Cosmos Plant: How To Develop Sensible Flowers

The reasons to add the cosmos plant to your garden are almost endless. They make great cut flowers, they’re low maintenance, and they attract butterflies and other beneficial insects. You can grow them in a dedicated flower bed or interplant them amongst your vegetable crops to take full advantage of the beneficial insects that they attract to aid in pest management. Cosmos are very easy to grow and produce tons of gorgeous flowers! 

They can be easily started from seeds indoors and transplanted in the early spring after the last frost. They will establish in spring and come into full bloom as a part of the summer garden. As a part of the aster family, cosmos flowers have a daisy-like appearance and come in a variety of colors. They are easily identified by their blooms and feathery, fern-like foliage. They come in dwarf varieties and tall varieties, and there is a cosmos for every location in your garden. 

The cosmos flower readily self-seeds and can create a stand that will come back year after year. The flower heads are also easy to collect at the end of the season if you’d like to collect seeds and sow them in another area of your garden. Sow cosmos seeds after all danger of frost, as they require temperatures near 70 degrees Fahrenheit and consistent moisture in order to germinate. Also, reference your seed packets for the specific germination requirements of the variety you intend to grow. 

Quick Care Guide

Cosmos. Source: Nico_Alonso

Common Name Cosmos, garden cosmos, Mexican aster, sulfur cosmos
Scientific Name There are many different cosmos varieties such as cosmos sulphureus, cosmos bipinnatus, cosmos atrosanguineus. They all have similar care requirements. 
Family Asteraceae, the same family as sunflowers and daisies.
Height & Spread 12 to 36 inches tall with a 3ft spread. Some varieties, however, can grow 50 inches tall! 
Light Full sunlight, but can tolerate light shade in areas with extreme heat
Soil Dry, arid soil. Accepts poor soil conditions.
Water Drought tolerant. 
Pests & Diseases Aphids, thrips, aster yellows, powdery mildew

All About The Cosmos Plant

C. sulphureusC. sulphureus. Source: Dinesh Valke

There are over 30 known species of the plant cosmos. Along with other plants like them, they are native to scrub and meadowland in Mexico, Central America, and South America. These daisy-like flowers – commonly referred to as the Mexican aster – extend as far north as the Olympic Peninsula in Washington. Cosmos bipinnatus has even naturalized in the Eastern United States and Eastern Canada. 

Most cosmos flowers are annuals. However, there are also perennial varieties such as C. atrosanguineas, also known as chocolate cosmos. Although they are perennial, they are still extremely frost tender and can only be grown as perennials in USDA zones 9-11. If they are well mulched, then they can potentially survive in USDA zones 7-8. Annual cosmos can be grown in USDA zones 2-11. 

Many gardeners grow cosmos flowers for their daisy-like shape. The cosmos flower sits atop slender stems that are 12 to 50 inches tall and decorated with fern-like feathery foliage. Young plants can be planted outside once the danger of frost has passed in the spring. From their first bloom, they will continue to produce flowers until the first fall frost. 

The Mexican aster is also edible, and the flower petals can be added to salads. They don’t add much flavor, but they do add a splash of color! The name cosmos comes from the Greek word Kosmos which means “order” and is said to refer to the orderly arrangement of their petals. 

Types of Cosmos

As mentioned above, there are over 30 different species of cosmos and multiple varieties and cultivars within each species. That being said, a few species of this plant type are noteworthy as they are more popular and more widely available to home gardeners.

Cosmos bipinnatus

A typical range of colors for the Cosmos bipinnatus species includes double blooms in shades of purple, pink, and white. Because of their flower color, this species is particularly good at attracting butterflies to your garden. They make an excellent addition to a butterfly garden. Their semi-double flowers will also self-seed profusely, which tends to attract birds looking for a snack at the end of the growing season. The sea shells cultivar even has tubular petals that liven up the garden.   

Cosmos atrosanguineas

Also known as chocolate cosmos, this is a perennial variety in USDA zones 9-11. In zones that receive frost, their tubers can be dug up from the flower bed like other plants with bulbed roots and moved to a frost-free area for storage over the winter. They can then be replanted in the spring (much like Dahlias). Their name comes from their color which generally ranges from dark red to black. They also have the largest flower head of all cosmos species at 5 cm in diameter. 

Cosmos sulphureus

C. sulphureus’ double blooms come in a variety of colors, but the most common color is yellow. Their young shoots are eaten raw or cooked in Indonesia and in Thailand, they are consumed in salads or brewed in teas. The bright lights mix has an array of yellow and orange associated with it. Because of their color, the daisy-like flowers have also been used to create an orange-yellow dye that was used to dye wool in pre-Colombian America and later South America. 

