Cosmos Plant Profile

Pink cosmos flowers
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Cosmos are freely flowering annual plants that are ridiculously easy to grow. If you're looking for a flower that will stay in bloom for months and can be grown by simply scattering seeds, cosmos are a great choice. The flowers sit atop long slender stems and form a cloud of color that not only looks attractive throughout the summer but also attract bees, butterflies, and birds to your garden.

Cosmos grow as easily in beds, and they make great cut flowers. The plants can handle drought, poor soil conditions, and general neglect. They even self-sow, but not to the point of becoming a nuisance. This is a truly low maintenance plant.

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 rough-textured flowers such as coneflowers and black-eyed Susans. Shorter varieties make very colorful, airy edging plants.

Cosmos flowers are daisy-like, with ray florets surrounding the center disc of florets in a shallow cup. There is a broad range of color and more every year. The leaves grow opposite and are either deeply lobed, pinnate, or bipinnate and feathery, depending on the variety. Since few pests bother cosmos, the plants look good all season.

Two Species of Cosmos

There are two types of cosmos typically grown in gardens. The Cosmos sulphureus is a North American native 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, 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 just about anywhere.

Botanical Name Cosmos sulphureusCosmos bipinnatus 
Common Name Cosmos, Mexican aster, and cut leaf cosmos
Plant Type Annual flower
Mature Size 1 to 6 feet high, 1- to 3-foot spread
Sun Exposure Full sun
Soil Type Well-draining soil
Soil pH 6.0 to 6.8; slightly acidic to neutral
Bloom Time Summer through fall
Flower Color Golden yellow, white, pink, magenta, orange, yellow, red, chocolate
Hardiness Zones 2 to 11
Native Area Mexico and the Southern United States

How to Grow Cosmos

For the best flowering, choose a site that gets full sun. Cosmos will grow in partial shade but will have fewer blooms and are less vigorous when planted in shady areas. They will grow in poor soil where many other flowering plants languish. Some cosmos varieties can flop over, so if you are not growing them closely or near other plants that can support them, you may need some type of staking.

There are virtually no pests or diseases that commonly affect cosmos.

Light

Cosmos require full sun and will thrive even under uninterrupted full sun in the hottest conditions. These plants are native to the arid regions of Mexico and the southern U.S., so they will thrive in conditions that mimic those found in these regions.

Soil

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.

Water

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.

Fertilizer

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. Soil that is too nutrient-rich can produce plants that are weak-stemmed, with sparse, late blooms.

Pruning

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.

Propagating Cosmos

These plants readily self-seed themselves, and it is also very easy to collect the dried seed at the end of the season to save for next year. Remember that seeds from hybrid varieties do not "come true" and may produce plants that that revert to the appearance of the parent species.

Growing From Seeds

Although nursery seedlings are available, cosmos are so easy to grow from seeds that it makes little sense no 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.

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.

Varieties of Cosmos

  • Bright Lights Mix: a blend of exuberant yellows, oranges, and reds
  • Cosmic Orange: a brilliant, clear 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 colors or individually, 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.