Cosmos (Cosmos bipinnatus) is a Mexican annual that flowers from July until the first frost, filling borders, beds, and patio pots with colour for months on end. The feathery foliage is attractive in its own right, and the flowers — singles, doubles, and fluted forms in every shade from pure white through blush pink and apricot to deep crimson — are among the best cut flowers any garden can produce. Cut a bunch for the kitchen table and more appear within days. Leave them alone, apart from deadheading, and they'll keep the bees and butterflies happy until October.

This guide covers everything you need to know to grow Cosmos from plug plants in UK gardens and containers. If you're looking for a specific variety, browse our Cosmos plug plants, which are dispatched from May, hand sown and grown by us.

What Are the Different Types of Cosmos?

Three species are commonly grown in UK gardens, but they are very different plants.

Garden Cosmos (Cosmos bipinnatus) is the one most people mean when they say "Cosmos." It's a half-hardy annual, growing anywhere from 60cm (compact types like the Sonata series and Xanthos) to well over a metre for standard varieties such as Sensation Purity and Apricotta. Flowers come in whites, pinks, crimsons, apricots, and bicolours. Some varieties produce single daisy-like flowers; others are fully double or have fluted, shell-shaped petals. All are excellent for cutting.

Sulphur Cosmos (Cosmos sulphureus) is shorter and stockier, with bolder foliage and flowers in warm yellows, oranges, and reds. It likes even more heat than garden Cosmos and is less commonly grown in the UK.

Chocolate Cosmos (Cosmos atrosanguineus) is not an annual at all — it's a tender perennial grown from tubers, more closely related to a dahlia than to the garden Cosmos in this guide. It produces small, dark maroon flowers with an unmistakable chocolate scent, but it needs winter protection and completely different cultivation. We won't cover it here.

All the Cosmos varieties we sell at Ashridge are C. bipinnatus, and this guide is written for that species. The growing advice applies to all of them, whether you're planting a compact Sonata White for a pot or a tall Dazzler for the back of a border.

Where Should I Plant Cosmos?

Full sun is essential. Cosmos need as much direct sunlight as possible so plant them where they can see the sun for at least six hours a day (when it is out). In anything less, they produce plenty of foliage but very few flowers. A south- or west-facing position is ideal.

A sheltered spot helps for the taller varieties, which can reach a metre or more and have soft, herbaceous stems that snap in strong wind. That said, Cosmos also benefit from decent air circulation — stagnant, humid corners encourage powdery mildew later in the season. The perfect position is open to the sun but not in a wind tunnel.

Now for the part that surprises most gardeners: don't improve the soil. Cosmos evolved in the poor, rocky, well-drained soils of Mexico and Central America. Rich, fertile ground — the kind you'd prepare for sweet peas or dahlias — produces exactly the wrong result: masses of lush green foliage and hardly any flowers. This is the single most common reason Cosmos disappoint.

Plant into your existing soil. If it's heavy clay, fork in some grit or sharp sand to improve drainage. If it's light and sandy, even better — Cosmos thrive in it. Don't dig in compost, don't add manure, and don't prepare a trench. The poorer and better-drained the soil, the more flowers you'll get.

If you grow sweet peas as well, this is the polar opposite of what they want. Sweet peas crave rich, moisture-retentive soil packed with organic matter. Cosmos want none of that. So just grow them in different places in your garden if you can.

How Do I Plant Cosmos Plug Plants?

When to plant: After the last frost. In most of southern England, that means mid-May. Further north, in exposed gardens, or at altitude, wait until early June. Cosmos are half-hardy annuals and a late frost will kill them.

Your Cosmos plugs will arrive hardened off and ready to plant. So there's no need for a week of shuttling trays in and out of the porch. Once the risk of frost has passed, you can plant them straight into the ground or outdoor containers.

To Plant in the Ground

Choose your sunny, well-drained spot. Water the plugs thoroughly before planting — it's much easier to get water into the rootball before it goes into the ground than after. Dig a hole just deep enough to cover the plug. Space plants 30–45cm apart: closer for a fuller, more mutually supportive planting; wider for individual bushier plants with better airflow. Firm the soil gently around the plug and give each plant a thorough soaking... not a quick sprinkle.

Water every few days for the first two to three weeks while the roots establish, assuming it doesn't rain. After that, Cosmos in the ground are remarkably drought-tolerant and rarely need watering in a normal UK summer.

Protect against slugs immediately. Young Cosmos are a slug magnet. Use whatever method you prefer — organic slug pellets, beer traps, copper tape, or evening patrols — but put protection in place on planting day. This vulnerability passes once the stems become woodier, usually within three or four weeks, but a single night without protection at planting time can reduce a row of plugs to stumps.

