This highly-sought after flower was first introduced commercially in 1885, in a seed catalog. However, most varieties produce sterile seeds, so these cosmos are propagated via tuberous roots, similar to the roots of dahlias. Native to Mexico and rumored to be extinct in the wild for decades, chocolate cosmos was found in old oak and pine forests in the early 21st century. In recent years, several new cultivars have been introduced. The chocolaty fragrance of chocolate cosmos is due to the presence of vanillin, an organic compound also found in cocoa. The scent is most pronounced on warm days. Plant them in containers near a patio or seating area to enjoy the fragrance more easily.
Light
Like regular cosmos, chocolate cosmos likes plenty of direct sunlight, at least six hours per day.
Soil
Well-drained, fertile soil will allow these beauties to flourish. If growing in containers, a mix of potting soil and topsoil should yield good results, perhaps adding some pebbles in the bottom of the container to ensure good drainage.
Water
Too much water can cause their roots to rot, so be careful not to overwater. A deep watering once per week in summer, unless there is a lot of rain, should be sufficient. Allow the soil to dry out slightly between waterings.
Temperature and Humidity
These flowers can be frost sensitive, so avoid planting before all danger of frost has passed. They can handle humidity and enjoy moist soil, but too much water at their roots can cause rot.
Fertilizer
Too much fertilizer might cause your chocolate cosmos to produce more foliage than blooms, but a bit of rose food applied in spring can help these flowers achieve and maintain their vibrant color through to autumn.
Propagating Chocolate Cosmos
The seeds of chocolate cosmos are sterile, so they won’t reseed or produce plants from seed. The only way to propagate these flowers is via a root cutting or planting. The roots are thick and tuberous; look for root sections that have “eyes” or buds of new growth for the most successful plant starts.
Overwintering
Chocolate cosmos are grown as an annual in colder climates, but the roots may be dug up in autumn and stored for winter as you’d store any other root annual such as a canna or dahlia, and replanted in spring. In warmer climates where they grow as perennials, give them a good coating of natural mulch to protect the roots for winter.
Common Pests and Diseases
The following problems may plague your chocolate cosmos: Powdery mildew, stem canker, Rhizoctonia stem rot, gray mold, and aphids. Give them plenty of space and air flow to prevent powdery mildew.