Cosmos bring airy foliage and bright, daisy-shaped flowers to borders, cutting gardens and pollinator plantings. The two species most often grown from seed are Cosmos bipinnatus, native to Mexico, and Cosmos sulphureus, native from Mexico into Central America. Both belong to the Asteraceae family. The genus and both species were formally published by botanist Antonio José Cavanilles in 1791.
These warm-season annuals grow readily from seed and perform best in full sun and well-drained soil without heavy fertilization. Cultivars range from compact plants suited to containers to tall, branching forms that produce armfuls of cut flowers through summer and into fall.
Cosmos Fast Facts
- Botanical names: Cosmos bipinnatus and Cosmos sulphureus
- Plant family: Asteraceae
- Life cycle: Frost-tender annual
- Native range: Mexico for Cosmos bipinnatus; Mexico to Central America for Cosmos sulphureus
- Mature size: 1 to 7 feet tall, depending on species and cultivar
- Light: Full sun
- Soil: Well-drained soil with average to low fertility
- Planting season: After the last spring frost, once the soil reaches 65°F
- Bloom season: Summer through the first fall frost
- Germination: 7 to 14 days under favorable warm conditions
- Spacing: 12 to 24 inches, depending on mature plant size
- Water needs: Consistent moisture during establishment, followed by moderate to low needs
How to Choose the Right Cosmos Varieties
Cosmos bipinnatus
This is the familiar garden cosmos with finely divided, threadlike foliage. Flowers commonly appear in white, pink, rose, red and lavender shades. Cultivars include compact and tall plants as well as single, semidouble and double flower forms. Taller selections are especially useful in cutting gardens and toward the back of a border.
Cosmos sulphureus
Often called sulfur cosmos or orange cosmos, this species has broader foliage that is less finely divided than that of Cosmos bipinnatus. Its flowers bring warm shades of yellow, orange and red to the garden. Both compact and tall selections are available, offering varied heights for different garden placements.
Choosing Cosmos for Space and Use
Select a compact or dwarf cultivar for containers, edging and smaller beds. Taller, branching cosmos are better suited to backgrounds, meadow-style plantings and repeated flower cutting. Tall plants may need support in exposed or windy locations. Cultivar height determines spacing, with small plants needing only 12 inches and tall branching types requiring up to 24 inches of room.
When to Plant Cosmos
Cosmos grow best after cold weather has passed. Wait until the danger of frost is over and the soil has warmed to at least 65°F before sowing outdoors or setting out transplants.
Starting Cosmos Indoors
Cosmos can be started indoors about four to six weeks before the last expected spring frost. This provides a helpful head start in short-season climates and gives you greater control over temperature, moisture and timing. No cold stratification or soaking is needed before sowing. Because seedlings grow quickly, avoid starting them any earlier, which can lead to tall, weak or leggy plants. Wait to transplant them outside until all danger of frost has passed and the soil is warm.
Planting Cosmos in Spring
Spring is the ideal time to direct sow cosmos outdoors. Wait until all danger of frost has passed and the soil temperature reaches 65°F. Sowing in cold, wet spring soil can reduce germination and increase the risk of seed decay. Direct sowing is the simplest and most reliable method in areas with a reasonably long warm season. No pretreatment or dormancy breaking is required.
Planting Cosmos in Summer
Early summer sowing can extend the bloom season and stagger cut-flower production, but temperature matters. Cosmos germinate best in soil around 70°F to 75°F, while germination is reduced below 60°F or above 80°F. Direct sow only when you can keep the seedbed evenly moist and enough warm weather remains before the first expected fall frost. Cosmos commonly begin flowering about 50 to 90 days after sowing, depending on the cultivar and growing conditions, so count backward from your average frost date. Where midsummer soil is excessively hot, seeds can be started in containers under controlled conditions and transplanted after hardening off when the worst heat eases, provided enough growing season remains.
Planting Cosmos in Fall
Because cosmos are frost-tender annuals, outdoor fall planting is generally unsuitable where freezing weather is expected. Compare the cultivar's expected 50 to 90 days to bloom with the time remaining before the first killing frost. In mild, frost-free climates, fall sowing may be possible while soil and air temperatures remain warm enough for germination and growth.
Planting Cosmos in Winter
Cosmos do not require winter cold or stratification, but container winter sowing can be used as a low-maintenance seed-starting method. In cold-winter regions, sow in covered, well-drained containers in January or February, after conditions are likely to remain cold enough to discourage premature germination. Once seedlings emerge, protect them from freezing nights and transplant after frost danger has passed and the soil has warmed. Direct dormant sowing into open garden soil is less predictable because seeds may be displaced by weather, eaten by animals, waterlogged or prompted to germinate during a false warm spell.
How to Plant and Grow Cosmos
Direct Sowing Cosmos Outdoors
- Choose the site: Select a well-drained location that receives full sun. Remove weeds and loosen the soil surface without adding rich fertilizer or large amounts of compost.
