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Entire healthy mature cosmos plant in a real garden bed, showing growth habit and scale.Primary identification image. 60–150 cm tall, 30–45 cm wide at maturity.

Cosmos

Cosmos bipinnatus Cav.

Also called: Garden cosmos

Family
Asteraceae
Season
Warm-season tender annual
Best grown in
Portland, Oregon

An airy, tall annual daisy for poor soil; open single flowers feed bees and hoverflies from midsummer to frost.

Safety: Not grown for eating.

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Life cycle & growth stages

  1. 1. SeedSow 5 mm deep after frost.
  2. 2. SeedlingEmerges in 7–10 days.
  3. 3. Mid-growthFeathery foliage; pinch for bushiness.
  4. 4. FloweringPink, white or crimson daisies from 8 weeks.
  5. 5. DeadheadRemove spent flowers weekly.

Growth stages are ordered chronologically; timing varies with cultivar, temperature, light and care.

Planting calendar for Portland, Oregon

Start indoors 3–5 weeks before last frost; transplant 1–3 weeks after last frost; direct sow 1–4 weeks after last frost; 55–70 days to maturity from sowing. Windows are derived from the region's average frost dates (last Apr 5, first Nov 5) and are estimates until you confirm your own dates in the calendar.

CosmosCosmos bipinnatus
PlantStart indoorsDirect sowAutumn sowTransplantTendHarvest
CosmosMar 1–Mar 15 (estimate)Apr 12–May 3 (estimate)—Apr 12–Apr 26 (estimate)May 3–Jun 6 (estimate)Jun 6–Nov 5 (estimate)
  • Start seeds indoors: Mar 1 – Mar 15
  • Transplant outdoors: Apr 12 – Apr 26
  • Direct sow: Apr 12 – May 3
  • Tend and monitor: May 3 – Jun 6
  • Harvest: Jun 6 – Nov 5

Spacing & support

In-row 30–45 cm; rows 45–45 cm. Planner footprint: 1 cell of 30 cm.

Side-profile diagram of mature cosmos: canopy 60–150 cm tall, 30–45 cm wide, medium root zone, support and harvest-access envelope.

Soil & water

Soil
Poor to moderate, well-drained soil
pH
6–7.5
Fertility
low
Water
Drought-tolerant; overwatering reduces flowers.
Frost
tender; killed by frost

Support & training

Stake tall cultivars in wind. Install at planting so roots are not disturbed later.

Cosmos trained on its support: stake tall cultivars in wind.

Harvest signs

  • Deadhead to prolong flowering
  • Cut flowers when just open

Common problems

Pests: aphids

Diseases: powdery mildew

Appearance alone is not a diagnosis; confirm with a regional extension guide.

Side-by-side: healthy cosmos leaf or fruit and an example of powdery mildew.

Container growing

  • Minimum 10 L (25 cm deep)
  • Use quality potting mix and water more frequently
Mature cosmos in a correctly sized container (10 L or more) with drainage visible.

Companion overview

1 reviewed relationship record (evidence: low).

Supported or conditional

None reviewed

Avoid or separate

None with evidence

Traditional claims only: Tomato.

Full companion profile

Crop rotation

Avoid planting cosmos or other daisy family crops in the same spot for 1 year. Rotation is about previous seasons, not about live neighbours.

Year 1Year 2Year 3AsteraceaeOther familyOther familyback after 1+ years

Shared risks in Asteraceae: lettuce downy mildew, aphids, slugs on lettuce.

Crop rotation hub

Sources, reviewer & update

Sources

Reviewer: Garden Planner editorial team (pending independent horticultural review)

Last reviewed: August 15, 2026 · version 1

Found an error or have feedback? Let us know.

Growing cosmos

Cosmos is sown directly after the last frost or started indoors 3–5 weeks earlier, and flowers from about 8 weeks until frost if deadheaded. Rich soil produces foliage instead of flowers; poor, dry soil suits it.

Cosmos seed beside a ruler or coin for scale.
Cosmos seedling at cotyledon and first-true-leaf stage, in soil, distinguishable from common weeds.
Cosmos at transplant-ready stage with root ball visible beside the cell or pot.

Plants reach 60–150 cm and are light and open, so they cast little shade. Single-flowered cultivars are visited by bees, hoverflies and butterflies; the planner treats cosmos as an insectary plant with general rather than crop-specific evidence.

Two or more mature cosmos plants with a tape measure showing in-row spacing (30–45 cm).
Cosmos flower detail or bolting stage.

Edible parts and safety

Not grown for eating. Not grown for eating.

Cosmos at end of season: seed head, dry pod, spent plant or removal stage.

Pests and diseases

Common pests: aphids. Common diseases: powdery mildew. These vary by region; a photograph is an example, not a diagnosis. See the pest and disease guides and your local extension service.

aphids on cosmos, with a wider damage-context view.

Timeline

Season timeline for cosmos: sowing, transplanting, flowering, harvest and bed occupancy aligned to frost dates.Text alternative: Start seeds indoors Mar 1–Mar 15; Transplant outdoors Apr 12–Apr 26; Direct sow Apr 12–May 3; Tend and monitor May 3–Jun 6; Harvest Jun 6–Nov 5.

Related plants

See also: Asteraceae family page · crop rotation · regional calendar · guides

Our Methodology & Sources

We synthesize peer-reviewed research, university extension publications, and trusted horticulture resources. Our evidence levels help you understand how strong the science is behind the guidance.

Evidence tiers

Supported evidenceConsistent results from multiple robust studies
Traditional claimHistorical use reported, limited scientific support
Context dependentMixed or variable results across conditions
No verified relationshipNo reliable evidence identified

Primary source types

  • Peer-reviewed journal articles
  • University extension publications
  • Government & research institute reports
  • Systematic reviews & meta-analyses

Last updated

August 24, 2026

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