Cosmos Plant Growing & Care Guide for Gardeners

Article by Dean Ravenscroft, Ph.D. - Updated 22nd Nov 2023

In this Gardener's HQ guide, we'll explore cultivating Cosmos plants in your garden, indoor spaces, and other settings.

Mexican Aster, Ulam Raja, & Cosmea: Cultivation & Garden Use

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 one to four feet (30 to 120 cm) in height (depending on species).

Common names for Cosmos include Cosmea and Mexican Aster.

Cosmos bipinnatus (Garden Cosmos) by Markles55.

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 / just lightly cover them). They prefer to grow in a sunny area of the garden that has poor soil with 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 70 to 75°F (21 to 24°C).

Cosmos sulphureus – Yellow Cosmos by Puzzler4879.

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 16 inches (40 cm) to encourage bushiness and more blooms. Cosmos will require watering during prolonged dry spells.

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 / just lightly cover them).

They prefer to grow in a sunny area of the garden that has poor soil with 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 70 to 75°F (21 to 24°C).

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 16 inches (40 cm) to encourage bushiness and more blooms. Cosmos will require watering during prolonged dry spells.

Common Questions

How many members does the Cosmos genus have?

The Cosmos genus includes around 20 species of flowering plants, known for their showy, daisy-like flowers.

Do members of Cosmos make a good garden or landscaping plant?

Yes, Cosmos plants are popular in gardens for their vibrant, long-lasting blooms, easy care, and ability to attract butterflies and bees.

Which Cosmos species are most frequently grown by gardeners?

The most commonly grown species are Cosmos bipinnatus (Garden Cosmos) and Cosmos sulphureus (Yellow Cosmos), cherished for their colorful and abundant blooms.

Are members of the Cosmos plant genus fragrant?

While not all Cosmos are fragrant, Cosmos atrosanguineus (Chocolate Cosmos) is known for its unique, chocolate-scented flowers.

What is the perfect location to grow Cosmos?

Cosmos thrive in full sun and well-drained soil. They're drought-tolerant once established, making them a good choice for sunny, dry areas.

Is Cosmos invasive in the USA?

Currently, Cosmos species are not considered invasive in the USA. However, they can self-seed prolifically if not properly managed.

How do I remove Cosmos plants from my garden?

Cosmos can be easily removed by hand-pulling or hoeing, and preventing the plants from setting seed can control their spread.

Summary

The Cosmos genus, part of the Asteraceae family, is native to Mexico. These annual plants are popular in gardens for their daisy-like flowers in various vibrant colors that bloom from summer to first frost, attracting butterflies and other pollinators.

To grow Cosmos, plant them in a sunny location with well-drained soil. They can be grown from seeds sown in late spring. They are drought-tolerant and require little care once established, but deadheading can encourage continuous blooming. Given their height, they are often used in the back of borders or in wildflower meadows.

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.