How to Grow and Care for Chocolate Cosmos

The Chocolate Cosmos, known botanically as Cosmos atrosanguineus, stands out in the garden for its unique sensory appeal. It produces velvety, deep maroon flowers that can appear almost black in certain light. These blossoms are known for a distinct fragrance reminiscent of chocolate, which becomes more noticeable on warm, sunny days. This plant is a tender perennial that grows from a tuberous root system, distinguishing it from the common annual cosmos varieties grown from seed.

Ideal Growing Conditions

For prolific blooms and the strongest fragrance, chocolate cosmos require a location that receives at least six to eight hours of direct sunlight per day. A spot sheltered from strong winds is also beneficial, as the stems can grow up to three feet tall and might be susceptible to damage. Insufficient light will result in leggy growth and a significant reduction in flowering.

The plant thrives in moderately fertile and, most importantly, well-draining soil to prevent the tubers from rotting. For gardens with heavy clay, amending the soil with organic compost improves its structure and drainage. Sandy soils also benefit from compost to help retain moisture and provide nutrients. A slightly acidic to neutral soil pH is preferred for optimal nutrient uptake.

This plant prefers consistent moisture during its growing season but is sensitive to overwatering. Water deeply when the top inch or two of the soil feels dry to the touch. This approach ensures the tubers have access to water without sitting in saturated soil. Once established, chocolate cosmos have a fair degree of drought tolerance, but regular irrigation produces a healthier plant with more abundant flowers.

Native to Mexico, this species flourishes in moderate temperatures and is considered hardy in USDA zones 7 through 10. For gardeners in colder regions, it is treated as an annual or the tubers must be lifted and stored for the winter. The plant does not respond well to frost, and temperatures below 50 degrees Fahrenheit can hinder its growth.

Chocolate cosmos are not heavy feeders, and excessive fertilizer can be counterproductive. To support blooming, a balanced liquid fertilizer low in nitrogen can be applied once or twice during the growing season.

Seasonal Maintenance

Throughout the flowering season from summer through fall, regular deadheading is beneficial. This practice involves snipping off spent blooms at the base of their stems. By preventing the plant from setting seed, its energy is redirected into producing new buds, which encourages a continuous display of flowers and prolongs the blooming period.

Early in the growing season, pinching back the plant can promote a fuller, more robust shape. When the plant has developed a few sets of leaves, pinch off the top pair. This action stimulates the plant to develop side branches from the nodes below the pinch point. The result is a bushier plant with more stems, which leads to more flowers and a sturdier structure.

Overwintering Chocolate Cosmos

In climates colder than USDA zone 7, you must overwinter the tubers to preserve the plant. Begin in the fall, after the first light frost causes the foliage to die back but before the ground freezes. This timing ensures the tuber has stored enough energy for dormancy. Cut the stems down to a few inches above the soil line to make handling easier.

With the foliage removed, carefully lift the tubers from the ground using a garden fork or spade. Dig a wide perimeter around the plant to avoid accidentally piercing or damaging the delicate tubers. Once excavated, gently shake off any loose soil, being careful not to break them. Avoid washing the tubers, as this can introduce moisture that leads to rot during storage.

After lifting, the tubers need a few days to cure in a warm, dry, and well-ventilated location away from direct sunlight. This allows any cuts on the surface to dry and harden, which helps prevent disease. After curing, place the tubers in a cardboard box or crate filled with a dry medium like peat moss, sand, or sawdust. This material insulates the tubers and prevents them from drying out completely. Store the container in a cool, dark, and dry place, such as a frost-free garage or basement, until spring.

Common Issues and Solutions

Aphids are a common pest that may appear on chocolate cosmos, often clustering on new growth or buds. These small insects can be managed by spraying them off with a strong jet of water from a hose. For more persistent populations, an application of insecticidal soap is an effective control method.

In high humidity or where plants are spaced too closely, powdery mildew can develop. This fungal disease appears as a white, powdery coating on the leaves and can weaken the plant. To prevent it, ensure proper spacing for ample airflow and water at the base of the plant to keep the foliage dry. A sunny location also helps deter this disease.

If a healthy-looking plant produces few or no blooms, the cause is often its environment or feeding regimen. The most frequent reasons are insufficient sunlight or an excess of nitrogen from fertilizer. Lush green foliage with a lack of flowers is a strong indicator of too much nitrogen. Moving the plant to a sunnier spot or reducing fertilizer applications can resolve the issue and encourage blooming.