How Do I Pinch Out Cosmos to Make Bushy Plants?

This is the single most valuable thing you can do for your Cosmos, and it takes five seconds.

Left to its own devices, a Cosmos plant grows a single tall stem with one flower at the top. That flower is lovely, but it's one flower. If you remove the growing tip early on, the plant branches from below the cut and produces multiple stems, each carrying its own flowers. Instead of one bloom, you get a bushy, productive plant covered in blooms.

When to pinch: When the plant reaches 15–20cm tall, or when it has developed three pairs of true leaves — whichever comes first. Our plugs arrive at 8–14cm, so let them settle for a week after planting, then pinch once they've put on enough growth. Don't rush it.

How to pinch: Find the growing tip at the top of the main stem. Using your thumb and forefinger (or clean scissors, or secateurs), remove the tip just above a pair of leaves. That's it.

What happens next: Two or more side shoots will develop from the leaf joints below the cut. Each of those shoots will eventually carry flowers. The plant becomes bushier, stockier, and dramatically more productive over the season.

The trade-off: Pinched plants start flowering roughly seven to ten days later than unpinched ones. This is a trade-off but it is worth making. The brief delay is nothing compared to the weeks of extra flowers you'll get from a well-branched plant.

If you've grown sweet peas, pinching Cosmos feels familiar — but Cosmos are even more responsive to it.

How Should I Water and Feed Cosmos?

Watering is straightforward. During the first two or three weeks after planting, check the soil every few days and water if it's dry. Once established, Cosmos in the ground rarely need extra water. They're genuinely drought-tolerant and dry conditions actually encourage flowering. If you hit a drought, a deep weekly soak will see them through. Water at the base of the plant, not over the foliage as wet leaves encourage powdery mildew.

Container-grown Cosmos are a different matter and are covered in the container section below.

Feeding is where most people go wrong with Cosmos.

In the ground: do not feed. We know this sounds counterintuitive. So many plants in the garden benefit from a fortnightly liquid feed, so why not Cosmos? Because nitrogen — the main ingredient in most general-purpose fertilisers — promotes leafy growth at the expense of flowers. Feed Cosmos in a border and you'll get a magnificent dome of feathery foliage with barely a bloom in sight. The soil they're planted in provides everything they need.

In containers: feed lightly. Pots contain a limited volume of compost, and nutrients deplete over a long flowering season. Use a high-potash liquid feed — tomato fertiliser or comfrey tea — every two weeks from when they start flowering. Apply at half the strength recommended on the bottle. Potassium promotes flowers; nitrogen promotes leaves. If you notice the plant producing lots of foliage but few flowers in a container, stop feeding altogether and let it settle.

This is one of the few plants where doing less genuinely produces better results.

Staking and Supporting Cosmos

Cosmos stems are green and herbaceous — not woody. They snap easily in wind, and the taller varieties can become top-heavy once they're loaded with flowers. A little support, installed before the plant needs it, makes a significant difference.

For individual plants or small groups: Push a bamboo cane or hazel stick into the ground next to each plant when it reaches about 30cm tall — well before it gets leggy. Tie the main stem loosely with flexi-tie or soft twine at 30cm intervals as the plant grows. Insert the cane before the roots spread; doing it later risks damaging the root system.

For beds and rows: Horizontal netting or mesh at around 30cm height, with a second layer of twine zig-zagged at 60cm as the plants grow, works well for larger plantings. Pea sticks or twiggy brushwood pushed in between the plants early in the season is another option — the Cosmos grow through it and are supported naturally.

Compact varieties may not need staking at all. The Sonata series, Xanthos, and Antiquity stay short enough (60–80cm) that they support themselves in most situations, particularly in a sheltered spot or a container. If you're not keen on staking, choose compact varieties and plant them where they're out of the worst of the wind.

How Do I Deadhead and Cut Cosmos?

You bet. Cosmos are cut-and-come-again flowers. Remove a spent bloom and the plant produces another — and another, and another, from July until the first frost. Stop deadheading, and the plant sets seed, diverts its energy away from flowering, and the display fades weeks earlier than it needs to.

How to deadhead: Cut the stem back to just above the next leaf joint or side branch. Don't just snap off the flower head — take a decent length of stem, which encourages the plant to branch from lower down and produce more flowering shoots.

Spotting the difference between buds and spent flowers catches out a lot of people. New buds are round and firm. Spent flower heads — the ones you want to remove — are elongated, slightly pointed, and soft when you squeeze them gently. If in doubt, leave it a day and look again.

Cutting for the vase: Cut stems early in the morning or in the evening, never in the heat of the afternoon. Cut when the flower is just opening for the longest vase life, or fully open for immediate impact. Strip the lower foliage, place in cool water straight away, and you'll get five to seven days in a vase. The feathery Cosmos foliage makes a beautiful filler in mixed arrangements too — cut a few extra stems of foliage even when you're not taking flowers.

