How to Grow Cosmos Plants in your Garden

Gardener's HQ Guide to Growing Mexican Aster, Ulam Raja, and Cosmea

Members of the Cosmos plant genus are half hardy perennials or half hardy annuals.

They bloom with daisy like flowers in rich red, yellow, orange, pink or white.

They generally have stalks of about 30 to 120 cm in height (depending on species).

Common names for Cosmos include Cosmea and Mexican Aster.

Cosmos bipinnatus
Cosmos bipinnatus (Garden Cosmos) by Markles55.

Cosmos sulphureus
Cosmos sulphureus – Yellow Cosmos by Puzzler4879.

Cosmos Growing and Care Guide

Common Names: Cosmos: Common; Garden; Chocolate; Wild; Klondike; Sulphur; Orange; yellow. Mexican Aster, Ulam Raja.
Family: Asteraceae.
Life Cycle: Half hardy perennial. Half hardy annual.
Height: 12 to 80 inches (30 to 200 cm).
Native: Americas.

Growing Region: Zones 1 to 10. Perennials in warm areas.
Flowers: Summer and autumn to last frost. Species dependent.
Flower Details: Yellow, orange, chocolate, pink, red. Daisy-like.
Foliage: Feathery. Smooth.

Sow Outside: 1/8 inch (3mm). Following last frost or autumn. Spacing 6 to 12 inches (15 to 30 cm); large species up to 40 inches (100cm).
Sow Inside: Germination time: three to eight days. Temperature 70°F (21°C). Five weeks before expected last frost. Transplant outdoors following the last frost.

Requirements: Full sunlight. Soil pH 6 to 8.5. Poor soils. Encourage branching by removing tips of tall varieties when they reach 16 inches (40cm). Stake tall plants. Supply winter mulch to perennials or bring indoors.

How to Grow Cosmos

If planning to grow Cosmea, Mexican Aster and other Cosmos members outdoors then the seeds should be sown out after the last frost of spring at a depth of 3 mm (1/10th inch / lightly cover).

They prefer to grow in a sunny area of the garden that has poor soil of PH 5 to 7.

If you are planning to fist grow Cosmos indoors as starter seedlings, then they should be started off about five weeks before they are due to be put out in spring (following the last frost).

They take about half to one and a half weeks to germinate at a temperature of about 21 to 24 degrees centigrade.

Caring for Cosmos in the Garden

Once growing it is best to pinch out the growing tips of the plant when it reaches an height of about 40 cm to encourage bushiness and more blooms. Cosmos requires watering during prolonged dry spells.

I hope that you enjoyed this guide on how to grow Cosmos plants. You may also enjoy the following Gardener's HQ growing guides: How to grow Liatris spicata and Senecio plants.