- Sow the seed: No soaking, scarification or cold stratification is needed. Plant seed about 1/8 inch deep and cover it lightly. Deep planting can reduce emergence.
- Provide warm conditions: Germination is most successful when the soil is around 70°F to 75°F. Seedlings generally emerge in 7 to 14 days under favorable conditions.
- Maintain moisture: Water gently and keep the upper soil evenly moist until seedlings emerge. Avoid saturating the bed or allowing the surface to form a hard crust.
- Thin the seedlings: Once seedlings are established, thin them to 12 to 24 inches apart according to the cultivar's mature size. Give taller branching varieties the widest spacing.
Starting Cosmos Indoors
- Fill the containers: Use clean cells or small pots filled with a moist seed-starting mix.
- Plant the seed: Sow about 1/8 inch deep and cover lightly. Keep the medium around 70°F to 75°F and evenly moist. Germination generally takes 7 to 14 days.
- Provide light: Move emerged seedlings into bright light promptly to encourage sturdy growth.
- Thin and grow on: Keep one healthy seedling per cell and water when the surface of the mix begins to dry.
- Harden the seedlings: Over 7 to 14 days, gradually expose the plants to outdoor shade, then increasing sun, wind and temperature changes. Protect them from frost throughout this process.
- Transplant outdoors: Plant after frost danger has passed and the soil has warmed. Seedlings should have true leaves and enough roots to hold the growing mix together when removed from the cell. Keep the root ball intact, space plants 12 to 24 inches apart based on expected mature size and water them in thoroughly.
How to Care for Cosmos
Watering Cosmos
Keep young plants evenly moist while they establish. Mature cosmos tolerate periods of dry soil, but they benefit from a thorough watering during extended dry weather. Containers dry faster than garden beds and should be checked more often. Avoid keeping the soil constantly saturated.
Soil and Feeding
Cosmos generally flower best without routine feeding. Rich soil and excessive nitrogen can produce large, weak-stemmed plants with abundant foliage and fewer flowers. Add fertilizer only when a soil test or a clear nutrient deficiency indicates that it is needed.
Pinching and Deadheading Cosmos
An early pinch can encourage branching in young plants. Once flowering begins, remove fading blooms regularly to encourage continued flower production and limit unwanted self-sowing. If you want to collect seed or allow volunteer seedlings, leave several healthy flower heads to mature late in the season.
Support, Airflow and Common Problems
Tall cultivars may lean or fall in strong wind and heavy rain. Support them with stakes, a plant ring or nearby plants before the stems begin to flop. Proper spacing improves airflow around the foliage. Aphids are occasional pests. Powdery mildew may occur when conditions favor infection, while gray mold is associated with cool, wet weather and damp plant tissue.
When and How to Harvest Cosmos
Cutting Cosmos Flowers
For fresh arrangements, cut a stem when the petals on its first flower are beginning to open but have not flattened completely. This early stage helps the flower last longer in a vase. Harvest during the cooler part of the day with clean, sharp shears and place the stems in clean water promptly. Leave healthy foliage and leaf nodes below the cut so the plant can continue branching and producing flowers.
Collecting and Storing Cosmos Seed
Allow selected flowers to remain on the plant until the heads are brown, dry and mature. Clip the dry heads into a paper bag, separate the seeds and allow them to finish drying in a protected, well-ventilated place. Store fully dry seed in a labeled paper envelope or bag in a cool, dark and dry location.
Seed saved from an F1 hybrid may germinate, but the next generation may not resemble the parent plant. Open-pollinated cultivars are generally the better choice for seed saving, but saved seedlings can still vary if nearby cosmos of the same species cross-pollinate. Keep this in mind if maintaining a particular flower form, height or color is important.
Frequently Asked Questions About Cosmos
Do cosmos need full sun?
Cosmos perform best in full sun. Plants grown with insufficient light may become weaker and produce fewer flowers.
Why are my cosmos tall but not flowering?
Rich soil, excessive nitrogen or insufficient sunlight can encourage leafy growth at the expense of flowers. Some cultivars also begin flowering later than others, taking up to 90 days to bloom, so they may just need more time.
Can cosmos grow in containers?
Yes. Choose a compact cultivar and a container with unobstructed drainage. Place it in full sun and check the soil regularly because containers can dry more quickly than garden beds.
Do cosmos come back every year?
Cosmos are annuals and are killed by frost. They may self-sow and produce volunteer seedlings the following season, but their return is not guaranteed and seedlings may differ from the original plants.
Do cosmos attract pollinators?
Yes. Cosmos flowers attract butterflies, bees and other visiting insects. Their open flower heads make them a useful addition to sunny pollinator gardens.
Choose a cosmos cultivar that fits your space, wait for warm soil and sow the seeds shallowly. With plenty of sun and restrained feeding, these cheerful annuals can keep the garden and flower vase bright for much of the growing season.
