Cosmos (Cosmos bipinnatus) are annual plants with colorful, daisy-like flowers that sit atop long, slender stems. The leaves are deeply lobed and feather looking depending on the type. Cosmos have varying heights and are available in many colors.
Growing cosmos is simple and low maintenance. They need full sun to flower productively with single or double blooms appearing on stems reaching 1 to 4 feet (0.5-1 m.) Cosmos do not need any special soil preparation. For best flowering avoid soil that is too rich. Rich soil will encourage foliage at the expense of blooms. They tolerate warm, dry weather and are drought-tolerant. As with all flowers, do not over water.
Cosmos may self-sow but these seeds will germinate very late in the season and may only produce very short-lived plants, making the bed appear weedy. Some seeds may overwinter in the ground germinating the following year, but they are unlikely to be true to the parent plant. Remember to deadhead (remove) flowers before they go to seed or to thin seedlings in the spring. Deadheading the plants will also speed up flower development, prolong flowering and aids branching. Cosmos will flower from July until October or the first frost.
Cosmos flowers can be used as cut flowers for the indoor display and backgrounds for other plants. Cosmos can even be used as screens to hide unsightly elements in the landscape. You can cut the flowers off anytime after blooming, but best to pick some right when the petals have opened. They will last more than a week in water. Strip the lower leaves and put in water. Remember not to pick in the heat of the day.
Cosmos seedlings and very young plants are very attractive to slugs and snails. You can protect with slug barriers. Pests like aphids, flea beetles, and thrips do enjoy cosmos but are easy to control with a strong spray or insecticidal soap. Avoid powdery mildew by ensuring the plants are well spaced and not too crowded. Do not over water. Cosmos can also suffer from fusarium wilt, which is a fungal disease. Fusarium will cause discoloration of the stems and foliage, as well as stunt growth. In case of fusarium, remove the plant affected. Soil may also be affected so you can replace the soil or avoid planting in that area for a few years.
Cosmos planted in the southern U.S. tend to be invasive and in that area you should consider keeping them as potted plants.
Cosmos plants may be featured at the back of a descending garden or in the middle of an island garden.
Taller varieties may need staking if not planted in an area protected from the wind. Growing cosmos by a fence will help with the support. There are several ways to stake the plants:
- Stake plants by growing them by a fence for support.
- Stake with a hazel or bamboo cane when it reaches 2 ft and using flex-tie or string to tie it in about 1 ft off the ground.
- Use pea or jute netting. Stretch the net horizontally over the young cosmos plants and between bamboo or hazel canes to support it. The cosmos will grow up and through the grid and are lightly supported. With taller cosmos, it is a good idea to add a second layer of netting. The first layer at about 1ft and the second at 2ft with the ultimate height of 3-4ft.