Cosmos are freely flowering annual plants that are easy to grow simply by sowing seeds in the garden after any danger of frost has passed. They will reach full maturity within two months. The flowers sit atop long slender stems and form a cloud of color that not only looks attractive throughout the summer but also attracts bees, butterflies, and birds to your garden.
Cosmos flowers are daisy-like, with ray florets surrounding the center disc of florets in a shallow cup. There is a broad range of colors and more cultivars developed every year. The leaves grow opposite on the stems and are either deeply lobed, pinnate, or bipinnate and feathery, depending on the species. Since few pests or diseases bother cosmos, the plants look good all season.
Cosmos are quintessential cottage garden flowers and mix well with just about everything. The taller varieties look good in the middle or rear of the border, with spiky flowers such as Agastache and goat's beard, and with flowers such as coneflowers and black-eyed Susans. Shorter varieties make very colorful, airy edging plants.
|Botanical Name||Cosmos sulphureus, Cosmos bipinnatus|
|Common Name||Cosmos, Mexican aster, cut-leaf cosmos|
|Plant Type||Annual flower|
|Mature Size||1–6 feet tall, 1–3 feet wide|
|Sun Exposure||Full sun|
|Soil Type||Well-draining soil|
|Soil pH||6.0–6.8 (slightly acidic)|
|Bloom Time||Summer through fall|
|Flower Color||Golden yellow, white, pink, magenta, orange, yellow, red, chocolate|
|Hardiness Zones||2–11 (USDA)|
|Native Area||Mexico and the Southern United States|
Cosmos grow easily in beds and make great cut flowers. When established, the plants can handle drought, poor soil conditions, and general neglect. They even self-sow. (Be careful, as Cosmos sulphureus is considered invasive in the southeast. Check with your local extension agent if you have any questions.) This is a truly low-maintenance plant.
While some pests, like aphids, flea beetles, and thrips, do enjoy cosmos, they're easy to control with a strong spray of water or insecticidal soap. Aster yellows, bacterial wilt, and powdery mildew may also affect cosmos. Space plants accordingly to ensure good airflow to avoid diseases.
For the best flowering, choose a site that gets full sun. Cosmos will grow in partial shade but will have fewer blooms and be less vigorous when planted in shady areas. They thrive under uninterrupted full sun in the hottest conditions. These plants are native to the arid regions of Mexico and Central and South America, so they will thrive in conditions that mimic the climate of these regions.
Cosmos plants prefer neutral to slightly acidic soil although they will grow in poor soil where many other flowering plants languish. Cosmos perform best in medium moisture, well-drained soils, but they will perform adequately even in dry soils. Avoid soils that are too rich, which can cause the plants to get too tall and flop over. You can prevent this from staking the plants or growing them close enough to other plants that can support them.
Once established, you should not need to water your cosmos plants at all unless there is a prolonged drought. Where water is limited, these are the last plants that require irrigation.
Temperature and Humidity
Hot weather is ideal for cosmos, and they thrive under nearly any humidity level.
Unless your plants seem to be struggling, there is no need for fertilizer. Cosmos can handle poor soil, and fertilizing can actually have a negative impact. Too much fertilizer can create strong plants with lots of foliage—but few blooms.
There are two types of cosmos typically grown in gardens. Cosmos sulphureus is native to Mexico, Central America, and northern South America. With golden yellow blooms, it is very drought tolerant and loves hot weather. The plant grows 2 to 6 feet tall and comes in double and semi-double flowers. Some of the more recent cultivars tend to be shorter, more orangy, and with smaller flowers.
Cosmos bipinnatus are the colorful daisy-like flowers that come in white, pinks, reds, and orange. At 1 to 4 feet in height, they are shorter than C. suphureus, and are available in several popular hybrid series. Although they are not quite as heat tolerant as Cosmos sulphureus, C. bipinnatus will grow well in just about any sunny space.
Some popular varieties of cosmos include:
- 'Bright Lights' Mix: a blend of exuberant yellows, oranges, and reds
- 'Cosmic Orange': a brilliant, semi-double orange flower with great drought tolerance
- Peppermint Candy: an award-winning variety with petals splashed in magenta and white
- Seashells Series: a pretty mix of pastel colors, with distinctive tubular petals
- Chocolate Cosmos (Cosmos atrosanguineus): The dark red flowers do indeed smell like chocolate. This is a perennial that is hardy to USDA Zone 7, but it is much more finicky than the annual cosmos. It grows from tubers, much like dahlias.
The only real maintenance cosmos plants need is deadheading which will prolong the flowering season. If you fall behind, simply shear the plants by about one-third, when most of the flowers have faded. This produces a second flush of leaves and flowers.
At the end of the season, you can cut off the plants at ground level, or pull them up, roots and all. However, leaving the plants in place may allow them to self-seed in the garden.
These plants readily self-seed, and it is also easy to collect the dried seeds at the end of the season to save for next year. Self-seeded volunteers can be carefully dug up and transplanted elsewhere in the garden when they are a few inches tall.
How to Grow Cosmos From Seeds
Although nursery seedlings are available, cosmos are so easy to grow from seeds that it makes little sense to overspend by buying nursery plants. You can start seeds indoors, four to six weeks before the last frost, but cosmos sown outdoors directly in the garden will quickly catch up. Cosmos typically germinate in 7 to 21 days at 75 degrees Fahrenheit, followed by flowering in about 50 to 60 days.
Loosen the soil to a depth of 8 inches, then plant the seeds and cover them with 1/4 inch of fine soil. Seed packets usually recommend precise spacing, such as at 2-foot intervals, but you will get a better display if you simply scatter the seeds and let the plants support each other as they grow. You can always thin if you need to, moving the extra plants to another part of the garden.
Wait until all danger of frost has passed before sowing or transplanting outdoors. Cosmos grow very quickly but can be killed by a late frost, so don't rush it.