Sulphur cosmos

Sulphur cosmos: how do you recognise it?

Habit

A loose, branched annual about 80 cm tall and 40 cm wide.

Leaves

Opposite, deeply cut into narrow segments, fresh green; broader than those of pink cosmos.

Flowers

Single or semi-double flower heads 4–6 cm across on long stems, from July to October.

Colour

Yellow, golden yellow, orange and orange-red, with a yellow centre.

Seed

Long, narrow seeds with small hooks at the tip that easily catch on clothing.

Flowering: July–October · Sowing: March–May · Plants: May–June

Why grow it?

• Flowers non-stop from July until the first frost, especially if you pick regularly.
• Bees and butterflies come and go on the open flowers.
• Grows well on poor, dry soil and copes with a dry summer.
• A cheerful, light cut flower for bouquets.
• Easy from seed: it germinates quickly and flowers a few weeks after planting out.

Sulphur cosmos: where does it grow best?

• Full sun: in shade it hardly flowers and the stems flop.
• Poor to ordinary, free-draining sandy or loamy soil, acid to chalky. Rich soil gives lots of leaves and few flowers.
• Dry to moist; once established it tolerates drought well.
• Allow 30–40 cm between plants; low cultivars can be closer.

Light

Sun

Soil type

Sand or loam

Acidity

Not fussy

Moisture

Dry or normal

Size

0.8 m tall, 0.4 m wide

Hardiness

Frost tender: plant out after the last frost (mid-May)

Goes well with

• Purpletop vervain: Purple flower heads on tall, airy stems, also loved by butterflies.
• Zinnia: Flowers at the same time in warm colours and is also a good cut flower.
• Mealy sage: Blue spikes as a cool contrast to the orange flowers.
• Globe thistle: Blue globes in the same dry, sunny spot, a magnet for bees.

Sulphur cosmos: when and how to plant it?

Plant in May–June.

1. Sow indoors in March or April at 18–20 °C, about 0.5 cm deep. The seedlings come up after about a week.
2. Prick them out into individual pots once they have two true leaves, and grow them on in a light, cool place.
3. Or sow in place in May in loose soil, then thin to 30–40 cm.
4. Plant out raised plants in late May or June, once frost is no longer expected, at the same depth as in the pot.
5. Water after planting, but do not feed: poor soil gives the best flowering.

Care

Watering

In the first weeks after planting, water in dry weather. After that only in prolonged drought; in pots, regularly.

Feeding

Not needed in open ground: too much feed gives leaves instead of flowers. In pots, a little feed for flowering plants now and then.

Pinching

Pinch out the tip of young plants once, when they are about 20 cm tall: they will branch better.

Picking

Pick flowers for the vase regularly and remove spent flowers: this keeps it making new buds.

Staking

In a windy spot, support tall cultivars with a few canes or twiggy sticks.

Tip: Sow indoors in April or in place in May in poor soil: too much feed gives leaves instead of flowers. Pick regularly and it will keep flowering.

Sulphur cosmos: when to prune it?

When: Cosmos is not pruned. You pinch young plants once in June, remove spent flowers from July to October and clear the plants after the first frost.

How:

• Pinch young plants once, just above a pair of leaves, when they are about 20 cm tall.
• Cut spent flowers with their stem back to just above a leaf or side shoot.
• In September, let a few flowers go to seed if you want to collect it.
• Pull up the plants after the first frost and put them on the compost heap.

Pruning calendar and basic rules

Propagation

Seed

In September, let a few flower heads dry on the plant and pick the seeds once they are brown. Store them cool and dry and sow indoors in March–April or outdoors in May.

Self-seeding

After a warm summer it sometimes self-seeds. Move the seedlings in May to where you want them.

Sulphur cosmos: how does it get through the winter?

Hardy to 0 °C: protect in harsh winters.

• Not hardy: the plant dies at the first frost. It is an annual and does not need overwintering.
• Leave a few spent plants standing: birds like to pick the seeds from the heads.
• Store your own seed cool and dry in a paper envelope until spring.

Diseases and pests

• Slugs: Can eat young seedlings in a single night. Plant out sturdy plants and pick off slugs in the evening.
• Aphids: On the young tips and buds. Hose them off with water; ladybirds and hoverflies help.
• Lots of leaves, few flowers: Soil too rich or too much feed. Plant in a poor, sunny spot and do not feed.
• Powdery mildew: White powder on the leaves in late summer. Leave enough space between plants and cut off affected leaves.

Through the year

• March: Sow indoors at 18–20 °C.
• April: Sow indoors. Prick out seedlings.
• May: Sow in place. Harden off raised plants outdoors.
• June: Plant out once frost is no longer expected. Pinch young plants once.
• July: First flowers. Bees and butterflies visit. Pick flowers.
• August: Full flower. Remove spent flowers. Water in long dry spells.
• September: Flowering. Collect seed from spent heads.
• October: Last flowers. Clear the plants after the first frost.

Good for wildlife

Bees

Nectar and pollen for honeybees and wild bees.

Butterflies

Nectar for butterflies.

Did you know?

• The name cosmos comes from the Greek ‘kosmos’, order or ornament. According to tradition, Spanish priests in Mexico chose the name for the regular arrangement of the petals.
• Sulphureus means ‘sulphur-yellow’ in Latin, after the yellow flowers of the wild plant.
• Cosmos is related to the dahlia, which also comes from Mexico.