Caring for Cosmos Plants

Chocolate CosmosChocolate cosmos. Source: wallygrom

Cosmos flower care is relatively easy, and once your flowers are established, you’ll enjoy their blooms all season long! Read to learn how to provide cosmos flowers with their ideal growing conditions and encourage more blooms. 

Sun and Temperature

Plant cosmos in full sun areas that receive 6-8 hours of direct sunlight per day. However, since they can be grown in USDA zones 2-11, this recommendation can vary depending on where you are. In cooler or more temperate regions, it’s best to follow the full sunlight requirement. It is best to provide your cosmos with light shade in areas that receive extreme heat during the summer. 

When you grow cosmos, extreme heat (above 90 degrees) for extended periods of time will drastically reduce the number of blooms. Germination occurs at 75 degrees, and 75-85 degrees is the ideal growing temperature for young plants. These frost-tender annuals cannot survive even light frosts of 32 degrees Fahrenheit. In hard frosts, they may die. At this point in the growing season, it’s best to let the plants go to seed and collect the seeds for next year. 

Water and Humidity

Water your growing cosmos in the morning or later in the afternoon after the heat of the day has passed to avoid losing moisture to evaporation. Avoid soaking the whole plant and water directly at the base. This will help avoid potential fungal issues with the foliage. This is especially important in areas with high humidity. 

If you grow cosmos in an area with a humid climate, you may also want to consider increasing the spacing between plants to help with air circulation and prevent disease. As the seeds germinate, it’s important to keep the soil moist. Continue to water young plants regularly while they become established. 

Once cosmos reach maturity and begin to grow cosmos flowers, they can become quite drought tolerant and can survive on a deep watering once every two weeks. Cosmos plants thrive in dry and arid conditions. 

Soil

Candystripe CosmosCandystripe Cosmos. Source: Dane Vandeputte

One of the things that makes growing cosmos so easy is their ability to tolerate poor soil conditions. They prefer well-draining, sandy, neutral to alkaline soil. The most important thing is good drainage. Cosmos and cosmos seeds don’t do well in standing water or continuously moist soil. 

Compost or organic matter can be added to the planting site prior to planting, although this is not required. When growing cosmos in pots, be sure to amend the potting soil with perlite or vermiculite to increase drainage since most potting soils are made to retain moisture. 

Fertilizing Cosmos

As mentioned above, cosmos plants can survive poor soil conditions and don’t require soil amendments in order to thrive! In fact, the addition of fertilizers can encourage foliage growth at the expense of blooms. This is especially true with nitrogen-heavy fertilizers. 

For this reason, it is not recommended to fertilize cosmos flowers. The addition of compost at the planting site at the beginning of the growing season, when you plant seeds, will provide all the nutrition that these plants require. 

Pruning Cosmos Plants

Cosmos plants benefit greatly from regular pruning of dead flower heads. Removing flowers as they fade will help encourage growth and more flowers. This will come easily if you’re one of the many gardeners that have added the cosmos flower to your landscape for use as a cut flower. Don’t be shy about cutting them back and bringing the flowers inside for a beautiful bouquet. Your plants will thank you by producing even more flowers. 

When you’re growing cosmos, aside from deadheading, they do not require additional pruning. However, cosmos flowers can grow several feet tall, and providing some taller varieties support like staking can be beneficial, especially in areas with high winds. In areas that receive heavy rains, some stem breakage may be unavoidable, even if you give cosmos plants support. 

Cosmos Plant Propagation

While cosmos can self-seed, they can easily be propagated from stem cuttings. Take a cutting just below a leaf node. Remove the lower foliage and either place the cutting into a glass of water or directly into the soil. If placed directly into the soil, you should expect to see new growth within 3 weeks. When placing the cutting into a glass of water, you will see root growth appear over a few weeks, and then you can choose to transplant your cutting into the ground. 

Cosmos don’t particularly like being transplanted as the roots can be sensitive. For this reason, growing cosmos plants from seed is the usual method of propagation. Scatter seeds onto a bare area and lightly rake them in with your hands. Once planted, this annual flower self-seeds and will continue to provide more cosmos flowers in that area for years to come. 

Repotting a Cosmos Plant

If you’ve chosen to grow your cosmos flowers in a container then you may be asking whether repotting is necessary. As mentioned above, the root ball can be sensitive to transplant, so it’s best to avoid this by growing it in the appropriate-sized container from the start. 

However, if you are growing a perennial variety, you may come to a time when you need to re-pot your cosmos plant, as it will self-seed and self-sow into the pot annually. Prepare your cosmos for transplant by giving it a deep watering the day before. This will help the soil around the roots stay intact and prevent the root ball from being damaged. Dig up your cosmos and move it into its new home, maintaining as much of the original root matter as possible. 

