When do cosmos bloom?

In zones 5–7, cosmos sown after last frost (late May) typically begin blooming in late July, about 8–10 weeks after germination. They continue until hard frost in October. C. sulphureus blooms a bit earlier and handles midsummer heat without stalling. For earlier bloom in cold climates, start seeds indoors 3–4 weeks before last frost and transplant after danger of frost has passed.

Do cosmos come back every year?

Both species are annuals — they complete their life cycle in one season and die. However, they self-sow prolifically wherever seeds ripen and fall on bare soil. Per University of Minnesota Extension, "cosmos often return year after year through self-sowing, giving the appearance of a perennial." Mulched beds suppress self-sown germination; leave bare patches near established plants if you want them to naturalize.

Can I save cosmos seeds?

Yes. Allow the last flush of flowers to fully mature into dry, dark brown seedheads, then collect and store seeds in a paper envelope in a cool, dry location. Germination rates typically remain high for 3–4 years when stored properly. Hybrid varieties like Sensation may not come true from saved seed — open-pollinated varieties like Purity and Rubenza are better candidates for seed saving.

Are cosmos deer-resistant?

Cosmos are generally avoided by deer — the fine foliage and somewhat bitter taste make them a low-preference browse. Per Rutgers NJAES's deer resistance ratings, cosmos are considered "rarely damaged" in landscapes with moderate deer pressure. That said, hungry deer in high-pressure areas will eat nearly anything.

How tall do cosmos get?

Standard C. bipinnatus varieties reach 3–4 feet in lean soil; in rich soil they can hit 5–6 feet and become prone to flopping. Dwarf series like Sonata and Daydream stay under 18 inches. C. sulphureus is naturally shorter — 18–30 inches in most conditions. Pinching seedlings once at 8–12 inches of height encourages bushier, self-supporting plants.

Home/Plants/Cosmos Care: Easy Annual Cut Flowers That Self-Sow

Species guide

Cosmos Care: Easy Annual Cut Flowers That Self-Sow

Annual Cosmos bipinnatus and C. sulphureus are among the easiest cut flowers to grow from seed, but they bloom later and stay compact when kept lean — this guide covers direct sowing, deadheading, and why fertilizing backfires.

By Thomas Joseph Published 2026-06-11 Updated 2026-06-13 Reviewed by Thomas Joseph 7 min read

Cosmos Cosmos bipinnatus with single daisy-like pink and white eight-petaled flowers and feathery thread-like foliage — Cosmos bipinnatus with its airy thread-like foliage. Image: AI-generated reference.

Which cosmos do you have?

Two cosmos species dominate home gardens, and they behave differently enough that identifying them is worth the effort before you plan your planting schedule.

Cosmos bipinnatus — garden cosmos

The classic cosmos. Ferny, thread-like foliage. Flowers in pink, rose, crimson, lavender, and white on stems 3–4 feet tall. Per University of Minnesota Extension, C. bipinnatus "prefers cool nights and may slow blooming in the hottest weeks of midsummer, then rebound in the cooler weather of late summer and fall." It is strictly an annual, killed by frost, but self-sows readily where conditions allow. Popular series include Sensation, Sonata (dwarf), and Purity (white).

Cosmos sulphureus — sulphur cosmos

More heat-tolerant than its cousin. Flowers in gold, orange, and red-orange on shorter, bushier plants (1.5–3 feet). Leaves are broader and darker green than C. bipinnatus. Per University of Minnesota Extension, sulphur cosmos "blooms well even in the heat of midsummer" and is a better choice for zones 7–9 where garden cosmos stalls in July. The Bright Lights and Ladybird series are widely available.

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Cosmos Care: Easy Annual Cut Flowers That Self-Sow

Which cosmos do you have?

Cosmos bipinnatus — garden cosmos

Cosmos sulphureus — sulphur cosmos