Chocolate Cosmos: Grow the Cocoa-Scented Tuber

Chocolate cosmos is the flower people bend down to sniff and then call someone over to try. Its small, velvety, deep maroon flowers really do smell of chocolate on a warm afternoon. It’s a tender plant, but if you can look after a dahlia, you can grow this.

This guide covers where to plant it, pots versus borders, feeding, how to keep the tuber through winter and how to make more plants.

What is chocolate cosmos?

Chocolate cosmos (Cosmos atrosanguineus) is a tender perennial from Mexico. It grows from a tuber, a fleshy storage root, and comes back each year if you keep it frost-free. It forms a low clump of leaves, then sends up wiry stems about 60-75cm tall. Each stem carries a single maroon flower about 4cm across.

The flowers look almost black in shade and glow dark red in low sun. On a warm, still day they smell of cocoa. The scent fades when it’s cool or breezy, so give it the sunniest spot you have.

For many years almost every plant in British gardens came from the same self-sterile clone, so it never set seed. That’s why the classic plant is sold as a pot plant or dormant tuber. Newer selections are around too. ‘Chocamocha’ is a compact form about 30-40cm tall that suits a pot on a patio table.

Gardener’s tip: If you see packets of “chocolate cosmos seed”, read the label closely. It may be a different plant or a newer strain, not the classic scented clone.

This is what you buy or store over winter: a small cluster of fleshy tubers, a lot like a mini dahlia.

Chocolate cosmos vs annual cosmos and dahlias

Chocolate cosmos shares a name with annual cosmos but is cared for like a dahlia. Annual cosmos (Cosmos bipinnatus) is sown in spring, flowers for one summer and dies with the frost. For those, see the guide to growing cosmos from seed.

Chocolate cosmos is a plant you keep. Its tuber has to be kept safe from winter frost, just like a dahlia. If you already grow dahlias, the guide to growing dahlias in the UK uses the same lift-and-store rhythm.

Where to plant chocolate cosmos

Plant chocolate cosmos in full sun with moist but free-draining soil. It loves warmth. In sun it flowers well, smells strongest and builds a good tuber for winter. In shade it gives few flowers and little scent.

Aim for at least six hours of sun a day. The foot of a south- or west-facing wall is ideal, as the warmth lifts the scent and shelter protects the thin stems. A sunny raised bed, the front of a warm border or a patio pot all work.

Plant out only once the risk of frost has passed, usually late May, a bit later in the north. Harden off young plants for a week first, standing them outside by day and bringing them in at night.

A sunny raised bed suits chocolate cosmos well. Good drainage keeps the tuber happy and the sun brings out the scent.

Growing chocolate cosmos in pots or in the border

A pot is the easiest way to grow chocolate cosmos in most UK gardens. It solves drainage on heavy soil, and you can carry the whole thing under cover for winter.

In a pot, use a container at least 25cm wide with drainage holes. Fill it with two parts John Innes No.2 to one part horticultural grit. Plant one tuber per pot. Put compact ‘Chocamocha’ in a smaller pot on a table, close to your nose. In autumn, move the pot into a frost-free shed or greenhouse. No digging needed.

In the border, plant 30-40cm apart, with the top of the tuber about 5cm below the surface, in groups of three. Dig in grit on heavy soil. The trade-off is winter, as you’ll usually need to lift the tubers.

Pots on a sunny balcony work brilliantly. When the first frost is forecast, just carry them indoors to a cool, frost-free spot.

Feeding, watering and deadheading for more flowers

Water steadily, feed with tomato food and deadhead often, and it will flower from July to the frosts. It likes moist roots in summer. If it dries out badly, buds can drop.

Water plants in the ground once or twice a week in dry spells. Pots need more, sometimes daily in hot weather. Keep the compost moist but never standing in water.

From midsummer, feed every two weeks with a high-potash feed such as tomato food. Potash encourages flowers and helps the tuber ripen for winter. Avoid nitrogen-rich feeds, which give leaves instead of flowers.

Snip off faded flowers just above the next leaf or side shoot every few days. This keeps new buds coming. The guide to deadheading flowers covers the basics.

Lean in on a warm afternoon to catch the scent. Regular deadheading keeps fresh flowers like this coming until autumn.

Is chocolate cosmos hardy?

No, chocolate cosmos is tender and needs protecting from frost. In a mild, sheltered city or coastal garden on light soil it may survive a normal winter in the ground under a thick mulch. On cold, wet or exposed sites it will usually die.

So plan for winter from the start. You have three choices:

• Lift and store the tuber, the safest option for plants in the border.
• Grow it in a pot and move it under cover, the least work.
• Mulch it deeply where it grows, only on mild sites with free-draining soil.

The guide to overwintering tender plants covers the general principles.

How to overwinter chocolate cosmos tubers

Overwinter chocolate cosmos like a dahlia: lift after the first frost, dry the tuber and store it frost-free.

1. Wait for the first frost to blacken the leaves, usually in October.
2. Cut the stems back to about 5cm.
3. Ease the tuber out with a fork, digging well clear so you don’t spear it. Brush off the soil.
4. Leave it upside down somewhere cool and airy for a few days to dry.
5. Pack it into a box of barely damp old compost or vermiculite, with the crown just showing.
6. Store it somewhere cool and frost-free, such as a shed, garage or cool spare room.

