Planting Cosmos Flowers – How to Plant, Grow, and Care For Cosmos

Cosmos are annual flowers that look lovely in informal planting or a tangle of flowers in a cottage garden setting. These beautiful plants are low-maintenance, summer-blooming flowers with brilliantly colored blooms atop tall, delicate stems. They are the perfect addition to a sunny meadow garden.

The flowers are an excellent food source for bees and make fantastic cut flowers for informal style bouquets for your home—a low-maintenance flowering choice for folks who want a vibrant display of flowers without much work.

Planting comos in The Garden

You will not be disappointed planting cosmos in your garden. The plants bear brilliantly colored flowers on tall, delicate stems. Although Cosmos’ sprawling nature makes them unlikely for formal beds, they shine in informal plantings.

Cosmos belong to the daisy family. Both cosmos species look like typical daisy-type flowers with yellow centers surrounded by showy petals called ray florets. Blooms can be single or double, and up to 4 inches across. Both species will bloom from early summer until the first frost.

Cosmos flowers are native to Central America and Mexico; hence, they are sometimes called Mexican Aster. There are two different species of cosmos:

c. bipinnatus (common garden cosmos). Tall varieties of cosmos bipinnatus will grow 4-6 feet with a spread of 1-3 feet. Dwarf varieties will grow 3-3.5 feet. Blooms are deep pink, pale pink, white, and magenta.

c. sulphureus (Yellow Cosmos). Taller varieties of cosmos sulphureus will grow 2-6 feet with a spread of 1-3 feet. Dwarf varieties will grow 1-2 feet with a 1-3 feet spread. Blooms are gold, yellow, and orange.

Tall varieties of both species of cosmos may need support to avoid being toppled during strong winds.

Grow cosmos flowers in the right environment

As it is with almost every flower, the trick to growing Cosmos is to ensure the proper environment for the flowers to thrive. These plants with daisy-like flowers bloom from early summer to late summer and sometimes into early fall.

Tall plants look lovely planted as a singular mass planting in the garden, or for the back of more extensive informal flower beds and borders. Shorter varieties of these annual plants are lovely for edges and the fronts of garden beds. They also look beautiful in containers and window boxes during the flowering season.

Hardiness

Cosmos flowers are tender annuals and will not survive even a light frost. However, they will bloom throughout summer so long as you remove the spent flowers.

Light

Cosmos are sun-loving plants that like their environment hot and dry. They do best in full sun but will tolerate some partial shade. Plant them in a sunny spot with afternoon sun for abundant blooms.

Soil

Like nasturtiums, cosmos prefer poor soil. Fertile soil will lead to plants with lots of green leaves at the expense of flowers—plant in dry non-fertile soils for best flowering.

The soil should be well-drained with a pH range from 6-8.

Planting and Propagation of Cosmos

Cosmos are one of the easiest plants to grow. You can start seeds indoors for earlier blooms or direct sow right into the garden. Nothing could be easier!

Planting

If sowing outdoors, be sure to wait until all danger of frost has passed.

Seeds can be started 4-6 weeks indoors in early spring before the last frost date in your area. Sow cosmos seeds in flats but barely cover the seeds with soil. The cosmos seeds need heat to germinate, so be sure to keep the soil warm. Seeds should germinate in four to seven days.

Space plants are 8-16 inches apart, depending on the height at maturity.

Propagation

In warm areas with mild winters, the cosmos will seed themselves yearly. You can also collect the seeds after the first frost when the flower head is dry. Keep the seeds in a dry seed packet or envelop for planting next spring.

Container gardening

Plant smaller cultivars like Sonata in containers. So long as the soil isn’t fertile, cosmos will grow well in containers.

Recommended Cosmos Varieties

There are so many varieties of cosmos to try; we’ve listed the ones that we have personally grown and loved.

Bright Lights – Classic Sulphur cosmos with flowers that glow in yellow, gold, orange, and scarlet shades. Ht 36-48″ tall.

Pied Piper – This is a sea-shell type flower with fluted petals in deep maroon and soft creamy pinky white. Ht. 36-40″ tall.

Double Click – This flower is terrific for cut gardens. Its ruffly fluffy powder puffy style blooms fill bouquets with loads of color and texture. The colors range from dark maroon to pink, mauves, and whites. Ht. 24-31″. Double click is our #1 go-to flower for our garden.

Purity – This is the flower you want to plant in your moonlit garden. The blooms of Purity are brilliantly bright white flowers that are especially beautiful when lit by the moon. Ht. 30-36″. This flower is an excellent addition to a moonlight garden.

