How to Grow and Care for Chocolate Cosmos

Common Name Chocolate cosmos
Botanical Name  Cosmos atrosanguineus 
Family Asteraceae
Plant Type  Perennial 
Mature Size  1-3 ft. tall, 1-2 ft. wide
Sun Exposure  Full
Soil Type  Well-drained 
Soil pH  Acidic 
Bloom Time Summer, fall
Flower Color  Red
Hardiness Zones 9-11 (USDA)
Native Areas  North America

Chocolate Cosmos Care

Here are the main care requirements for growing a chocolate cosmos:

  • Give plenty of direct full sun, at least six to eight hours daily, sheltered from the wind.
  • Plant in fertile, slightly acidic, well-drained soil.
  • Water moderately but deeply once a week and let the soil dry out between waterings.
  • Avoid temperatures below 50 degrees F.
  • Fertilize in the spring with a fertilizer for flowering plants.




Eva Lechner /


Like regular cosmos, chocolate cosmos needs at least six hours of full sun daily to prosper.


Well-drained, fertile soil with a pH between 5.5-6.0 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 to ensure good drainage.


Once established, the plants need only moderate amounts of water. In dry periods, water deeply about once per week. Allow the soil to dry out slightly between waterings. Be mindful of overwatering; too much water causes the roots to rot.

Temperature and Humidity

The plants are frost sensitive, so avoid planting before all danger of frost has passed. Chocolate cosmos are hardy as perennials in Zones 9 and above, but with heavy mulching and winter protection, you may have luck growing them as perennials in Zones 7 and 8.

Chocolate cosmos is tolerant of high humidity.


These flowers are not heavy feeders. Too much fertilizer might cause your chocolate cosmos to produce more foliage than blooms. A springtime application of fertilizer for roses and other flowering plants once in the springtime is usually sufficient.

Types of Chocolate Cosmos

After the rediscovery of the plant by plant breeders, several cultivars of chocolate cosmos have been introduced, including:

  • C. atrosanguineus’ Pinot Noir’: the first cultivar released in New Zealand in 1996. It has small flowers.
  • C. atrosanguineus’ Choca Mocha’ or ‘Chocamocha’: a dwarf cultivar about 12 inches tall. The deep maroon, dahlia-like flowers last through the fall.
  • C. atrosanguineus’ Black Magic’: a tidy, compact cultivar growing 12 to 24 inches tall. The large or small flowers are a deep velvety burgundy almost black color.


To boost flower production and vigorous branching, pinch young plants once they reach a height of 8 inches.

After the first round of flowering finishes, cut the plants down to 12 to 18 inches. This encourages new growth and potentially more flowers.

If in USDA zones 9 to 11, cut the chocolate cosmos back to the base in late fall after the foliage has yellowed.

Propagating Chocolate Cosmos

Dividing an existing chocolate cosmos is an easy way to propagate the plant when the tubers get crowded. The tuberous roots are handled similarly to the roots of dahlias. 

  1. In the fall or spring, dig up the plant with all its roots.
  2. Gently shake it to remove any excess soil. Cut it into sections in a way so that each tuberous root section has at least two “eyes” or new growth buds and some healthy-looking stem tissue.
  3. Dust the cuts with horticultural charcoal powder (optional). It prevents roots from getting waterlogged.
  4. Plant the tubers at the same depth as the original plant, about one inch deep. Water immediately. Mulch the planting area to preserve soil moisture and suppress weeds.

How to Grow Chocolate Cosmos From Seed

Collecting seeds from your chocolate cosmos for propagation is not recommended, as the seeds of the plants are sterile, unlike seeds that you purchase from a seed company, which have undergone special cultivation.

  1. Start the seeds indoors in early March or direct sow them outdoors in early May after the danger of frost has passed.
  2. Sow seeds about 18 to 14 inch deep.
  3. Keep the soil evenly moist. At a temperature between 70 and 75 degrees F, the seeds germinate in 7 to 14 days.
  4. Plant potted seedlings outdoors when the weather has warmed up. Space plants 10 inches apart. If you have started the plants outdoors, thin out any extra seedings as needed.

