While some pests, like aphids, flea beetles, and thrips feed on 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. Taller varieties look good in the middle or rear of the border with goat’s beard, coneflowers, and black-eyed Susans. Shorter varieties make very colorful, airy edging plants.
Light
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. These plants will also thrive under uninterrupted full sun in the hottest conditions, much like their native habitat: the arid regions of Mexico and Central America.
Soil
Cosmos plants prefer a neutral soil with a pH of 6.0 to 8.0, although they will grow in poor soil where many flowering plants languish. They perform best in medium moisture, well-drained soils, but they will perform adequately in dry soils. Avoid planting in a rich soil; it can cause the plants to get too tall and flop over. You can prevent drooping by staking the plants or growing them close to other plants that can support them.
Water
Once established, you will not need to water your cosmos plants 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 in any humidity level.
Fertilizer
Fertilizing can negatively impact cosmos. Cosmos can handle poor soil. Too much fertilizer can often lead to strong plants with lots of foliage but few blooms. Unless your plants seem to be struggling, these plants do not need fertilizer.
Types of Cosmos
There are over 25 species of cosmos. However, three species are most commonly used in gardens and landscaping. 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 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 C. sulphureus, C. bipinnatus will grow well in just about any sunny space. Chocolate cosmos are a separate species: Cosmos atrosanguineus. The dark red flowers smell like chocolate. This perennial is hardy to USDA zone 7, but it is higher maintenance than annual cosmos. Like dahlias, it grows from tubers. Other common cosmos cultivars include:
‘Bright Lights’ mix: This variety boasts a blend of exuberant yellows, oranges, and reds.‘Cosmic Orange’: This brilliant, semi-double orange flower has great drought tolerance.‘Peppermint Candy’: An award-winning variety, the petals are splashed in magenta and white.‘Sea Shells’ series: A pretty mix of pastel colors, it has distinctive tubular petals.‘Ladybird’: This cosmos is a shorter variety that blooms in red, yellow, orange, or gold, averaging 18 to 24 inches tall.
Pruning
The only real maintenance cosmos plants need is deadheading which will prolong the flowering season. If you fall behind, shear the plants by about one-third, when most flowers have faded. This kind of pruning produces a second flush of leaves and flowers. By the end of the season, you can cut off the plants at ground level or pull them up, roots and all. However, if you leave the plants in place, they may self-seed for the following growing season.
Propagating Cosmos
Cosmos plants readily self-seed. It’s best to propagate these plants after the threat of frost is gone. Although sowing seeds is the best and easiest way to propagate this plant, you can also propagate via stem cutting. When you take stem trimmings, it stimulates more leaf and flower growth. Besides seed, stem cutting is the best way to propagate this plant. Here’s how you do it:
How to Grow Cosmos From Seeds
Start seeds indoors, four to six weeks before the last frost. Or if you can sow cosmos outdoors directly in the garden well after the threat of frost is gone. Cosmos grow very quickly but can be killed by a late frost, so don’t rush it. They 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. 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, or you can scatter the seeds and let the plants support each other as they grow. You can always thin them out later, moving the extra plants to another part of the garden.
Potting and Repotting Cosmos
When growing cosmos in pots, make sure the container has bottom drainage holes. Cosmos can’t handle overly wet, soggy soil. Plan on growing one cosmos plant per gallon of your container. If growing in pots, do not enrich the soil, it makes the plants grow tall, leggy, and droopy. Also, tall varieties will need staking in containers. At the very least, plan on using at least a heavy, 12-inch diameter container.
Overwintering
Cosmos is an annual. If left outside in frosty temperatures, they will die. However, at the end of the growing season, if you allow the dead flower heads to drop their seeds, cosmos seeds will go dormant and sprout when the soil warms up again in the spring. If you have a potted cosmos in a container and want to keep your cosmos alive over the winter season, you will need a bright full sun growing lamp for at least 7 hours a day. You will need to snip off any blooms as they form. This plant’s life cycle ends with flowering when it drops its seeds for the next growing season.
How to Get Cosmos to Bloom
Cosmos plants need full sun to bloom. Even the hint of shade, can restrict flowering. Also, to encourage more blooms, you need to deadhead the old blooms. For faster blooms, prune between the main stem and a leaf. The lower you cut in the stem, the longer it takes to grow more flowers.
Common Problems With Cosmos
Cosmos are easy to grow and maintain over the growing season. They are usually resistant to disease, and most insects; however, some pests can become a nuisance and affect their growth.
Wilting or Leaf Discoloration
If your plant has ample water and is not wilting from a lack of hydration, there are two possible causes. A plant that is wilting with leaf discoloration might have a common fusarium fungal infection. If you dig up the plant and notice a pink mass on the roots, then the plant likely has fusarium. The whole plant is beyond help, will die, and should be disposed of to stop the fungus spread. If you dig up the roots and they look healthy, the plant may have a bacterial wilt infection. The bacteria cause the stems to wilt at their base. This plant will die and should be disposed of.
Yellowing Leaves and Leaf Drop
Powdery mildew mainly affects plants in the shade. Fungus spores fly through the air and attach to a host plant in a shady spot. It creates a powdery white coating on leaves and causes leaves to yellow and fall off. To prevent powdery mildew, provide your plants good circulation, bright light, and avoid getting water on the leaves. If your plant has fungus, use a horticultural fungicide according to the package instructions.
Flowers Distorting or Stunting in Growth
As a member of the aster family, cosmos can get aster yellows, a disease spread by leafhoppers (a tiny grasshopper-looking insect). The leaves will get yellow mottling on the leaves, and the flowers will appear distorted or stunted. Dispose of these plants since there is nothing you can do help them recover.