Rubenza – The darkest red of all the cosmos. Flowers are present on tall, delicate stems and have heavy contrast with the bright green lacy foliage. They are an absolute beauty in the garden planted en masse. Ht. 36–48″

Cosmos look lovely planted with hostas, snapdragons, forget-me-nots, zinnias, dahlias, and marigolds.

Caring for cosmos

The trick with any flower you grow in your garden is to ensure that the plants have just the right conditions to thrive. Not all plants like the same things, so learn to care for each species individually.

Water

Cosmos require 1 inch of water a week, either through rain or manual watering.

Fertilizer

No need to fertilize your cosmos plants; they like poor soils and will not bloom well if fertilizer is added to the soil. Planting cosmos in fertilized soil will lead to large leafy plants with few blooms.

Flower Cuttings & Drying – Ways to enjoy your cosmos flowers

Growing cosmos is a wonderful pastime in the summer. The more you cut them the more they bloom. You can have fresh bouquets weekly in your home by planting out a dozen or so seeds. We love our cut flower garden and I like to save some of my blooms for drying for crafts in the fall and winter.

Cut Flowers

Cosmos make for a beautiful cut flower. Every year, we grow “double click” and “purity” in our cutting garden. Cut flowers last about 4-6 days, but stems are loaded with blooms that will open individually over a week. Be sure to use floral preservatives in the water.

Cosmos generally do not produce heavily scented blooms except for cosmos atrosanguineus (Chocolate Cosmos). These dark red flowers have a unique chocolate scent.

See: The Best Flowers for the Cutting Garden

Drying

Cosmos do not air dry well, but can you dry them with silica gel. Cosmo flowers also press very well, although drying with silica gel will help retain the color far better than pressing.

Check out our post on How to Dry Flowers and our follow-up posts How To Make Dried Flowers Last and How to Dry Rose Petals.

Common Insect Pests

Cosmos, like most annuals, can have issues with many different types of garden pests:

aphids

Aphids will distort the growth of any plant they infest. The leaves may be curled or stunted and may turn yellow.

Spray the underside of leaves with a forceful jet of water, or use insecticidal soap to get rid of aphids.

Japanese beetles

Japanese beetles will eat leaves and destroy flowers. They are small, 1/2 inch long shiny metallic green beetles with copper-colored wings.

To control beetles, handpick them from the plants and crush or drop them into a pail of soapy water.

If you have a heavy infestation, spray infested plants with pyrethrum every 3-5 days. Pyrethrum is harmful to bees and pollinators, so be sure to spray after 9:00 pm and before 5:00 am to avoid harming the beneficial insects.

Borers

If your cosmos plants are suddenly wilting and falling over, look closely at the stem for small round holes.

Remove and destroy any plants infected with borers.

Preventative measures are the best defense against borers. Clean up all weeds and debris, and burn or destroy them.

Bt (Bacillus thuringiensis) is effective early in the season.

Common Diseases that Impact cosmos

A number of fungal and bacterial diseases can impact Cosmos. Understanding what to look for when diagnosing illness is the first step to help control the spread in your gardens.

Bacterial Wilt

If your plants turn yellow, wilt, and die, you likely have a case of bacterial wilt. You can confirm bacterial wilt in the cosmos by cutting a stem close to the root. If it oozes a mass of grey bacteria, you have the fungal disease known as bacterial wilt.

Overcrowing and overwater are two common causes of bacterial wilt in the garden. Avoid planting Cosmos too close together, or in waterlogged areas.

Remove infected plants entirely from the garden, including the soil where the plant was growing. Do not compost, but place it in a tied plastic bag and add it to the garbage.

After cutting any plant infected with bacterial wilt, be sure to sanitize your garden tools.

Stem Canker

Stem canker is easy to spot on cosmos. First, the stems develop dark brown spots that turn grey, a sure sign of Canker. Eventually, the Canker will envelop the stem and break it, causing the plant to wilt and die.

To control the presence of stem canker in your garden, remove all infected plants and destroy them. Do not add to the compost bin.  

Be sure to sanitize all tools and equipment used to remove stem canker to prevent it from spreading.

Powdery Mildew

If you see white spots on cosmos leaves, the cause could be powdery mildew. While usually not a threat to plants’ health, powdery mildew can cause leaves to curl and wither.

To control, remove affected leaves quickly to contain the pathogen.

Growing cosmos in your home garden

Planting Cosmos is an easy way to get beautiful, colorful blooms in your flower garden. They don’t need much care or expensive fertilizers and will remain low maintenance all season long. Hopefully, you’ve learned everything you need to get started with one of my favorite flower species!

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