Ornamental · Asteraceae
Cosmos
Cosmos bipinnatus
In full sun and poor soil, cosmos opens daisy-form flowers from summer to frost.
Full sun
USDA 2–11
2–4 ft · 0.6–1.2 m
Overview
Cosmos is a flower that thrives on neglect, and up close it shows why it earns the space. A fast-growing annual from Mexico, it climbs from seed in one season to flat, daisy-form blooms whose ray petals open in clear pink, crisp white, and deep magenta. Each flower rides at the tip of a slender stem above foliage so finely cut it reads as a green haze. Sow it once and it tends to return on its own.
The leaves are the giveaway. Cosmos carries thread-fine, fern-like foliage, the blades divided into narrow segments that catch the light and sway with any breeze. The plant flowers best in lean, dry soil, and richer ground only swells those leaves while the flowers thin out. Stems run 2 to 4 feet tall, and the tallest lean and sprawl under their own slim weight without support.
The open faces draw a steady traffic. Bees and butterflies settle on the wide, flat blooms all season, working the central disc. A row along a fence or at the back of a border keeps fresh stems coming, and a few heads left to ripen dry into seed and scatter their own volunteers for spring.
Care
The short version: full sun and lean, well-drained soil, with medium water and almost no feeding. Pinch off the faded daisy heads to keep new buds opening, and stake the tall, slim stems before they bend over.
Light
Cosmos wants full sun on its open faces, the more the better. In shade the slim stems stretch tall and weak, lean toward the light, and carry far fewer of those daisy blooms.
Water
Give medium, even moisture while the fine roots take hold, then ease off, since established cosmos shrugs off a dry spell. Soggy ground is the one thing to avoid, as it rots the soft stems and roots from the base up.
Soil
Plant in lean, average, well-drained soil and leave it unimproved. Cosmos tolerates poor and even occasionally dry ground, while rich, fertile beds and high nitrogen swell the leaves at the cost of the flowers.
Humidity
Ordinary outdoor air suits cosmos, so there is no number to chase. Airflow through the feathery foliage matters more, since crowded, still air settles powdery mildew and mold onto the leaves.
Feeding
Feed rarely, if at all, because cosmos blooms best a little hungry. Rich soil and high-nitrogen feeds push tall, floppy stems and dense leaves with hardly a flower among them.
Pruning
Pinch off the faded daisy heads through the season to keep fresh buds opening, and cut away any damaged or diseased growth. A few late heads left to ripen dry to seed and let the plant sow itself for next year.
Size & spacing
Stems reach 2 to 4 feet (61 to 122 cm) tall with a spread of 2 to 3 feet (61 to 91 cm) of fine foliage. Set them at that spacing so air threads through the leaves, and stake the tall, slim stems before they bend.
When to plant
Cosmos is a warm-season annual with soft, frost-tender stems that the first freeze blackens. Start seeds indoors 6 to 8 weeks before your last frost, or sow into warm ground, and the open daisy faces follow from early summer until 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 what shows up on it traces to soil that is too rich or air that sits too still. The newest growth at the tips and the lower stems carry the first marks. Match what you see below and step in before it spreads.
Yellowed, stunted plants with greenish or warped flowers
Twisted, stunted growth and oddly green, malformed flower heads point to aster yellows, a phytoplasma that leafhoppers carry from plant to plant. There is no cure, so any plant showing the warped foliage and deformed blooms gets dug out and destroyed before it spreads. Holding down leafhoppers and weedy host plants is the only real defense, and cool, wet seasons sharpen it.
Fix:
- Remove and destroy infected plants promptly, as there is no cure
- Control leafhoppers that spread the phytoplasma
- Remove nearby weedy host plants
Soft brown stem bases with gray or white mold and collapse
Stem bases gone soft and brown, topped with fuzzy gray or cottony white mold, mean stem rot, Botrytis, or white mold has taken the lower stem, all of them at home in cool, wet, poorly drained ground. The mold-furred plants and the debris around them get pulled and binned. Sharper drainage, wider spacing, and watering at the base instead of overhead keep the fungus off the stems.
