How to Plant, Grow, and Care For Flowering Cosmos Plants

Whether you’re a newbie plant-lover or an experienced gardener, you’ll enjoy the daisy-like blooms and funky foliage of cosmos. Cosmos seeds germinate fast and the plant will bloom like crazy throughout the Summer into the Fall! The best part is, these plants actually prefer poor fertility soils, so if you have a spot in your garden that the veggies just don’t care for, try planting some cosmos for cheer and to attract pollinators! 

No matter your circumstance, there’s probably a cosmos variety out there waiting to fulfill your needs in the garden or landscape. Try planting a dwarf variety of C. sulphureus to spice up a patio pot or maybe a tall, airy variety of C. bipinnatus to provide a splash of pink, purple, or white in the background of your landscaping. Wanting a conversation piece for your garden? Try growing the moody and scented Chocolate Cosmos (C. astrosanguineus) with its rich, dark, and you guessed it chocolate-colored blooms.  

Kids and adults alike will love this beautiful and lively accent flower in the garden. The very thought of these quirky plants brings to mind the sounds of bees buzzing through the air and butterflies fluttering from bloom to bloom. The flowers nod and wave with the breeze to give your garden a romantic, wildflower feel. Easy-to-grow from seed and beautiful what more can you ask for? 

History of Flowering Cosmos

As the Spanish traveled through present-day Mexico, conquering and studying the region, they found many splendid plant specimens, one of which was the quirky and remarkable flower known today as cosmos. In the late 18th century, interest in botany in Europe practically blossomed.

Claire Shaver Haughton records in her book Green Immigrants: The Plants That Transformed America that Charles III of Spain organized a botanical exploration of Spanish America in the 1780s. Many of the plant specimens from this exploration were sent to the botanical garden in Madrid, but some of these plants and seeds were also sent to favored aristocrats. 

Don Antonio Cavanilles, a Spanish aristocrat famous for his garden of exotic plants, was the first to grow cosmos in Europe. He then shared his cosmos seeds with the Marchioness of Bute, who was the wife of the English ambassador to Spain.

When the Butes were recalled to London in the late 18th century, cosmos seeds were amongst the collection of Mexican seeds that the marchioness took back to London. Cosmos were not cultivated for commercial sale until the middle of the 19th century, however. 

Cosmos in the US

In the United States, cosmos still had yet to be grown in American gardens. In 1897, the Plant Introduction Center in Washington, D.C. was founded, which led to the discovery of many plants and flowers that could be of economic value and adapted to varied climates.

During a project to find plants adapted to the arid climate of the American southwest, botanists began to take an interest in Mexican plants, and this was where cosmos was discovered for Americans. The first cosmos plant was introduced to the United States in 1898 from Mexico. 

Who Named the Cosmos Flower?

So, where did this fun, quirky plant get its name? Well, Spanish priests often grew cosmos in their mission gardens. Because of the flower’s even placed and orderly petals, it was named after the Greek word “kosmos,” which means “ordered universe in harmony.”  

Today, plant breeders have since created new hybrid cosmos varieties that are tall, light, and airy plants for backgrounds of landscapes, with blooms two to three inches across or more in a variety of colors.  Similar to Gloxinia, Zinnia, and Cockscomb, you can find these flowers with a wide range of color variations.

Propagation of Cosmos 

Cosmos are easy to sow from seed! In fact, they are prolific re-seeders in the garden and have even escaped home gardens and landscaping to be found “in the wild” in various places. To plant cosmos, bury the seeds ⅛” deep, whether you’re starting the seeds in cell trays indoors or into the garden directly.

Start your seeds indoors about 4 weeks prior to your estimated last frost. They will germinate quickly, sometimes within days. Plants can become leggy without ample light. 

Sow seeds in the garden after your last expected frost in the Spring, planting them in a row at ⅛” depth. You can also broadcast cosmos seeds and lightly rake them into the soil. Make sure to thin your seedlings so that they are at least 9 to 12 inches apart.

