Flowers & bulbs

How to Grow Cosmos: Planting, Care & Bloom

Quick answer

Start indoors about 4 weeks before outdoor transplant (Purdue HO-14). Direct sow or transplant after frost danger is past when soil has warmed to at least 65°F (NC State). Available space about 12 inches–3 feet; height often 2–3+ ft. Deadhead for more blooms; plants readily reseed (NC State).

Cosmos at a glance

PlantStart indoors: 4 wk before last frost; Direct sow / transplant: last frost
SunAt least half-day direct sun; full sun preferred (≥6 hours).
SpacingNC State: rake seeds into loose soil (not deep); available space about 12 inches–3 feet; height about 2–3+ ft depending on type.
Days to bloomGAP (days); bloom season July-Oct. (Purdue HO-80)
-5-4-3-2-1LF+1+2+3
■ Start indoors   ■ Plant out   ■ Harvest
Weeks relative to last frost (LF). Colored weeks show when our ZIP planner schedules each step.
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Updated

Focus here is Cosmos bipinnatus (garden cosmos), a warm-season annual.

Pink Cosmos bipinnatus flowers
Cosmos (Cosmos bipinnatus). Photo: George Chernilevsky, Wikimedia Commons, CC BY 4.0 (resized).

Sourced from university extension guides. For dates where you live, use the ZIP planting planner.

Planting timing

Purdue HO-14-W Starting Seeds Indoors lists cosmos at 4 weeks before outdoor transplant. (Purdue HO-14-W) NC State: “Sow the seeds outdoors after the danger of frost is past and when the soil has warmed to at least 65°F (65 degrees).” (NC State cosmos) Planner: Start indoors last frost −4; Direct sow / transplant last frost 0.

Depth and spacing

NC State: rake seeds into loose soil (not deep); available space about 12 inches–3 feet; height about 2–3+ ft depending on type. Germination about 7–21 days; keep moist about 5–10 days after seeding.

Sun, soil and water

At least half-day direct sun; full sun preferred (≥6 hours). Drought- and heat-tolerant once established (NC State). Fertilize lightly if at all — rich soil can mean lush foliage and fewer flowers; we omit printed synthetic rates.

Care and bloom

Deadhead for more blooms; plants readily reseed (NC State). Summer–fall flowers; Purdue HO-80 lists bloom about July–October for planning.

Pests and diseases

NC State lists aphids; also stem canker, powdery mildew, gray mold, southern blight. Aphids: insecticidal soap or oil applied thoroughly — UC IPM: “Oils and soaps work by smothering aphids” (UC IPM aphids). Cultural spacing and sanitation for blight. Use any product as the label directs.

Related: ZIP planner · organic pest control · zinnias

Frequently asked questions

When do I plant cosmos?

Direct sow after frost when soil is at least 65°F (NC State). Start indoors about 4 weeks before transplant (Purdue).

Do cosmos reseed?

Yes — NC State notes they readily reseed.

How tall do they get?

Often about 2–3+ ft; allow about 12 inches to 3 feet of space (NC State).

Sources