Growing and Caring for Cosmos: A Comprehensive Guide
Care and Maintenance
Cosmos are renowned for their ease of care, requiring minimal attention to thrive. Once established, these resilient plants can withstand drought conditions, poor soil, and general neglect. They even possess the remarkable ability to self-sow, ensuring a continuous display of vibrant blooms season after season.
Cosmos are relatively pest-resistant, but some common culprits include aphids, flea beetles, and thrips. These pests can be easily controlled with a strong spray of water or insecticidal soap. Diseases that may affect cosmos include aster yellows, bacterial wilt, and powdery mildew. Ensuring good airflow and promptly removing infected plants can help prevent these issues.
Types of Cosmos
There exists a diverse range of cosmos species, with over 25 varieties known. However, three species are most commonly cultivated in gardens and landscaping:
- Cosmos sulphureus: Native to Mexico and Central America, this species boasts golden yellow blooms and a drought-tolerant nature. It grows 2 to 6 feet tall and produces double and semi-double flowers.
- Cosmos bipinnatus: Known for its colorful daisy-like flowers in shades of white, pink, red, and orange, this species reaches a height of 1 to 4 feet. It offers a wide variety of hybrid series and is slightly less heat tolerant than C. sulphureus.
- Chocolate cosmos (Cosmos atrosanguineus): A separate species with dark red flowers that emit a chocolatey fragrance. It differs from annual cosmos by being a perennial hardy to USDA zone 7 and requiring more maintenance.
Propagation
Cosmos plants readily self-seed, making them effortless to propagate. The best time to propagate is after the threat of frost has passed. While sowing seeds directly in the garden is the most common method, propagation via stem cutting is also possible:
- Cut a stem with 3 to 5 leaf nodes below the last leaf node.
- Remove the leaves at the last leaf node, leaving the node intact.
- Bury the cut tip of the stem in moist potting soil, ensuring the last leaf node is above the soil line.
- Water generously and keep the soil moist.
- Transplant the rooted cutting to its desired location once new leaf growth appears.
Growing from Seeds
Cosmos seeds can be started indoors 4 to 6 weeks before the last frost or sown directly outdoors once the threat of frost has subsided. These fast-growing plants typically germinate within 7 to 21 days at 75 degrees Fahrenheit, followed by flowering in about 50 to 60 days.
To sow seeds outdoors, loosen the soil to a depth of 8 inches and plant the seeds 1/4 inch deep. Space the seeds 2 feet apart or scatter them and allow the plants to support each other. Thin out the seedlings later and transplant the extras to another location.
Potting and Repotting
When growing cosmos in containers, ensure the container has drainage holes to prevent waterlogging. Plant one cosmos plant per gallon of container volume. Avoid enriching the soil in pots, as it can lead to leggy and droopy growth. Stake tall varieties in containers for support.
Overwintering
Cosmos is an annual that will die after flowering and dropping its seeds. However, if you wish to keep a potted cosmos alive over winter, provide a bright, full-sun growing lamp for at least 7 hours daily and remove any blooms that form.
Blooming
Cosmos plants require full sun to bloom profusely. Even partial shade can restrict flowering. Deadheading spent blooms encourages new blooms and prolongs the flowering season. Pruning between the main stem and a leaf results in faster blooming.
Common Problems
Wilting or Leaf Discoloration
- Fusarium fungal infection: Causes pink masses on roots and plant death.
- Bacterial wilt: Causes stems to wilt at their base and plant death.
Yellowing Leaves and Leaf Drop
- Powdery mildew: Creates a white coating on leaves and causes them to yellow and fall off.
Flowers Distorting or Stunting in Growth
- Aster yellows: A disease spread by leafhoppers that causes leaf yellowing and distorted or stunted flowers.
Troubleshooting
- Why are my cosmos plants wilting? Check for signs of fusarium or bacterial wilt infection.
- How can I get my cosmos to bloom more profusely? Ensure full sun and deadhead spent blooms regularly.
- What is causing the yellowing leaves on my cosmos plants? Examine for signs of powdery mildew.
- How can I prevent diseases in my cosmos plants? Provide good airflow, avoid overwatering, and promptly remove infected plants.
Additional Tips
- Cosmos make excellent cut flowers.
- Taller varieties look stunning in the middle or rear of borders, while shorter varieties create colorful edging.
- Cosmos sulphureus is invasive in the southeastern United States; check with local authorities for restrictions.
