Cosmos

Zone 1b Save Garden

🌱 60d to harvest Upright

Feathery, cloudlike blooms open in a vivid wash of color—silky petals with a crisp, satiny sheen that catches the light from ever…

Crop Dates

Milestone	Date
Start Indoors	May 11th
Last Frost	Jun 1st
Transplant / Sow Outdoors	Jun 1st
Harvest Begins	Jul 31st
Harvest Ends	Aug 1st

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)	7

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Frequently Asked Questions

What pest or disease commonly affects Cosmos bipinnatus, and how can I control it at home?

Asteraceae plants like Cosmos bipinnatus are commonly bothered by aphids, especially on soft new growth and flower buds. Spray the stems and undersides of leaves with a strong jet of water, then follow with insecticidal soap if you still see colonies after 2–3 days. Watch also for powdery mildew in humid conditions—space plants well, water at the soil line, and remove heavily affected leaves early to slow spread.

How often should I water Cosmos bipinnatus during the main growing phase (after seedlings establish)?

After establishment, water deeply about 1–2 times per week, enough to moisten the root zone but not keep the soil constantly wet. Cosmos bipinnatus prefers evenly moist soil at first, then performs best with drier intervals once plants are actively growing—water when the top 1 inch of soil feels dry. Avoid frequent light watering, which encourages weak growth and increases the chance of mildew.

How do I know when Cosmos bipinnatus is ready to harvest?

Cosmos is ready to harvest once the flower heads are fully open and show crisp, evenly colored ray petals—harvest these for cut flowers. For seed, leave a few blooms to mature until the seed heads turn dry and tan-brown and the seeds are firm; harvest before they begin to disperse. Plan on about 60 days to maturity from sowing under good conditions, with flowers appearing well before full seed readiness.

Feathery, cloudlike blooms open in a vivid wash of color—silky petals with a crisp, satiny sheen that catches the light from every angle. Cosmos ‘Cosmos’ forms airy, upright stems topped with long-lasting flowers, offering a delicate crunch of texture in the garden and a graceful presence in arrangements. Grow it for abundant cut-flower color and effortless massing in beds and borders, where its airy foliage makes every planting feel light and luminous.

Sowing Tips

Transplant Conditions

Wait until soil reaches 60°F and nights stay above 50°F before moving outdoors. Harden off for 7 days first.

Expert Note

For earlier blooms, start cosmos indoors in plug trays, then transplant only once nights are consistently mild (and expect it to be happier with direct sow in warm areas).