mildew and mold.
Feeding
Feed it rarely, if ever, since cosmos blooms best a little hungry. Skip the rich soil and high-nitrogen feeds, or you'll end up with tall, floppy plants and barely a flower to show for it.
Pruning
Deadhead the spent flowers through the season and you'll keep the new buds coming, plus snip out anything that looks damaged or diseased. Leave a few late heads to ripen, though, and the plant reseeds itself for next year.
Size & spacing
Plants run 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 can move through, and stake the tall ones before they tip over.
When to plant
Cosmos is a warm-season annual, and frost is the one thing it can't take. So you can start seeds indoors 6 to 8 weeks before your last frost, or just sow them straight into the ground once it warms up, and you'll get flowers from early summer all the way 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 honestly, most of what goes wrong comes down to soil that's too rich or air that's too still. Keep an eye on the new growth and the lower stems, since that's where trouble shows up first. Find your match below and step in before it spreads.
Yellow, stunted plants with green or misshapen flowers
Twisted growth and odd greenish blooms? That points to aster yellows, a phytoplasma that leafhoppers carry from plant to plant. There's no cure for it, so dig out and destroy any plant that's showing it before it can spread. Keeping leafhoppers and weedy host plants down is really your only 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, plants caving in
Soft brown stem bases topped with fuzzy gray or cottony mold mean stem rot, Botrytis, or white mold has set in, and they all love cool, wet, badly drained ground. Pull and toss the affected plants along with any debris around them. Better drainage, more spacing, and watering at the base instead of overhead keep these fungi from getting going.
Fix:
- Remove and destroy affected plants and debris
- Improve drainage and reduce moisture around stems
- Avoid overcrowding and overhead watering
Sticky new growth covered in tiny green or black bugs
Those soft little clusters on the bud tips? Those are aphids, and they go straight for the tenderest growth, leaving sticky honeydew behind. A hard jet of water knocks them right off, and you can bring in insecticidal soap if they keep coming back. Easing up on nitrogen helps too, since the soft growth it brings is exactly what they're after.
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 creeping across the leaves
That dusty white stuff is powdery mildew, a fungus that moves in when plants are crowded and the air can't move. Just thin or space the plants to open them up, and pull off the worst of the coated leaves. It usually shows 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 turns into lush leaves and weak, sprawling stems with hardly any flowers, the soil's too rich and the nitrogen's too high. So stop feeding, stake whatever has flopped, and let lean ground sort it out. Cosmos blooms best when it's 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 if you've got pets: cosmos is safe across the board. The ASPCA lists it as non-toxic to cats, dogs, and horses, and there's no poison in any part of it for the other animals either. So if a pet or a grazing animal sneaks a nibble? No harm done.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats None
Cosmos is safe for cats, so a curious nibble won't cause any poisoning at all.
Dogs None
Dogs can brush against it or chew on it without harm, since none of it's toxic to them.
Horses None
The ASPCA clears cosmos for horses, so grazing near it isn't a problem.
Rabbits Safe
Rabbits can sample the leaves and flowers with no risk of poisoning whatsoever.
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, and those are perfectly safe.
People Safe
Cosmos is harmless to people too, though it's grown for looks rather than the table.
Varieties
Cosmos comes in a handful of distinct types, from tall border plants to tidy little dwarfs and a few with really unusual flower shapes. Here are the ones worth knowing.
Sensation
Sensation's the classic tall cosmos, hitting 3 to 4 feet with big 3 to 4 inch pink and white daisy flowers.
Sonata
Sonata's a compact dwarf series at around 18 to 24 inches, so it's perfect for containers and the front of a border.
Double Click
Double Click gives you plush double and semi-double blooms instead of the usual single daisy shape.
Seashells
Seashells has petals that roll into fluted, tubular rays, which makes them look just like little shells.
Cosmos sulphureus
Cosmos sulphureus is a related sulphur cosmos in warm yellow, orange, and red, and that's what sets it apart from C. bipinnatus.
How to propagate
If you've never grown anything from seed, cosmos is a great place to start, since it's about as easy as it gets. And once it's settled 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 is about as easy as flowers come from seed. Just sow it straight into the garden once the soil hits at least 65°F (18°C), or start it indoors 6 to 8 weeks before your last frost, and barely cover the seed, since burying it deep keeps it from sprouting. It'll come up in 7 to 21 days and flower within 7 to 10 weeks, and if you leave a few heads to ripen, it'll seed itself for next year.
Myths
Cosmos needs rich, fertile soil and steady feeding to bloom well.
It's actually the other way around. Rich ground and extra nitrogen give you 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.
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 has to be replanted from new seed every year.
Leave it be and cosmos drops seed and comes right back on its own. Let a few spent flowerheads ripen and the plants reseed themselves, filling the same patch again the next season.