How to Grow Cosmos from Seed UK

Cosmos is one of the easiest flowers to grow from seed in the UK. A packet of Sensation Mix sown in March will produce armfuls of daisy-like blooms from June right through to the first frosts. The plants ask for very little: sun, thin soil, and a weekly pass with the deadheading scissors.

This guide covers both Cosmos bipinnatus (the common annual) and Cosmos atrosanguineus (chocolate cosmos), including sowing times, variety choice, and how to keep them flowering for five months straight. For a full monthly plan, see our seed sowing calendar.

When to sow cosmos seeds in the UK

Cosmos is frost-sensitive, so timing depends on whether you sow indoors or direct.

Indoors: Sow from early March to mid-April. Use modular trays or 7.5cm pots filled with peat-free seed compost. Sow seeds 5mm deep — they need light to germinate, so don’t bury them deeply. Place on a warm windowsill or in a propagator at 18-20C. Seeds germinate in 7-14 days. Starting indoors gives you a six-week head start and flowers by June.

Outdoors (direct sow): Wait until mid-May in southern England or late May in northern England and Scotland. Soil must be above 10C and the last frost must have passed. Rake soil to a fine tilth, sow thinly in rows 45cm apart, and thin to 30-45cm spacing once seedlings have two true leaves.

Latest sowing date: Early June. Direct-sown seeds germinate so quickly that you can still get flowers by September from a June sowing, though the season will be shorter.

For timing alongside other annual flowers, see our guide to how to sow seeds indoors.

How to sow and grow cosmos from seed

Indoor sowing — step by step:

1. Fill a modular tray or small pots with peat-free seed compost. Water until evenly moist.
2. Place one or two seeds per cell, 5mm deep. Do not cover with compost — scatter a thin layer of vermiculite instead to retain moisture without blocking light.
3. Place in a propagator or on a warm windowsill at 18-20C.
4. Seeds germinate in 7-14 days. Remove the propagator lid once shoots appear.
5. When seedlings have two true leaves, pot on into 9cm pots using peat-free multipurpose compost.
6. Pinch out the growing tip when plants reach 20-30cm. This produces a bushier plant with more flowers.
7. Harden off over two weeks in late April or May — move pots outside during the day and bring in at night.
8. Plant out after the last frost at 30-45cm spacing in a sunny border.

Direct sowing outdoors: Rake soil to a fine tilth, removing any large clods. Sow seeds thinly in shallow drills 5mm deep and 45cm apart. Water gently. Thin to 30-45cm once seedlings are 10cm tall. Slugs are the main threat at this stage — see our guide to spring gardening jobs for advice on protecting young seedlings.

Soil and feeding: This is the most important thing to get right. Cosmos flower best in poor, free-draining soil. Rich soil or regular feeding produces dark green, leafy plants with few flowers. Do not add manure or nitrogen-rich fertiliser. If your soil is heavy clay, improve drainage by forking in grit. On chalk or sandy soil, cosmos thrives without any amendment at all.

Best cosmos varieties for UK gardens

Not all cosmos are the same. The table below covers the most reliable varieties for UK conditions, grown in RHS trials at Wisley.

For cutting: Sensation Mix and Double Click both hold well in a vase for 7-10 days. Cut stems early in the morning before the heat builds, and recut at an angle underwater before placing in a clean vase with cool water.

For wildlife: Dazzler and the single-flowered forms in Sensation Mix are excellent for bee-friendly garden planting. The open-faced flowers allow hoverflies and bumblebees easy access to nectar. Cosmos is consistently listed by the RHS as an excellent late-summer pollinator plant.

For small spaces: Cupcakes stays compact at 60-75cm and suits a large container (minimum 30cm diameter). Use a peat-free multipurpose compost and water regularly — pots dry out faster than border soil. For more ideas on small-space growing, see our guide to container vegetable and flower gardening.

How to deadhead cosmos for continuous flowering

Deadheading is the single most effective thing you can do to extend the flowering season. Left unpicked, spent flowers set seed and the plant redirects energy away from producing new blooms.

How to deadhead: Snip off spent flower heads just above the next set of leaves or buds. Do not just remove the petals — remove the entire seed head. Check plants every week from June onwards. Once you get into the habit it takes five minutes per plant.

Cutting for the vase also counts as deadheading. The more you cut, the more cosmos flowers. A well-cut plant can flower continuously for four to five months.

Allowing self-seeding: If you want cosmos to self-seed for next year, stop deadheading on a few stems in September. Leave those stems to set seed and drop. Seedlings appear the following May. This is one of the easiest ways to get free plants year after year, though seedlings from F1 varieties will not come true to type.

Cosmos month-by-month growing calendar

Cosmos pairs well with annual sunflowers for a cutting garden. See our guide to growing sunflowers in the UK for timing that runs alongside cosmos.

Growing chocolate cosmos (Cosmos atrosanguineus)

Why we recommend Sensation Mix for UK cutting gardens: After 30 years of trialling annual flowers, Sensation Mix consistently delivers the best combination of vigour, colour range, and vase life for UK conditions. Stems cut in the morning last 7-10 days in a clean vase — longer than most named single-colour varieties — and one packet sown in March fills a generous cutting row by June.

Chocolate cosmos is a different species from the annual cosmos most gardeners know. It is a tender perennial that grows from small tubers and produces dark maroon-red flowers with a faint chocolate scent — most noticeable on warm afternoons.

