Cosmos are the easiest tall flower you can grow from seed: sow them in spring, and by midsummer you have feathery, fern-like plants covered in daisy flowers in pink, white and crimson, swaying above the border and covered in bees and butterflies. They flower until the first frost and they ask for almost nothing.
The secret of cosmos is neglect. They flower best in poor soil — rich soil and fertiliser give you tall, leafy plants with few flowers, which is the opposite of what you want. They are also brilliant in a wildflower meadow or a cottage garden, where their airy growth weaves between other plants, and they make lovely cut flowers that last well in a vase.
Cosmos at a glance
| Growing factor | What cosmos need |
|---|---|
| Sun | full sun |
| Soil pH | 6.0–7.0 |
| Days to germination | 7–14 |
| Days to maturity | 60–70 |
| Spacing | 12–18 inches apart |
| Water needs | moderate |
| Cold tolerance | tender |
| Family | Asteraceae |
Starting seeds
Cosmos are direct-sown where they are to grow — they dislike transplanting, so skip the seed trays. Sow after the last frost, when the soil has warmed: in Zones 3–5 that is late spring, in Zones 6–8 mid-spring, and in Zones 9–10 early spring, with a second sowing possible in late summer. Zones 11–13 are frost-free, so you can sow in succession through much of the year; check your local extension service for the best local sowing dates. Seeds germinate in 7–14 days in warm, moist soil. Sow at the spacing in the at-a-glance table above rather than thickly and then thinning hard — although thinning is still needed if germination is patchy, sow thinly to start. Cover lightly, keep the soil evenly moist until seedlings appear, and mark the row so you do not weed out the seedlings by mistake. Our seed starting calculator and germination time calculator help you plan sowing dates, and the seed spacing calculator works out row spacing.
Transplanting and planting out
Cosmos are sown where they grow, so there is no transplanting step — instead, thin the seedlings once they are a few inches tall, leaving the strongest one at each 12–18-inch spacing. Thin on a damp day, water the row first so the remaining seedlings are not disturbed, and snip out the extras rather than pulling them, which can damage the roots of the ones you keep. Our plant spacing calculator works out exactly how many plants fit your bed, and the square foot garden planner helps lay them out.
Soil and feeding
Cosmos grow best in fertile, well-drained soil with a pH of 6.0–7.0. Dig in compost before planting — it feeds the soil and helps it hold moisture without getting soggy — and mulch after planting to keep the roots cool and the weeds down. Feed during the growing season with a balanced fertiliser at the label rate, roughly once a month while plants are actively growing, and stop feeding as the season ends. Our fertiliser calculator works out the right amount for your bed, and the compost ratio calculator helps you make good compost to dig in.
Watering
Cosmos have moderate water needs: keep the soil evenly moist while plants are establishing and flowering, and water deeply rather than little and often so the roots grow down. Water at the base in the morning so the foliage dries during the day, mulch to hold moisture in, and watch for wilting in hot, dry spells. Our watering calculator gives a starting point for your bed size, and the watering schedule generator can build a routine around it.
Sunlight and temperature
Cosmos need full sun — at least six hours of direct sun a day. Cosmos are tender — frost kills them or damages them badly, so they are grown as warm-season plants in most zones and planted out only once frosts are over. In frost-free Zones 11–13 they can grow for much of the year. Use the frost date calculator to find your last frost date. Our sunlight calculator can check whether your spot gets enough sun.
Pests and diseases
The pests and diseases to watch for on cosmos are aphids, slugs, thrips. Cosmos are rarely troubled by pests. Aphids can appear on young plants, slugs may eat seedlings, and thrips can mark the petals in hot, dry weather, but a healthy cosmos usually outgrows any damage. Check plants weekly, looking under leaves as well as on top, and act early. Hand-pick larger pests, knock aphids off with a strong spray of water, and remove badly affected leaves. Keep the area weed-free and avoid wetting the foliage when you water, since damp leaves invite fungal disease. Our pest identifier helper can help identify what you are seeing, and the plant disease checker helps narrow down leaf and stem problems.
Companion plants
Good companions for cosmos are lavender, marigolds, garlic, chives. Lavender, marigolds, garlic and chives are good companions, and cosmos also works well planted among vegetables, where it draws in pollinators and beneficial insects. Our companion planting finder can suggest more pairings for your bed plan.
Harvesting
Cut cosmos for the vase when the flowers are fully open, in the morning, and put stems straight into water — they last about a week. Deadhead regularly, snipping off spent flowers, and the plants will keep blooming until frost. If you want cosmos to self-seed, leave a few flowers to set seed at the end of the season and let them drop where they fall. Pull plants up once frost kills them. Our harvest date calculator can estimate your picking date from your sowing date, and the vegetable yield estimator helps you work out how much a bed will produce.
Common problems and fixes
| Problem | Likely cause | Fix |
|---|---|---|
| Tall plants, no flowers | soil too rich or too much fertiliser | Stop feeding and grow cosmos in poor soil. |
| Plants flop over | tall growth in wind | Pinch young plants and stake tall varieties in exposed spots. |
| Seedlings eaten | slugs | Protect seedlings and sow a few extra to allow for losses. |
Tall plants, no flowers
Cosmos in rich, fertilised soil grow into tall green plants with hardly a flower. This is the classic cosmos mistake: they flower best when they are slightly hungry. Stop feeding, do not dig in fresh manure, and the flowers will come.
Plants flop over
Cosmos can reach well over a metre, and a windy site flattens them. Pinch the tips of young plants to make them bushier and sturdier, and grow them in a drift so they support each other, or stake the tallest ones.
Seedlings eaten
Slugs love young cosmos seedlings and can clear a row overnight. Sow a few more than you need, protect seedlings until they are sturdy, and water in the morning so the soil surface dries by night.
Frequently asked questions
How long does it take to grow cosmos?
From planting to maturity takes 60–70 days. Seed germination takes 7–14 days, and the total time depends on whether you start from seed, a nursery plant, or a bulb or cutting.
How far apart should I plant cosmos?
Space cosmos 12–18 inches apart. That spacing gives each plant room for air movement, which keeps disease down, and room to reach its full size without crowding its neighbours.
What soil pH do cosmos need?
Cosmos prefer a soil pH of 6.0–7.0. Test your soil before amending it — most garden soils are close enough, and guessing with lime or sulphur can do more harm than good.
Do cosmos need deadheading?
Yes, regular deadheading keeps cosmos flowering for months. Snip off spent flowers and the plant produces new ones; left to set seed, it slows down and stops blooming.
Will cosmos self-seed?
Yes, cosmos self-seed freely in most gardens. Leave a few flowers to ripen at the end of the season and seedlings will appear next spring — thin them out or move them where you want them.