Check the tubers every month. Throw away any that go soft or mouldy. If one starts to shrivel, lightly mist the compost.

On a mild site with free-draining soil, you can try leaving it in the ground. Cut it back after the frost and cover the crown with 15cm of dry mulch, such as bark or straw. It’s a gamble on cold or heavy ground.

Tubers tucked into barely damp compost in a cool shed. A quick monthly check catches any that start to rot.

Starting chocolate cosmos back into growth in spring

In March, pot up stored tubers in gritty compost and give them gentle warmth. Throw away any that feel soft or hollow. A healthy tuber is firm.

Plant each one with the crown near the surface and water sparingly. A heated propagator or warm windowsill speeds things up. Keep the compost just moist until shoots appear.

Grow the plants on somewhere frost-free, then harden them off and plant out in late May. Tubers started early indoors flower sooner than ones left to wake in cold ground.

How to take chocolate cosmos cuttings

Make more chocolate cosmos by taking basal cuttings in spring. Dahlia tubers split into chunky pieces, but chocolate cosmos tubers are thin and don’t divide well. Cuttings are the safer route.

1. Wait until the shoots on a sprouting tuber are 5-8cm long.
2. Cut each one off near its base with a clean, sharp knife.
3. Remove the lowest leaves and dip the cut end in rooting hormone if you have it.
4. Push it into gritty, free-draining compost.
5. Keep it warm, lightly covered and out of direct sun.

They usually root in a few weeks. Once you see new leaves, pot them on and grow them frost-free until early summer. The guide to plant propagation by cuttings, division and layering explains the basics.

Take shoots from the base of a sprouting tuber in spring. One tuber can give you several new plants.

What to grow with chocolate cosmos

Chocolate cosmos looks best against something pale or bright. The dark flowers can vanish in a border on their own, so give them contrast.

Silver leaves such as artemisia make the maroon look richer. Lime-green foliage works too. For a hot scheme, pair it with deep orange and scarlet flowers or a dark dahlia. Airy grasses such as stipa make the floating flowers look natural.

Plant it where you pass close by, beside a path or by a bench, so you catch the scent. More dark-flowered ideas are in the guide to black flowers for a gothic garden, and more perfumed plants in the plants hub.

Silver artemisia and soft grasses make the dark flowers stand out instead of disappearing into the border.

Month-by-month chocolate cosmos calendar

Common chocolate cosmos problems

Most chocolate cosmos problems come from cold, wet winters, too little sun or slugs.

• Losing the plant over winter. This is the most common loss. Lift the tuber or keep it in a pot under cover, unless your garden is mild and free-draining.
• Few flowers or weak scent. Usually too little sun or too much nitrogen. Move it to a hot, open spot and switch to tomato feed.
• Slugs on new shoots. Young spring growth is a favourite, just as with dahlias. The tips in stopping slugs eating dahlias work here too.
• Tubers rotting in store. Damp, warm or frosty stores cause rot. Dry tubers before storing, keep the compost barely damp and check monthly.

Frequently asked questions

Is chocolate cosmos a perennial?

Yes, it is a tender perennial that grows from a tuber. It comes back each year if you keep the tuber safe from frost. In most of the UK that means lifting it in autumn and storing it, or growing it in a pot you can move under cover.

Does chocolate cosmos really smell of chocolate?

Yes, the flowers have a real cocoa scent. It’s strongest on warm, still afternoons and fades in cool or windy weather. You usually need to lean in close, so grow it beside a path or in a pot on a table.

How is chocolate cosmos different from ordinary cosmos?

They are different species with different lifecycles. Ordinary cosmos (Cosmos bipinnatus) is an annual you sow from seed each spring. Chocolate cosmos (Cosmos atrosanguineus) is a tender perennial grown from a tuber, with dark maroon, chocolate-scented flowers.

How do you overwinter chocolate cosmos in the UK?

Treat it like a dahlia. After the first frost blackens the leaves, cut the stems to about 5cm and lift the tuber. Let it dry for a few days, then store it in barely damp compost somewhere cool and frost-free. In a mild garden on light soil, a thick dry mulch may be enough.

Can you grow chocolate cosmos in pots?

Yes, and a pot makes winter care much easier. Use a pot at least 25cm wide with gritty, free-draining compost and stand it in full sun. When frost is forecast, move the whole pot into a frost-free shed, porch or greenhouse and keep it nearly dry until spring.

Why won’t my chocolate cosmos set seed?

The classic chocolate cosmos is self-sterile and sets no seed. For many years almost every plant in gardens came from the same clone. That’s why it’s sold as a plant or tuber and increased by cuttings, not sown from seed.

How do you propagate chocolate cosmos?

Take basal cuttings from a sprouting tuber in spring. Cut shoots 5-8cm long from the base, pot them into gritty compost and keep them warm and out of direct sun. The thin tubers don’t split up well like dahlia clumps, so cuttings are the more reliable way.

Now your chocolate cosmos is sorted, read our guide to the best scented plants for UK gardens to build a fragrant corner around it.