Ornamental · Asteraceae
Cosmos
Cosmos bipinnatus
Hand it full sun and poor soil, and cosmos rewards you with blooms from summer until frost.
Full sun
USDA 2–11
2–4 ft · 0.6–1.2 m
Overview
If you have ever worried about killing a plant, cosmos is the one to relax with. This is a fast-growing annual from Mexico that climbs from a tiny seed to a soft cloud of daisy-shaped flowers in just one season, glowing in pink, white, and deep magenta above feathery, fern-like leaves. Sow it the once, and it often returns the next year without any help from you.
The lovely thing is how little it expects in return. Cosmos truly prefers lean, dry soil, so you can let go of any fear about getting feeding just right, because the more you pamper it, the more leaves you get and the fewer flowers. Plants settle anywhere from 2 to 4 feet tall, and the taller ones may lean a little, which a simple stake easily fixes.
Bees and butterflies will find it quickly and work the open flowers all season long. A line of cosmos along a fence or at the back of a bed keeps you in cut flowers, and you can either snip blooms for a vase or leave a few to ripen. Either way you end up with seedlings popping up on their own come spring.
Care
The short version: give cosmos full sun and lean, well-drained soil, water moderately, and feed almost never. Pinch off faded flowers to keep more coming, and pop a stake beside the tall ones before they lean.
Light
Cosmos loves full sun, so give it the brightest spot you have and it will thank you. Tucked into shade it grows tall and weak, reaches toward the light, and offers up far fewer flowers, so sunshine really is the kindest gift here.
Water
Keep the soil at a medium, even moisture while young plants find their feet, then you can relax and water less, because settled cosmos copes happily with a dry stretch. The one thing worth avoiding is soggy ground, since that rots the stems and roots from the bottom up.
Soil
Plant cosmos in lean, average, well-drained soil, and try to resist the urge to spoil it. It is quite content in poor and even occasionally dry ground, and that is good news, because rich beds and high nitrogen only hand you leaves where you wanted flowers.
Humidity
Everyday outdoor air suits cosmos just fine, so there is no humidity number to fret over. What helps far more is good airflow, since crowded, stagnant air is what tends to invite powdery mildew and mold.
Feeding
Feed cosmos rarely, if at all, because it honestly blooms best when it is a little hungry. Skip the rich soil and the high-nitrogen feeds, or you may end up with tall, floppy plants and hardly a flower, which is easy to avoid once you know.
Pruning
Pinch off the faded flowers through the season and fresh buds keep arriving, and snip away any growth that looks damaged or sickly. If you leave a few late heads to ripen, the plant kindly reseeds itself for the year ahead.
Size & spacing
Plants reach 2 to 4 feet (61 to 122 cm) tall and spread out 2 to 3 feet (61 to 91 cm). Give them that breathing room so air can move around them, and slip a stake beside the tall ones early, before they have a chance to topple.
When to plant
Cosmos is a warm-season annual, and a little frost is the only thing that troubles it, so there is no rush. You can start seeds indoors 6 to 8 weeks before your last frost, or simply scatter them outside once the ground warms, and then you get flowers from early summer right through 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 genuinely forgiving, and almost everything that goes wrong comes back to soil that is a touch too rich or air that sits too still. Take a friendly look at the fresh growth and the lower stems now and then. Find your plant in the list below and step in early, and you will likely set it right.
Yellow, stunted plants with oddly green or misshapen flowers
Twisted growth and strange greenish blooms point to aster yellows, a phytoplasma that leafhoppers carry from plant to plant. This one has no cure, so it is kindest to dig out and destroy any plant showing it before it can spread. Keeping leafhoppers and weedy host plants down is your best defense, and cool, wet seasons tend to 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 as plants slump
Soft brown stem bases topped with fuzzy gray or cottony mold mean stem rot, Botrytis, or white mold has moved in, and all of them love cool, wet, poorly drained ground. Pull and bin the affected plants and the debris around them so they cannot linger. Better drainage, wider spacing, and watering at the base instead of overhead keep these fungi from taking hold.
