Tall, floppy plants that fall over

Too-rich soil or too much water encouraging leggy growth. Stop feeding, ease off water, and pinch young plants to branch lower.

Lush foliage but few flowers

Soil too fertile, especially high nitrogen. Grow in lean soil and skip fertilizer entirely.

Crowding and wet, humid conditions. Space wider for airflow and water at the base, not overhead.

Soft new shoots attract sap-feeding aphids. Blast off with water or invite ladybugs; avoid high-nitrogen feed.

Gardening

A breezy, airy annual that blooms from summer to frost on lean soil. Thrives on neglect and feeds bees and butterflies all season.

When to plant cosmos where you live

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Sun

Full sun

Hardiness zones

Zones 3–11

Spacing

12–18 in apart for airflow and sturdy stems.

Soil

Lean, well-draining soil; pH 6.0–7.5. Rich soil gives leaves, not flowers.

Watering

About 1 in/week once established; cosmos are drought-tolerant and dislike soggy roots.

Plant / sow (typical)

May – Jun

Harvest (typical)

Jul – Oct

Starting

Direct-sow after the last frost once soil is warm, or start indoors 4–6 weeks earlier. Deadhead often to keep the flush going.

Companions

Good with Tomato, Squash, Most vegetables. Keep apart from .

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Tall, floppy plants that fall over. Too-rich soil or too much water encouraging leggy growth. Stop feeding, ease off water, and pinch young plants to branch lower.
Lush foliage but few flowers. Soil too fertile, especially high nitrogen. Grow in lean soil and skip fertilizer entirely.
Gray mold or rot on stems. Crowding and wet, humid conditions. Space wider for airflow and water at the base, not overhead.
Aphids clustered on new growth. Soft new shoots attract sap-feeding aphids. Blast off with water or invite ladybugs; avoid high-nitrogen feed.