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Cosmos

Cosmos bipinnatus

Flowers

Airy, daisy-like flowers in shades of pink, white, and crimson. Thrives in poor soil and attracts hoverflies that eat aphids.

Overview

SunFull Sun
WaterLowCalculate watering amount
Height60–150 cm
Spread45 cm
Spacing45 cm
Row Spacing60 cm
Germination7–10 days
To Harvest50–80 days

Companion Planting

Bad Companions

None

Toxicity

SeverityNon-toxic

Cosmos bipinnatus is listed as non-toxic to dogs, cats, and humans by the ASPCA; the petals are edible and sometimes used in salads.

Planting Tips

LocationFull sun (at least 6 hours of direct sunlight); sheltered from strong winds as tall stems can topple.
SoilAverage to poor, well-drained soil; overly rich soil encourages foliage at the expense of flowers.
Sowing Depth0.5 cm
Planting Depth5 cm
WateringWater moderately until established, then only during prolonged dry spells; cosmos are drought-tolerant once rooted.

1 cm of water per m² equals 10 liters. Use the Watering Calculator for exact amounts.

FertilizingLittle to no fertilizer needed; a light application of balanced fertilizer at planting is sufficient — excess nitrogen reduces blooming.

Care Tip: Deadhead spent flowers regularly to prolong blooming; pinch young plants at 30 cm height to encourage bushier growth.

Common Pests

Aphids

Symptoms: Clusters of small green or black insects on stems and flower buds; sticky honeydew residue and curled new growth.

Treatment: Spray with a strong jet of water or apply insecticidal soap; introduce ladybugs as biological control.

Prevention: Avoid excessive nitrogen fertilization; encourage beneficial insects by planting companion herbs nearby.

Spider Mites

Symptoms: Fine webbing on undersides of leaves; stippled, yellowing foliage that may dry out and drop.

Treatment: Spray foliage with water to raise humidity; apply neem oil or miticide if infestation is severe.

Prevention: Keep plants well-watered during hot, dry periods; avoid dusty conditions around plantings.

Slugs and Snails

Symptoms: Irregular holes in leaves and ragged edges, especially on young seedlings; slimy trails visible in the morning.

Treatment: Hand-pick at dusk or use iron-phosphate-based slug pellets around plants.

Prevention: Clear debris around plants; use copper tape or gravel barriers around seedling beds.

Common Diseases

Powdery Mildew

Symptoms: White, powdery coating on leaves and stems; affected leaves may yellow, curl, and drop prematurely.

Treatment: Remove and destroy affected foliage; apply a fungicide such as sulfur spray or potassium bicarbonate solution.

Prevention: Ensure good air circulation by spacing plants 30-45 cm apart; avoid overhead watering in the evening.

Aster Yellows

Symptoms: Stunted growth, green petal-like structures replacing flowers (phyllody), and yellowed or distorted foliage.

Treatment: No cure exists; remove and destroy infected plants immediately to prevent spread.

Prevention: Control leafhopper vectors with row covers or insecticidal soap; remove nearby weed hosts.

Gray Mold (Botrytis)

Symptoms: Fuzzy gray-brown fungal growth on flowers, buds, and stems, especially during cool, wet weather.

Treatment: Remove affected plant parts promptly; apply a copper-based fungicide in persistent wet conditions.

Prevention: Avoid overhead watering; space plants for adequate airflow and remove spent blooms before they rot.