Fix:
- Remove and destroy affected plants and debris
- Improve drainage and reduce moisture around stems
- Avoid overcrowding and overhead watering
Sticky bud tips crusted with tiny green or black insects
The soft, pinhead clusters massed on the tenderest bud tips are aphids, and they leave the leaves below glossed with sticky honeydew. A hard jet of water knocks them off the stems, and insecticidal soap clears any that crawl back. Easing off nitrogen helps too, since the soft, succulent growth it forces is exactly the tissue they pierce.
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, powdery dusting spreading over the leaves
The chalky white film coating the upper leaf surfaces is powdery mildew, a fungus that settles where plants crowd and air stalls. Thin or space the plants to let air through the foliage, and strip off the most heavily dusted leaves. It tends to bloom late in the season, so a sunny, breezy spot keeps it off the leaves.
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 bend over and carry few blooms
When cosmos runs to dense foliage on weak, sprawling stems with few flowers, the soil is too rich and the nitrogen too high. Stop feeding, prop the bent stems, and let lean ground reset it. Cosmos opens the most flowers when it is a little starved, so plain, average soil is exactly what brings them out.
Fix:
- Stop fertilizing and avoid high-nitrogen feeds
- Stake or otherwise support tall plants
- Grow in lean, average soil
Toxicity
For pet owners the news is good: cosmos is safe all around. The ASPCA lists it as non-toxic to cats, dogs, and horses, and no part of the plant, petal, leaf, or stem, holds poison for the other animals either. A stray nibble from a pet or a grazing animal does no harm.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats None
Cosmos is safe for cats, and a curious nibble at the petals or leaves causes no poisoning.
Dogs None
Dogs can brush against the foliage or chew a stem without harm, since none of it is toxic to them.
Horses None
The ASPCA clears cosmos for horses, so grazing near the plant poses no danger.
Rabbits Safe
Rabbits can sample the fine foliage and the flowers with no 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 the slim seeds from the ripe heads, which are perfectly safe.
People Safe
Cosmos is harmless to people, though it is grown for the flowers rather than the table.
Varieties
Cosmos comes in a handful of distinct strains that differ most in flower size, petal shape, and color, from tall single daisies to ruffled doubles and curled, shell-like rays. These are the ones worth knowing.
Sensation
Sensation is the classic tall cosmos, reaching 3 to 4 feet and opening large 3 to 4 inch single daisies in pink and white.
Sonata
Sonata is a compact dwarf series at roughly 18 to 24 inches, carrying the same daisy faces on shorter, sturdier stems.
Double Click
Double Click bears plush, layered double and semi-double blooms in place of the open single daisy.
Seashells
Seashells curls each ray petal into a fluted tube, so the flower looks rimmed with little shells.
Cosmos sulphureus
Cosmos sulphureus is a related sulphur cosmos in hot yellow, orange, and red, brighter and warmer toned than C. bipinnatus.
How to propagate
Cosmos is one of the easiest flowers to raise from seed, the slim, dark seeds sprouting readily into thread-leaved seedlings. Once it is 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 simple as flowers get from seed, the slim dark seeds sprouting fast. They go straight into the garden once the soil reaches at least 65°F (18°C), or indoors 6 to 8 weeks before the last frost, barely covered, since deep burial keeps them from sprouting. Thread-leaved seedlings push up in 7 to 21 days and flower within 7 to 10 weeks, and a few heads left to ripen dry to seed for next year.
Myths
Cosmos needs rich, fertile soil and steady feeding to bloom well.
The opposite holds. Rich ground and extra nitrogen swell tall, floppy leaves and yield very few flowers, so cosmos blooms best in lean, average soil with little to no feeding.
Cosmos flowers are toxic to pets.
There is nothing to fear in those petals. The ASPCA lists cosmos as non-toxic to cats, dogs, and horses, so a curious nibble does no harm.
Cosmos has to be replanted from new seed every year.
Left alone, cosmos drops its slim seeds and comes back on its own. A few spent flowerheads left to ripen dry to seed, and the plants reseed themselves into the same patch the following season.