Make deadheading a weekly habit at minimum, and daily during the peak of flowering in August if you can manage it. Every spent flower you remove is replaced by new buds.

Growing Cosmos in Containers

Cosmos make excellent container plants, and a pot of them on a sunny patio is one of the easiest summer displays you can create. The rules are slightly different from growing in the ground, though, so here's what to do.

Pot size: Minimum 30cm diameter and 30cm deep. Bigger pots are better — they hold more moisture, give the roots more room, and are less likely to blow over when the plant is in full sail. A heavy terracotta or stone pot is more stable than a lightweight plastic one, but will need watering more frequently. Make sure there are drainage holes and that you prevent them from blocking up — use broken bits of pot or smallish stones before you put the compost in. Cosmos hate bad drainage.

Compost: Standard peat-free multipurpose compost is fine. Don't over-enrich it. If you want to improve drainage slightly, mix in a handful of perlite or grit (roughly one part grit to three parts compost), but it's not essential.

Which varieties? Compact types are the natural choice for containers. The Sonata series — Sonata Carmine, Sonata Pink, Sonata White — stay around 30–50cm tall and produce flowers in proportion to their size. Xanthos is another excellent pot plant — it reaches about 60cm and produces unusual pale butter-yellow flowers without needing staking. Antiquity is good too. Standard-height varieties can go in large containers if you stake them, but they're more work and more vulnerable to wind.

How many plants per pot? For a 30cm pot, plant three to five plugs. For a 40cm pot or larger, nine or ten give you that full, abundant look that Cosmos does so well. Dense planting works in containers — the plants support each other and the display is far more impressive.

Watering: This is where container culture of Cosmos differs most from growing in open ground. Check the compost daily and water when the top couple of centimetres feel dry. In hot weather, you may even need to water twice a day. Water thoroughly and let the excess drain away — never leave pots standing in saucers of water.

Feeding: Use a high-potash liquid feed (tomato fertiliser or comfrey tea) every two weeks during flowering, at half the recommended strength. This replaces the nutrients that deplete in the limited compost volume. If the plant starts producing lots of leaf and few flowers, stop feeding.

Staking in pots: If you're growing a taller variety, push bamboo stakes into the compost at planting time, before the roots spread. Wrap twine between the stakes to create a support cradle. Compact varieties in sheltered spots shouldn't need this.

At the end of the season: After the first frost, pull out the spent plants and compost them. After you have washed it, the pot is then free for autumn bulbs, wallflowers, or winter pansies.

Common Cosmos Problems and How to Fix Them

Cosmos are remarkably trouble-free. This section is deliberately short.

Slugs and snails are the main threat, but only to young plants. Protect your plugs from the day you plant them. Once the stems are established and slightly woody — three or four weeks after planting — slug damage drops away sharply. If you find overnight damage on newly planted Cosmos, act immediately: slug pellets, beer traps, copper barriers, or torchlit evening patrols.

All leaf, no flowers is almost always caused by soil that's too rich or by feeding a border-planted Cosmos. The solution is to stop feeding and wait. The plant will eventually flower once it's used up the excess nitrogen. Next year, plant into unimproved soil and don't feed. This is the most common Cosmos problem and it has the simplest fix.

Powdery mildew — a white, dusty coating on the leaves — typically appears in late summer, especially in humid conditions with poor air circulation. It's rarely serious enough to affect flowering significantly. Good spacing, watering at the base rather than overhead, and decent airflow all help prevent it. If it does appear, remove the worst-affected leaves. In severe cases, cut the whole plant back by a third — it will rally and produce fresh, clean growth that flowers until the frost. Mildew-affected Cosmos foliage is safe to compost at home, according to the RHS — common foliar fungi break down fully during decomposition. Please note this does not apply to foliage and prunings from plants like roses, apples, pears, and quince, where fungi such as black spot and blight will overwinter very happily in a compost heap.

Leggy, floppy plants usually mean too little light or too many plants crammed together. Cosmos need full sun and enough space for air to move between them. If the damage is done, stake what you can and pinch out the growing tips to encourage branching lower down.

Aphids occasionally appear on growing tips. Squash them by hand, spray them off with a jet of water, or use a soft soap spray. They're rarely a serious problem for Cosmos.

What Should I Do with Cosmos at the End of the Season?

Cosmos are annuals. The first hard frost — the kind that blackens the foliage — marks the end of the season. In a mild autumn, this might not come until late October or even November.