Potting and Repotting Chocolate Cosmos

Chocolate cosmos grow well in containers. Drainage is vital with chocolate cosmos. Plant the seeds or tubers in well-draining soil and pots with ample drainage holes.

To help the water drain out of the containers, consider using small or medium-sized rocks at the bottom to ensure the water has an escape route. These plants do not like overly wet soil, so porous clay or terra-cotta pots are a plus for their ability to wick away extra water in the soil.


Chocolate cosmos are grown as an annual in colder climates, but the roots may be dug up in autumn and stored for winter the way you store dahlias or cannas, and replanted in spring. Keep the tubers cool and dry.

If growing them in a container, trim the stems to the base once they’ve entered dormancy and move the container to a frost-free basement or garage. Once the threat of frost has passed and temperatures warm to about 50 degrees F, gradually bring the container back outside to warm up and start the process of regrowth.

In warmer climates where they grow as perennials after you have cut the plants back, cover them with mulch to protect the roots against the winter cold.

Common Pests and Plant Diseases

Chocolate cosmos is susceptible to powdery mildew, stem canker, Rhizoctonia stem rot, gray mold, and aphids. Give the plants plenty of space and airflow to prevent powdery mildew.

How to Get Chocolate Cosmos to Bloom

Chocolate cosmos may take a couple of years to establish, but they will produce more flowers each season as they spread.

Bloom Months

Chocolate cosmos typically blooms in summer around June or July.

How Long Does Chocolate Cosmos Bloom?

Chocolate cosmos produces flowers until temperatures start to dip in fall; the plant doesn’t tolerate temperatures below 50 degrees F.

What Do Chocolate Cosmos Flowers Look and Smell Like?

Chocolate cosmos flowers are dark red-brown, sometimes almost black, velvety flowers on the ends of long reddish-brownish stems with green leaflets at the base of the stem. The chocolaty fragrance of chocolate cosmos is due to vanillin, an organic compound also found in cocoa. The scent is most pronounced on warm days.

How to Encourage More Blooms

Once the first round of blooms finishes, cut back the cosmos to 12 to 18 inches to stimulate another flush of flowers.

Deadheading Chocolate Cosmos Flowers

Chocolate cosmos benefit from deadheading to keep the blooms neat and produce new flowers until the first frost.

Common Problems With Chocolate Cosmos

Chocolate cosmos are relatively easy to grow, only susceptible to common garden variety problems.

Not Flowering

Cosmos plants are sun lovers and light sensitive. They need full sun all day and 12 hours of darkness to produce flowers. Ambient light is problematic for them. Other variables to check—do not overwater or overfertilize. Too much fertilizer will yield bushy foliage, pulling energy away from the flower heads.

Yellowing Leaves

The leaves will be your best indicator if the plant is getting too much water; the leaves will start yellowing or getting pale in color. Let the plant dry out between waterings. Do the finger test, going down about an inch or two; if your finger comes out with moist soil on it, let the plant dry out some more before watering.


If the seedlings, flowers, or leaves are wilting, your chocolate cosmos plant is likely underwatered and needs a good drenching with room-temperature water to reinvigorate it.


  • Chocolate cosmos’ dramatic look and fragrance make these flowers highly sought after for garden beds, containers, and floral arrangements. They pair exceptionally well with all shades of pink blooms, creamy white flowers, and blue hues to offset the brown tones. They hold up well as cut flowers too. Plant them near a patio or seating area to enjoy the fragrance more easily.

  • Chocolate cosmos have become rare in their native habitat in Mexico. Another factor that makes them rare is that they produce sterile seeds, so their spread is primarily through dividing underground rooting tubers or from specially cultivated seeds.

  • Despite the appetizing scent of the flowers, the plants are not edible.

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