Troubleshooting Your Cosmos Plant

Orange C. sulphureusOrange C. sulphureus. Source: Mauricio Mercadante

Although they are relatively easy to care for, there are some pest and disease issues that can affect cosmos. Read on to learn how to identify and treat these issues. 

Growing Problems

Most growing problems with cosmos plants appear as a lack of lovely flowers. If you notice that your cosmos isn’t flowering, then there are two likely suspects: over-fertilization and/or a lack of sunlight. Cosmos need full sunlight in order to produce flowers. If your plant is not producing, you may want to consider the planting location and assess whether or not it can be moved to an area that receives more sunlight. 

Cosmos plants also prefer lean soil rather than rich soil. This means the addition of fertilizers can actually cause more harm than good. This is especially true if you’ve applied a nitrogen-heavy fertilizer that can encourage more foliage at the expense of producing flowers. This can be another cause for fewer flowers.  

Pests

Some pests, like aphids, flea beetles, and thrips feed on cosmos plants, but they’re generally easy to control with a strong spray of water or insecticidal soap. These bugs all have a similar appearance as small white or greenish bugs that appear in groups, usually on the undersides of foliage. 

In a more advanced aphid infestation, neem oil can be a useful treatment. To help prevent and control aphid infestations, you may consider attracting beneficial predatory insects to your gardens, like ladybugs and predatory wasps. Ladybugs will happily feast on aphids and make short work of any potential infestations. These good bugs can be attracted to your garden by planting plenty of flowering and fragrant plants such as mint, dill, marigolds, sweet alyssum, yarrow, etc.

Diseases

Being in the Asteraceae family means that cosmos flowers are prone to Aster yellows disease. Infected plants have yellow stems, stunted growth, and small malformed flowers. The insect aster leafhopper can carry the pathogen. 

Once a plant has been infected, there is no way to cure it, and it’s best to remove the infected plant to prevent the spread to nearby healthy plants. Leafhoppers prefer to overwinter in perennial weeds such as thistles and dandelions. If you have leafhopper problems, then removing these perennial weeds can reduce their habitat. They can also be controlled with insecticides. 

Powdery mildew appears as a white flour-like powder on the leaves. To prevent powdery mildew issues, avoid overhead watering and always bottom water your plants. Neem oil may also be sprayed as a preventative measure to reduce the colonization of spores on foliage. 

In advanced cases of mildew, it may be best to remove and destroy infected plant material to prevent it from spreading to nearby healthy plants. After removing infected material, spray neem oil or a liquid copper fungicide onto the remainder of the plant and nearby plants to reduce the risk of further spread. 

The same goes for bacterial wilt disease. This disease causes the base and stems of the cosmos to wilt and brown. There is no cure for the bacterial pathogen that causes the disease, and affected plants should be dug up and disposed of. Prevent the disease by using only disease-free seed and keeping the bed free of plant debris. 

Frequently Asked Questions

Cosmos bipinnatus flowersCosmos bipinnatus flowers. Source: nousku

Q: Can cosmos come back every year?

A: Yes! The cosmos flower easily self-sows and will likely form a dense patch in a flower bed where it will come back year after year. Most varieties are annuals and will need to be resown each year, although chocolate cosmos are perennials that grow from tubers and are hardy to USDA zone 7. 

Q: Is cosmos full sun or shade?

A: Cosmos prefer full sun but can tolerate and even benefit from partial shade in areas with extreme heat or extended periods of hot weather. 

Q: Where do cosmos grow best?

A: The cosmos flower grows best in a sunny spot with well-draining soil. 

Q: What do I do with my cosmos after flowering?

A: They make excellent cut flowers! They can be cut and brought inside for a beautiful bouquet. Otherwise, cut them back as the blooms fade, and you’ll be rewarded with even more cosmos flowers. 

Q: How long do cosmos plants last?

A: Cut flowers will last for 7-10 days. Cosmos plants will continue to bloom with regular deadheading until the first frost. 

Q: Do cosmos grow well in pots?

A: Yes, they can grow well in pots if provided with the appropriate drainage. 

Q: How long does cosmos bloom?

A: Cosmos will begin blooming in early summer, and with regular deadheading, they will continue to bloom until the first frost. 

Q: Why are my cosmos so tall?

A: Cosmos, in general, will grow to be several feet tall, but certain taller varieties can grow up to 6ft tall! If growing a taller variety, it’s best to plant them in an area of your garden where they won’t shade out shorter plants. 

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