At the end of the season, if you wish for your cosmos to reseed, you’ll find success with casually letting the plants go to seed and tilling these plants into the soil in the Fall. In the Spring, you’ll have many new cosmos seedlings popping up! 

Most cosmos varieties can get very large, so make sure to space your plants out well. A few of the shorter or dwarf varieties can be spaced about 6 inches apart, but for the most part, you’ll want your plants to be at least 9 to 24 inches apart. In most cutting or landscaped gardens, it’s recommended to space cosmos at 9 to 12-inch spacing. 

Be aware that if you live in a humid climate you may need to space your plants out more for good air circulation to prevent disease. However, planting at least two or more plants together makes for a nice grouping of elegant and airy flowers,  and it also will help your plants support each other as they get larger. 

How to Grow Cosmos

If you are looking to grow cosmos flowers, there are a couple of different methods to do so successfully. How successful you are will depend on your skill level as a gardener, soil type, and sun exposure. Let’s take a look at the most common methods of planting and growing these beautiful flowers.

Deadheading and Pinching

Pinching your cosmos when they are at least 4 to 8 inches tall will promote side branching to make a bushier plant. Pinch just above a set of leaves. If your cosmos becomes stressed for some reason, the plants will try to start blooming when the plant is rather small. Pinching off this initial bloom will promote more stems and therefore more blooms. 

As the plant blooms, make sure to deadhead faded flowers to promote new growth. You can cut faded flowers at the base of their stem or deeper into the plant to promote more blooms! 

Soil and Fertility

Cosmos are really not picky about soil type, as they prefer well-draining and moderate to poor soils. In fact, if you plant cosmos in too fertile of an area they will often grow rampant, meaning they will grow so rapidly that they start to flop over. Branches may break as a result.

Cosmos may also bloom later if the soil is too high in nitrogen because the plant will focus on vegetative growth rather than flower bud growth. It is not recommended to fertilize cosmos or to amend the soil with compost. 

Sunlight

Cosmos prefer full sun (8+ hours of direct sunlight per day). This plant is day-length sensitive and prefers to bloom during the short days of late Summer and Fall. However, it will bloom as long as it has had ample time to establish itself.  

Cutting Back

If your Spring-sown cosmos have been blooming prolifically throughout the Summer but have gone to seed or are appearing rather worn, you can cut the plant back to about 12 to 18 inches tall to encourage a flush of new and healthy growth that will bloom in late Summer to early Fall. Leave the pruned seed heads on the ground so that new seedlings will germinate the following Spring. 

When to Harvest 

Cosmos flowers are rather short-lived in a vase, so it is best to cut the flowers for an arrangement during the “cracked bud” stage. This is when the bud has begun to open and the petals are starting to lift from the center disk, but the petals are still together. The flower will open quickly in a warm area. 

To harvest, cut at the base of the stem of that flower or just above a set of leaves. Don’t be afraid to make a deep cut into the plant. This will promote new stems that are longer. You can cut off any side buds that have not “cracked” yet. They will not open. 

You can also dry cosmos flowers. Press them in a book or plant press to make pendants and dried flower art in a frame. You can also use them as lovely potpourri toppers. Harvest these flowers when they are fully open. 

To harvest seeds, wait until the flower head has dried out and seeds have begun to form, then collect the long, thin, dark seeds and place them in a paper bag in a cool, dry area. Cosmos readily reseeds so you may not need to save seed if you would like to plant them in the same area the following year.  

Varieties of Cosmos 

There are several different popular varieties of Cosmos flowers. Let’s take a look at the most common varieties to see which one you’d be best to plant in your garden or around your home.

Cosmos bipinnatus

Most of these varieties come in colors of purple, pink, and white, though there are some newer yellow and cream colors available. C. bipinnatus ranges from 1 to 6 feet tall. ‘Candy Stripe’ grows to almost 3 feet tall with splashes of pink and deep rose over white. ‘Rubenza’ is a ruby-red colored flower that grows 2 feet tall with a bright yellow center disk and 3” flower heads.