Planting: Buy tubers in spring. Plant outdoors from late May after the last frost in a sheltered, south-facing spot in well-drained soil. Plant 5-7cm deep. In cooler northern gardens, grow in a container that can be moved under cover in autumn.

Growing conditions: Chocolate cosmos needs warmth and good drainage. It dislikes waterlogged soil and cold winds. A sunny, sheltered wall gives the best results. It does not self-seed — propagation is by division of tubers in spring.

Overwintering: Chocolate cosmos is not frost-hardy. Lift tubers in October before the first hard frost, shake off soil, and allow to dry for a few days. Store in dry compost or vermiculite in a cool, frost-free shed or garage. Replant the following May. The care is similar to dahlias — see our guide to growing dahlias in the UK for overwintering tips that apply equally to chocolate cosmos.

Varieties: Chocamocha is the most widely available named variety in the UK. It is more compact (45-60cm) and more reliably scented than unnamed seedling-raised plants. The RHS has awarded Chocamocha the AGM (Award of Garden Merit) for consistent garden performance.

Common cosmos problems and how to fix them

Few flowers, lots of leaves: Almost always caused by soil that is too rich or feeding with nitrogen. Do not add manure or balanced fertiliser. Move to a poorer, more free-draining site next year.

Leggy, floppy plants: Caused by insufficient light or sowing too early without adequate light. Pinch out the growing tip at 30cm to encourage branching. Stake taller varieties loosely with a single cane in exposed positions.

Aphids: Blackfly and greenfly congregate on soft new growth in warm weather. Rub off by hand or spray with a diluted washing-up liquid solution. Avoid systemic insecticides, which harm the pollinators cosmos attracts. Attracting beneficial insects helps naturally — companion planting with marigolds or nasturtiums near cosmos can reduce aphid pressure.

Powdery mildew: A white coating on leaves, common in late summer when nights turn cool. It rarely kills established plants. Improve air circulation by thinning crowded stems. Remove badly affected leaves.

Slug damage on seedlings: Seedlings are vulnerable before they reach 10cm. Protect with copper tape around containers, or delay planting out until plants are larger. This is the same approach used for other tender annuals — see our guide to growing sweet peas for more on protecting seedlings outdoors.

Collecting and saving cosmos seeds

Cosmos seed is easy to save and store. The seeds are long, slender, and unmistakeable once you know them.

1. Allow a few flower heads to fully dry on the plant in September.
2. When the petals have dropped and the seed heads look papery and brown, cut the stems.
3. Hold the seed head over a paper bag and rub to release the seeds.
4. Spread on a plate and allow to dry for a further week indoors.
5. Store in a labelled paper envelope in a cool, dry, dark place. Seeds remain viable for 2-3 years.

Note that F1 hybrid varieties (including some in Sensation Mix) do not come true from saved seed. Open-pollinated varieties like Purity and Dazzler are reliable choices if seed saving is your goal.

For advice on composting plant material after clearing out cosmos at the end of the season, see our guide to how to make compost.

Now you’ve mastered cosmos from seed, read our guide on growing dahlias in the UK for the next step.

Frequently asked questions

When should I sow cosmos seeds in the UK?

Sow cosmos indoors from March to April at 18-20C for flowers from June. Sow outdoors direct from mid-May once the last frost has passed. Southern England can direct sow from early May; Scotland and northern England should wait until late May.

Do cosmos need full sun or shade?

Cosmos need full sun — at least six hours of direct sunlight daily. They will grow in partial shade but produce fewer flowers and lean toward the light. A south or west-facing border gives the best results in the UK. Plants in heavy shade become leggy and flower poorly.

Should I feed cosmos plants?

No — cosmos thrive in poor, free-draining soil and do not need feeding. Adding nitrogen-rich fertiliser produces lush foliage at the expense of flowers. If your soil is very thin or sandy, a single low-nitrogen feed at planting is the maximum. On average UK garden soil, no feeding is needed at all.

How tall do cosmos grow?

Height depends on variety. Dwarf types like Cupcakes reach 60-75cm. Standard varieties such as Sensation Mix grow to 90-120cm. Taller selections like Purity and Daydream can reach 1.2-1.5m. Taller plants may need light staking with a single cane in exposed or windy positions.

How do I stop cosmos from going leggy?

Pinch out the growing tip when seedlings reach 30cm. This forces the plant to produce side shoots and results in a bushier habit with more flowers. Deadhead weekly throughout summer to keep energy going into new blooms rather than seed production. Growing in full sun also reduces legginess significantly.

Do cosmos self-seed in UK gardens?

Yes, cosmos self-seeds readily in warm, dry summers. Leave a few spent flower heads on the plant in September. Seedlings appear the following May and need thinning to 45cm spacing. Self-sown plants often flower slightly later than those raised indoors from March. Open-pollinated varieties come truest from self-sown seed.

What is chocolate cosmos and is it hard to grow?

Chocolate cosmos (Cosmos atrosanguineus) is a tender perennial with dark maroon, lightly chocolate-scented flowers. It grows from small tubers planted outdoors after the last frost in a sheltered, sunny spot. Lift tubers in October before the first hard frost and store in dry compost in a frost-free shed over winter — the same method used for dahlias. It is more demanding than annual cosmos but worth growing for its unusual scent and long flowering season.