How to Grow Cosmos bipinnatus & Care Guide

Plants of the genus Cosmos (Cosmos bipinnatus), which come in a rainbow of hues and can grow to a height of several inches, are a staple of many summer gardens.

When single or double blooms develop on stalks ranging from 1 to 4 feet, the effort put into growing and caring for the cosmos is more than worth it (0.5-1 m.).

Plants like the cosmos can be found at the bottom of a sloping garden or in the middle of a solitary island.

If you don’t plant taller varieties in a sheltered area, you may need to stake them.

Flowers from a Cosmos plant can be used as cut flowers for an interior arrangement or as a backdrop for other plants.

The cosmos can also be utilized as screens to conceal eyesores.

How to Grow Cosmos Flowers

Flowers like the cosmos do best when planted in unaltered soil.

Soil that is low to ordinary in quality and temperatures that are hot and dry is ideal for growing the cosmos. Growing cosmos from seed is the standard method.

If you want to plant cosmos somewhere, you should scatter the seeds there on the ground.

Once established, this annual flower will continue to spread its seeds and bloom in the area for years to come.

The cosmos plant has daisy-like flowers perched atop tall branches with delicate foliage.

Deadheading is a technique that can be used to maintain a healthy cosmo flower garden.

Inducing development lower on the flower stalk makes for a more robust plant and more blooms.

To encourage the same effect in a growing cosmos plant, you can care for your plant by cutting blooms for interior usage.

Varieties of Cosmos

There are annual and perennial cosmos plants, and together they number more than 20 different types.

The annual cosmos plants most commonly grown in the United States are Cosmos Sulphureus, sometimes known as yellow cosmos, and Cosmos Bipinnatus, also known as the Mexican aster.

Compared to the more frequent Mexican aster, the yellow cosmos is shorter and more compact.

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The Chocolate Cosmos, or Cosmos Atrosanguineus, is another unique species.

Get some cosmos started this year if you want them to self-seed in your flower bed next year.

This frilly flower is ideal for direct sowing into a sparse patch of garden that could use some tall, colorful blooms that require little maintenance.


By removing spent blossoms, you can get more blooms out of your cosmo plants and extend their blooming period.

To catch up if you’ve fallen behind, prune the plants by roughly a third after most of the blossoms have faded.

By cutting back branches in this way, you might encourage a second round of growth.

When the season is over, you can dig up the plants by their roots or trim them to the ground.

If the plants are allowed to remain in place, however, they may produce seeds for the following season.

Propagating Cosmos

The cosmos plant is able to easily reproduce by sending out its own seeds. Reproduce these plants when all danger of frost has passed.

Seeds are the most reliable and simple method of propagation for this plant, however, stem cuttings can also be used.

Taking cuttings from the stem encourages the development of new leaves and flowers.

The best technique to reproduce this plant, besides using seeds, is by cutting stems. This is how you get things done:

  • You’ll also need a pot of sterile, well-draining potting soil and a pair of sterile pruning shears or scissors.
  • Soak some potting soil and place it in a small container, no more than 3 inches in diameter. Make a small hole in the soil by pressing it straight down with the tip of a pencil. The hole should be approximately an inch or two deep.
  • Try to find a cosmos branch with three to five nodes where leaves have formed. Make a clean slit below the terminal leaf node. Leave the final leaf node intact for fresh growth and trim the leaves off at that point.
  • Put the snipped end of the stem into the crater you formed with the pencil. The lowest leaf node must be kept above ground. Compact the soil around the base of the stem to keep it standing straight.
  • Water heavily and continuously. Within three weeks, you should see the first signs of leaf growth. If you do, you can transplant the entire root ball straight from the container.
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How to Grow Cosmos From Seeds

Plan ahead and start seedlings indoors four to six weeks before the last frost.

Alternatively, if you can wait until the danger of frost has passed, you can sow cosmos straight in the garden outdoors.

Despite their rapid expansion, cosmos are vulnerable to late-season cold and should not be planted hastily.

At 75 degrees Fahrenheit, they usually germinate in 7-21 days and bloom 50-60 days later.

Dig down at least 8 inches to a point where the soil is loose.

Spread the seeds out and cover them with fine soil, at least 1/4 inch deep.

Most seed packets advise planting at 2-foot intervals, but you can also scatter the seeds and let the plants grow closer together if you like.

They can be thinned out later by relocating the unused plants to a different area of the garden.

Potting and Repotting Cosmos

If you want to grow cosmos in a pot, make sure it has holes at the bottom for water to flow out. Overly moist, soggy soil is detrimental to cosmos growth.

The recommended ratio for growing cosmos is one plant per gallon of container space.

Do not replenish the soil if growing in containers; doing so causes plants to become lanky and droopy.

As an additional note, taller cultivars will require staking if grown in pots.

To begin, you should think about utilizing a container with a minimum of a 12-inch-diameter and a strong base.


Every year, you can look forward to the new issue of Cosmos. In freezing weather, they will perish.

However, if you wait until the end of the growing season to harvest your cosmos, you’ll be wasting your time and energy.

The seeds need to go dormant throughout the winter so that they can germinate in the spring.

Maintaining a containerized cosmos during the winter requires a bright full-sun growing lamp for at least seven hours every day.

Flowers should be cut off as soon as they appear.

When this plant flowers, it releases its seeds for the following season, marking the conclusion of its life cycle.

How to Get Cosmos to Bloom

Flowers on a cosmo plant require a sunny location. Flowers are sensitive to light and can be stunted by even a little bit of shade.

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It’s important to deadhead spent flowers to promote new growth and further blooming.

Prune between the main stem and a leaf for quicker flowering.

It takes more time for the plant to produce new blossoms if you make your cut lower on the stem.

Common Problems With Cosmos

Growing and caring for the cosmos throughout the season is simple.

They generally do well in the face of disease and most insects, while some pests can become a nuisance and stunt their development. 

Wilting or Leaf Discoloration

Your plant may be experiencing one of two problems if it is not suffering from a lack of water.

Fusarium leaf spot is a symptom of a common fusarium fungal infection, which can cause a plant to wilt and change its leaf color.

  The presence of a pink mass on the plant’s roots during excavation is diagnostic of fusarium.

The plant as a whole is hopeless, will die, and must be thrown away to prevent the spread of the fungus.

Even though the roots appear healthy upon excavation, bacterial wilt may be affecting the plant.

 Root rot is caused by bacteria and manifests as a wilting of the stems. Sadly, this plant is dying and needs to be thrown away.

Yellowing Leaves and Leaf Drop

Shady plants are more likely to be affected by powdery mildew.

Six fungal spores float through the air before landing on a host plant in a shady area.

The leaves become covered in a white powder and eventually turn yellow and fall off.

Powdery mildew can be prevented by giving plants plenty of air and sunshine and keeping wet off the leaves.

Apply a horticultural fungicide to the infected area of the plant as directed on the product label.

Flowers Distorting or Stunting in Growth

The aster yellows virus is transmitted by leafhoppers and can cause significant damage to the cosmos because they are in the aster family (a tiny grasshopper-looking insect).

 The flowers may look deformed or stunted, and the foliage will develop a yellow mottling.

There is no hope for these plants, so please get rid of them.

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