Sea Shells

Family: Asteraceae Annual flowering plant

Planting Schedule

Provide your location or planting zone to unlock dates and add Sea Shells to a garden.

Feathery, seashell-like petals unfurl in a soft, airy bloom that seems to glow in the sun—Sea Shells Cosmos is a delight for the senses from first bud to full show.

Expect a light, crisp texture with ruffled, shell-formed florets and a graceful, upright habit that makes every stem feel like a fresh bouquet-ready treasure. Grow for long-lasting cut stems and pollinator-friendly color that draws butterflies and beneficial insects all season long.

Light: Full SunMaturity: 60 DaysHabit: Upright

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Sea Shells Planting Dates by USDA Zone

Estimated US hardiness-zone windows for sow, transplant, and harvest. Enter a US ZIP above for exact local dates, or open a zone below.

Zone	Start Indoors	Sow / Transplant	Harvest
Zone 1a	Protected Environment Only
Zone 1b	Direct Sow	Jun 1st	Jul 31st – Aug 1st
Zone 2a	Direct Sow	May 25th	Jul 24th – Aug 29th
Zone 2b	Direct Sow	May 15th	Jul 14th – Aug 25th
Zone 3a	Direct Sow	May 15th	Jul 14th – Sep 1st
Zone 3b	Direct Sow	May 19th	Jul 18th – Sep 21st
Zone 4a	Direct Sow	May 15th	Jul 14th – Sep 24th
Zone 4b	Direct Sow	May 13th	Jul 12th – Sep 27th
Zone 5a	Direct Sow	May 8th	Jul 7th – Sep 30th
Zone 5b	Direct Sow	May 4th	Jul 3rd – Oct 5th
Zone 6a	Direct Sow	May 1st	Jun 30th – Oct 10th
Zone 6b	Direct Sow	Apr 25th	Jun 24th – Oct 16th
Zone 7a	Direct Sow	Apr 20th	Jun 19th – Oct 20th
Zone 7b	Direct Sow	Apr 10th	Jun 9th – Oct 28th
Zone 8a	Direct Sow	Apr 1st	May 31st – Nov 5th
Zone 8b	Direct Sow	Mar 23rd	May 22nd – Nov 12th
Zone 9a	Direct Sow	Mar 6th	May 5th – Nov 26th
Zone 9b	Direct Sow	Feb 9th	Apr 10th – Dec 9th
Zone 10a	—	Oct 1st	Nov 30th – Feb 28th
Zone 10b	—	Oct 1st	Nov 30th – Feb 28th
Zone 11a	—	Oct 1st	Nov 30th – Feb 28th
Zone 11b	—	Oct 1st	Nov 30th – Feb 28th
Zone 12a	—	Oct 1st	Nov 30th – Feb 28th
Zone 12b	—	Oct 1st	Nov 30th – Feb 28th
Zone 13a	—	Oct 1st	Nov 30th – Feb 28th
Zone 13b	—	Oct 1st	Nov 30th – Feb 28th

Crop Details

Trait	Value
Days to Maturity	60
Sun Requirements	Full Sun
Growth Habit	Upright
Support Needed	None
Planting Depth	Normal
Germination Temp (°F)	75
Min Soil Temp (°F)	60
Min Night Temp (°F)	50
Harden Off (days)	Not Required

Frequently Asked Questions

What pest or disease most commonly affects Cosmos bipinnatus (“Sea Shells”), and how do I treat it at home?

Cosmos bipinnatus is often hit by powdery mildew in warm, humid weather and when plants are crowded. Improve airflow by thinning seedlings to about 12–18 in (30–45 cm) apart, watering at the soil line, and removing any heavily dusted leaves. If mildew keeps spreading, spray a potassium bicarbonate or horticultural wettable sulfur product according to the label and avoid overhead watering for several weeks.

How often should I water Cosmos bipinnatus during peak growth (around weeks 3–8)?

During active growth, keep the soil evenly moist but not soggy—water when the top 1 in (2–3 cm) of soil feels dry. For most home beds, that’s roughly 1–2 times per week depending on heat, with deeper soaking so roots develop rather than frequent light watering. Once established and flowering, let the soil dry slightly between waterings to reduce mildew risk.

How can I tell when to harvest Cosmos bipinnatus (“Sea Shells”) for best blooms?

Harvest when the flower heads are fully open and the petals are crisp, with pollen visible in the center but without browning and shriveled petals. For the longest vase life, cut in the morning on days when plants are dry, leaving several sets of foliage for continued branching and more blooms. Cosmos doesn’t “finish” like a vegetable—keep harvesting every few days once it starts flowering (about 60 days to first maturity), and stop only when blooming slows.