The ‘Double Click’ series is a fully double cosmos with fluffy mum-like flower heads. These flowers come in pink or burgundy shades and white that grows between 24 to 30 inches tall. ‘Apricot Lemonade’ is all the rage now with its shifting color. Petals start out soft apricot with a pale lavender base and reverse, then they become pale yellow at maturity. If grown in the garden, this variety has an average height of 27 inches. 

Cosmos sulphureus

This species can range from 8 inches to 7 feet in height. Their center disks are often fluffier than C. bipinnatus. There are a few dwarf varieties, such as ‘Mandarin’, topping out at 8 inches with bright orange petals. ‘Bright Lights’ mix is an early flowering cosmos that can grow up to 3 feet tall with a range of bright colors. ‘Limara Lemon’ is another dwarf variety at 8 inches with lemon-yellow flowers and a superb floriferous habit.  

Cosmos astrosanguineus

This species is also known as Chocolate Cosmos. The flower is a dark purple-brown with a unique chocolate fragrance. This species is actually a perennial that forms a fleshy tuberous root that can be treated much like dahlias, where the tuber is lifted and replanted after the last expected frost in Spring. This species is very rare. Seeds and plants are rather expensive, but it is exquisite nonetheless. ‘Chochamocha’ is a recommended variety. 

Pest Prevention 

There are few problems with cosmos, but some pests and diseases can be noted. Aphids can quickly infest a cosmos plant and spread disease. To get rid of aphids, douse the plant with a forceful stream of water or release beneficial insects into your garden that feed on aphids, like ladybugs. It is probably not worth it to spray an insecticide for aphids on your cosmos. 

Plant Diseases

Aster yellows is a disease carried by leafhoppers that can cause a plant to become yellow, stunted, and malformed. Once infected, the plant will never recover, so it should be destroyed by burning or burying. Do not compost the infected plant. Leafhoppers transfer the pathogen by feeding on infected plants.

Powdery mildew can also be a problem in cosmos plantings during warm, humid conditions, particularly in late summer in most areas. Planting cosmos farther apart or in an area that is sunny and with excellent air circulation will help reduce the incidence of this disease.  

Plant Uses 

Tall cosmos varieties are best used in the backgrounds of landscape beds. They can also be mixed into borders because of their light and airy feel. Dwarf varieties are excellent in pots or along borders with other small flowers, like pansies or shorter zinnia varieties. 

Cosmos are beautiful fillers in a fresh-cut flower bouquet. However, their short-lived vase-life requires you to harvest them just when the bud has cracked open. When harvested correctly, you can enjoy these cheery, daisy-like flowers for at least a week in a vase.  

Cosmos are also excellent for attracting pollinators, so planting with certain vegetable crops that require insect pollination can be beneficial. Be aware that cosmos will also attract aphids. This could be good because it can be used as a trap crop to pull aphids away from certain crops, but you may not want to grow cosmos near any plant that is heavily susceptible to aphid infestation and damage, like roses. 

Cosmos bipinnatus extracts from flowers have been used medicinally to treat jaundice, intermittent fever, and abnormal enlargement of the spleen in traditional herbal remedies. Some studies indicate that the flower extract has significant antioxidant effects. 

Frequently Asked Questions 

Final Thoughts 

If you’re looking for a cheerful, airy, and easy-to-grow accent in your garden, then look no further! Cosmos, with its quirky foliage and colorful, daisy-like blooms, looks like it stepped right out of a Dr. Seuss book. Seeds germinate quickly and once the plant begins to flower it will be pumping out blooms with rapidity. These plants will bloom well into the Summer and Fall for your enjoyment!

Cosmos readily self-seeds for next year, so you’ll be enjoying these bright blooms for many years to come if you’d like! This is an excellent flower to grow for the beginning gardener or for kids, but even an experienced gardener will find it hard not to smile when they see this charming native of Mexico.