Fix:
- Remove and destroy affected plants and debris
- Improve drainage and reduce moisture around stems
- Avoid overcrowding and overhead watering
Sticky new shoots covered in tiny green or black bugs
Those little soft clusters gathering on your bud tips are aphids, and they head straight for the tenderest growth, leaving a sticky honeydew as they go. Do not worry, a firm spray of water usually knocks them off, and insecticidal soap helps if they keep returning. Easing back on nitrogen makes a real difference too, since the soft growth it brings is just what they 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 soft white, dusty film creeping across the leaves
That powdery coating is powdery mildew, a fungus that moves in when plants sit crowded and the air cannot circulate. The fix is gentle: thin or space the plants to open them up, and pick off the worst of the dusted leaves. It usually arrives late in the season, so a sunny, breezy spot keeps it nicely 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 lean over and barely flower
When cosmos turn into lush leaves and weak, sprawling stems with few flowers, the soil is simply too rich and the nitrogen too high, which is an easy thing to put right. Stop feeding, gently stake any that have flopped, and let lean ground do the rest. Cosmos flowers best when it is a touch starved, so plain, average soil is exactly what you are after.
Fix:
- Stop fertilizing and avoid high-nitrogen feeds
- Stake or otherwise support tall plants
- Grow in lean, average soil
Toxicity
Here is something to set your mind at ease: cosmos is safe through and through. The ASPCA lists it as non-toxic to cats, dogs, and horses, and no part of the plant holds any poison for the other animals either. If a pet or grazing animal sneaks a nibble, there is no harm done.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats None
Cosmos is safe for cats, and even a curious nibble will not cause any poisoning, so you can let them explore.
Dogs None
Dogs can brush past or chew on the plant without any harm at all, since none of it is toxic to them.
Horses None
The ASPCA clears cosmos for horses, so a horse grazing nearby has nothing to fear from it.
Rabbits Safe
Rabbits can sample the leaves and flowers freely, with no risk of poisoning to worry about.
Livestock Safe
Cattle, goats, and sheep can graze around cosmos quite safely, with nothing for you to fret over.
Birds Safe
Garden and wild birds often pick at the ripe seed heads, and those are perfectly safe for them.
People Safe
Cosmos is harmless to people too, though it is grown for its looks rather than for eating.
Varieties
Cosmos comes in a friendly handful of types, from tall border beauties to neat little dwarfs and a few with charming, unusual flower shapes. Here are the ones worth getting to know first.
Sensation
Sensation is the classic tall cosmos you picture, rising 3 to 4 feet with big 3 to 4 inch pink and white daisy flowers.
Sonata
Sonata is a compact dwarf series at around 18 to 24 inches, perfect for pots or the front edge 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 curl into fluted, tubular rays, looking just like tiny little shells.
Cosmos sulphureus
Cosmos sulphureus is a related sulphur cosmos in warm yellow, orange, and red, which sets it apart from C. bipinnatus.
How to propagate
Cosmos may be the kindest flower of all to grow from seed, and once it has settled into your garden it happily keeps itself going, reseeding year after year with no fuss.
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 easygoing as flowers come from seed, so this is a lovely place to begin. Sow it straight into the garden once the soil reaches at least 65°F (18°C), or start it indoors 6 to 8 weeks before your last frost, just barely covering the seed, since burying it too deep keeps it from sprouting. Seedlings appear in 7 to 21 days and flower within 7 to 10 weeks, and if you let a few heads ripen they will happily seed themselves for next year.
Myths
Cosmos needs rich, fertile soil and steady feeding to bloom well.
It is actually the other way around, so you can stop worrying about feeding. Rich ground and extra nitrogen give you tall, floppy leaves and barely any flowers, while cosmos blooms best in lean, average soil with little to no feeding at all.
Cosmos flowers are toxic to pets.
You can set this worry aside completely. The ASPCA lists cosmos as non-toxic to cats, dogs, and horses, so a curious nibble from your pet will not do them any harm.
Cosmos has to be replanted from new seed every year.
Left to itself, cosmos drops seed and comes back on its own, which saves you the work. Leave a few spent flowerheads to ripen and the plants reseed themselves, filling the same patch again the following season.