The garden cosmos flower, Cosmos bipinnatus; AKA Mexican aster

Hailing from the volcanic mountainous region of central Mexico, the common garden cosmos, Cosmos bipinnatus, is one of the most widespread annual flowers in the world, being found on all continents except Antarctica. From its humble origins it has been cultivated and bred for less than a hundred years and yet today the variety of flower forms available is truly staggering.

Cosmos bipinnatus is a weedy annual herb found on disturbed fields and roadsides across the globe. Growing from 0.3 – 2.0 meters or more high, it tends to have a rambling habit, particularly as it grows taller. The complex pinnate leaves are composed of very narrow leaflets that are twice divided (bipinnate) and held in opposing pairs, giving them an airy, feather-like appearance.

The flower heads are borne singly on long stems (peduncles) held well above the plant, are usually 5-7 centimeters across, but can be up to 10 centimeters in selected varieties. Flower heads are made up of two flower types – small centrally clustered disc flowers surrounded by a ring of ray flowers with lobed petals (corollas) up to 5 centimeters long. Disc flowers typically are bright yellow and the corollas of the ray flowers range from pure white thru various shades of pink and purple. The dark brown, elongated seed is produced in large quantity, enough to make this a potential weed in warmer climates.

The natural distribution of C. bipinnatus is somewhat obscure with most sources (including efloras) stating “Mexico and the southwestern U.S.”, though in all likelihood it is originally from central Mexico. Regardless, nowadays the species in both its cultivated and self seeding forms can be found all around the globe in open fields, roadsides, or any other human disturbed ground. North of the Mexican border it has been recorded in 36 states of the U.S. as well as two Canadian provinces, where apparently it is self-seeding, at least on some sites. It has also been recorded throughout all of Japan south of Hokkaido, where it is both self-reproducing and cultivated.

So what is the attraction of this short lived herb? It’s irresistible, variably colored flowers of impeccable symmetry, are the obvious answer. It is said they were given the name “cosmos” by Spanish priests who grew them on mission grounds due to the perfect symmetry of the flowering heads (cosmos comes from the Greek word κόσμος, meaning the order of the universe, or the opposite of chaos). Since those early days of cultivation, breeders have taken this lowly annual herb and created an astounding range of flowers.

Keeping up with all the names of varieties that have been made is a large task. Here is a representative list of some of the more well known ones:

Picotée – white petals with pink to red marginal bands.
Sensation Series – large flowers, color varying from carmine through pure white. Average height, up to 2 meters.
Sea Shells – tubular trumpet-like fluted petals in variable shades of pink.
Sonata Series dwarf mix – Fleuroselect Awarded variety with large flowers on a relatively short plant (up to 60 cm). Like the Sensation Series, flower color is variable. Also given the Award of Merit by the Royal Horticultural Society.
Candy Stripe – similar to Picotée with pink to red margins.
Double Take – large, double to semi-double flowers up to 10 cm.
Gloria – bi-color flowers with a dark pink to red center.
Psyche Mix – Semi-double, frilly flowers from burgundy to white color.
Purity – pure white flowered variety.
Pinkie – as the name suggests, a pink flowered variety.
Radiance – flowers with a pink to red center, similar to Gloria.
Dazzler – deep carmine red flowers with yellow centers up to 10 cm.
Cupcakes – petals fused together, forming a bowl shape, white flowered fading to pink.

Despite being a frost tender plant from decidedly southern origins, C. bipinnatus is rather easy to grow and flower in a wide range of climates as long as some basic rules are followed. Perhaps the most important cultural pointers are to grow them in absolute full sun and resist fertilizing at all. Watering as well should be done only if natural rainfall is not adequate to keep them from wilting. The reason for all this is that if plants are fertilized, over-watered, or given too much shade they will grow tall and spindly, and have far fewer flowers. None of this is very surprising since the habitats it evolved in were probably relatively dry, open sites on fairly poor soils.

Here are some specifics about growing them. In cold weather climates with cool springs, germinate the seed indoors under bright light in temperatures above 18 C (65 F), and transplant outdoors once average temperatures are consistently warm. In warmer climates, directly sow the seed anytime after temperatures are consistently warm. Cosmos prefers hot conditions, so warmer is better, but again, avoid over-watering and apply no fertilizer to the growing beds. Seed typically takes a week or two to sprout, and flowering can commence within 5 weeks after germinating under warm conditions. Seed should only be superficially covered for good germination.

The growing site should receive as much sun as possible, with full fun being optimal. It cannot be overstated that these plants should not be pampered at all if you desire more flowers on shorter plants. This is particularly true if you are growing them in less than full sun conditions. Even varieties that are rated as “dwarf” will grow very tall and flower sparingly if you do not follow these guidelines. That being said, the soil should remain evenly moist throughout their growing cycle.

To promote flowering, deadhead plants, or simply prune them down to encourage new bud formation. Cosmos flowers make great cut flower bouquets. At the end of the flowering season, before the first frosts hit, growers can harvest the seed by taking the entire flowering head with mature seed and put these into a brown paper bag for drying. At a later time, separate the seed and store in a sealed, dry container until sowing time. Under good conditions this plant is self-sowing, meaning it will appear again on its own. Despite this ability, cosmos is not considered an invasive plant in most areas and is easily controlled through mechanical removal. That is a real bonus for an annual plant that produces copious seed – usually such plants are notorious weeds, but not with cosmos.

Besides having wonderful flowers blooming over a long period of time, this plant has other positive attributes, notably attracting beneficial insects to your garden such as lacewings, hover-flies, parasitic wasps, and of course bees. These help keep harmful insects such as aphids in check and improve overall conditions in your garden. A trip to a flowering field is amazing just for the shear number of flying insects alone. Luckily, C. bipinnatus doesn’t suffer from too many ailments, but can be subject to bacteria wilts, leaf spots, and powdery mildew occasionally. If grown under proper conditions these usually aren’t a problem.

Cosmos bipinnatus is an annual flower that is both easy to grow and bloom, and yet not weedy in nature. The garden cosmos has come a long way from its simple beginnings, and deserves a place in any flower lover’s garden.