Saving seed is optional but simple. If you'd like to try, allow a few flower heads to stay on the plant until the seeds are dry, dark brown, and papery. Pick them on a dry day, shake the seeds out, and store them in a paper envelope somewhere cool and dry. A note of caution: only open-pollinated varieties come true from seed. F1 hybrids and named cultivars — which includes all the Cosmos we sell — may produce something different, often perfectly nice, but not the same variety you started with.

Clearing up: Pull up the entire plant, roots and all. Unlike sweet peas — which are legumes and fix nitrogen into the soil via their roots — Cosmos roots have no special value. The whole plant goes on the compost heap.

The cleared ground is then free for autumn and winter: plant spring bulbs, sow a green manure, or simply mulch and let it rest until next year. If you plan to grow Cosmos in the same spot again, do nothing — the soil needs no preparation.

Cosmos: Month-by-Month Calendar

March – April
Order your Cosmos plug plants from Ashridge. Choose your planting position — sunny, well-drained, unamended soil, weed-free. Prepare containers if you're growing in pots: clean pots, fresh compost, drainage checked.

May
Receive and unpack your plugs. Your Cosmos will arrive hardened off. We keep an eye on the long-range weather forecast and try to deliver when there's no longer a risk of frost. That is mid to late May in most of the UK. So you should be able to plant them out straight away. Space 30–45cm apart in the ground, or three to ten per pot depending on size. Water in thoroughly. Protect against slugs from day one.

June
Pinch out the growing tips when plants reach 15–20cm tall. Install support canes or netting for taller varieties before the plants need them. Water every few days if the weather is dry. Continue watching for slugs on younger plants.

July
The first flowers appear. Begin your deadheading routine — cut spent blooms back to a leaf joint. Cut flowers for the vase. Do not feed Cosmos planted in borders. Start a fortnightly liquid feed (half-strength, high-potash) for container plants only.

August
Peak flowering. Deadhead or cut at least once a week, daily if you can manage it. Continue the fortnightly feed for containers. Watch for powdery mildew — remove affected leaves and improve airflow if it appears.

September
Flowering continues strongly. If plants are looking tired, leggy, or mildew-ridden, cut back by a third for a fresh flush that will carry on until the frost. Continue deadheading. Ease off the container feed.

October
Flowers until the first hard frost, which may not come until late in the month in mild areas. If you want to save seed, let a few heads dry on the plant now. Enjoy the last vases of the season.

November
After the first killing frost, pull up spent plants and compost them. Clean and store containers, or replant for winter with bulbs, wallflowers, or pansies.

Frequently Asked Questions

Do Cosmos Come Back Every Year?

No. Garden Cosmos (Cosmos bipinnatus) are annuals — they complete their life cycle in one season and are killed by the first hard frost. You need to plant fresh each year, either from seed or from plug plants.

The exception is Chocolate Cosmos (Cosmos atrosanguineus), which is a tender perennial grown from tubers. It can come back year after year if the tubers are lifted in autumn and stored frost-free over winter, or grown in a sheltered spot with a thick winter mulch. But Chocolate Cosmos is an entirely different plant to the garden Cosmos in this guide.

Cosmos do self-seed freely, though. If you leave a few spent flower heads on the plant at the end of the season, seedlings often appear the following spring — usually in late May or June. These self-sown plants tend to be tougher and slightly later-flowering than those grown from plugs, and named cultivars do not come true. It's a pleasant bonus rather than something to rely on.

How Tall Do Cosmos Grow?

It depends on the variety. Compact types like the Sonata series and Xanthos reach 50–80cm. Standard varieties — Sensation Purity, Apricotta, Dazzler, Daydream — grow to around a metre, sometimes more in ideal conditions. All tall varieties benefit from staking or support.

Do Cosmos Need Full Sun?

Yes. Six hours of direct sunlight a day is the minimum. In less than that, you'll get plenty of the attractive feathery foliage but far fewer flowers. A south- or west-facing border is the ideal position.

Do Slugs Eat Cosmos?

They certainly do — but only the young plants. Freshly planted Cosmos plugs are very vulnerable to slug and snail damage, particularly overnight in damp conditions. Protect them from planting day onwards. Once the stems have hardened off and become slightly woody, after about three to four weeks, slugs lose interest. It's a short-term problem with a simple solution: protect early, relax later.

Can I Grow Cosmos in Pots?

Absolutely. Cosmos do well in containers provided the pot is at least 30cm across, they get full sun, and you water regularly. Compact varieties like Sonata and Xanthos are the easiest choices. Feed container Cosmos with a half-strength high-potash liquid feed every two weeks — this is the one situation where feeding is helpful. See Growing Cosmos in Containers above for the full details.

Browse our full range of Cosmos plug plants, hand sown and UK-grown at our nursery in Wiltshire since 1949.