Learn About Cosmos

How to Sow

Cosmos seed should be sown directly in the garden after frost.

  • Direct sow seeds in average soil in full sun after danger of heavy frost.
  • Select a location in full to part shade with good rich well-drained organic soil protected from wind.
  • Prepare the bed by turning the soil under to a depth of 8 inches. Level with a rake to remove clumps of grass and stones.
  • Most plants respond well to soils amended with organic matter. Compost is a wonderful form of organic matter with a good balance of nutrients and an ideal pH level, it can be added to your planting area at any time. If compost is not available, top dress the soil after planting with 1-2 inches of organic mulch, which will begin to breakdown into compost. After the growing season, a soil test will indicate what soil amendments are needed for the following season.
  • Sow seeds 4 inches apart and ¼ inch deep
  • Firm soil lightly, water and keep evenly moist.
  • Seedlings will emerge in 7-10 days.
  • Thin plants to stand 8-10 inches when seedlings are 1 inch high.

How to Grow

  • Keep weeds under control during the growing season. Weeds compete with plants for water, space and nutrients, so control them by either cultivating often or use a mulch to prevent their seeds from germinating. 
  • Mulches also help retain soil moisture and maintain even soil temperatures. For annuals an organic mulch of shredded leaves lends a natural look to the bed and will improve the soil as it breaks down in time. Always keep mulches off a plant’s stems to prevent possible rot.
  • Keep soil evenly moist but not wet.
  • After plants are about 6 inches tall, a light fertilizer may be applied. Keep granular fertilizers away from the plant crown and foliage to avoid burn injury. Use low rates of a slow release fertilizer such as Flower-tone, as higher rates may encourage root rots.
  • Tall cultivars may need staking to prevent their thick, hollow stems from breaking due to heavy rain or wind.
  • Pinch off spent flowers to encourage continuous bloom. Pinching stem tips can reduce height and encourage branching but isn’t necessary. Deadheading is recommended since it lengthens the bloom season. Cosmos plants that aren’t deadheaded will self-sow in warm regions.
  • Monitor for pests and diseases. Check with your local Cooperative Extension Service for pest controls recommended for your area.
  • Remove plants after they are killed by heavy frost in fall to avoid disease issues the following year.

Growing tips

  • Cosmos plants can withstand heat and drought conditions.
  • Tall cosmos add a soft, airy touch to the back of an annual or mixed border. You can also use them as a fast-growing annual hedge or in a cutting garden. Since the taller cultivars are not always uniform in height, they’re perfect for adding an informal look to cottage gardens. Shorter cultivars of both species are ideal near the front of the border, in containers, or as edgings.
  • Cosmos makes beautiful cut flowers.
  • Because cosmos is easy to grow and the seeds are large and easy to handle, it is a good choice for a children’s garden.

Common Pests and Problems

Common Disease Problems

Aster Yellows: Plants are stunted, develop witch's brooms (excessive growth), petals turn green and become deformed. This virus-like condition is spread by leafhoppers. Burpee Recommends: Remove infected plants and control leafhoppers. Remove weeds in the area which serve as alternate hosts to the disease.

Bacterial Wilt: This causes yellow streaking on the foliage. It is soil borne and spread by flea beetles. Burpee Recommends: Rotate crops and control flea beetles. Remove and destroy infected plants.

Curlytop: This is a virus disease that is characterized by yellowing, stunting and eventual death of plants. It is spread by leafhoppers. Burpee Recommends: Control the leafhoppers which spread the disease and remove and destroy infected plants.

Damping Off: This is one of the most common problems when starting plants from seed. The seedling emerges and appears healthy; then it suddenly wilts and dies for no obvious reason. Damping off is caused by a fungus that is active when there is abundant moisture and soils and air temperatures are above 68 degrees F. Typically, this indicates that the soil is too wet or contains high amounts of nitrogen fertilizer. Burpee Recommends: Keep seedlings moist but do not overwater; avoid over-fertilizing your seedlings; thin out seedlings to avoid overcrowding; make sure the plants are getting good air circulation; if you plant in containers, thoroughly wash them in soapy water and rinse in a ten per cent bleach solution after use.

Powdery Mildew: This fungus disease occurs on the top of the leaves in humid weather conditions. The leaves appear to have a whitish or greyish surface and may curl. Burpee Recommends: Avoid powdery mildew by providing good air circulation for the plants by good spacing and pruning. Contact your Cooperative Extension Service for fungicide recommendations.

Common Pest and Cultural Problems

Aphids: Greenish, red, black or peach colored sucking insects can spread disease as they feed on the undersides of leaves. They leave a sticky residue on foliage that attracts ants. Burpee Recommends: Introduce or attract natural predators into your garden such as lady beetles and wasps which feed on aphids. You can also wash them off with a strong spray, or use an insecticidal soap.

Flea Beetles: These small hopping beetles feed on plant foliage and may spread diseases. Burpee Recommends: Rotate crops. Use floating row covers to prevent damage to young foliage. Remove and destroy plants.

Leafhoppers: Leafhoppers cause injury to leaves and stunt growth. They also spread disease. Burpee Recommends: Remove plant debris. Use insecticidal soaps. Consult your Cooperative Extension Service for other insecticide recommendations.

Lygus Bugs (Tarnished Plant Bug): Lygus bugs are ¼ inch long and are green or brown with yellow markings. Nymphs are flightless and smaller than the adults. They suck on stem tips and flower buds and inject a toxic that deforms roots, stems and ruins flowers.  Burpee Recommends: Because lygus bugs over winter in garden debris, remove all debris after the first frost. Contact your Cooperative Extension Service for insecticide recommendations.

Thrips: Thrips are tiny needle-thin insects that are black or straw colored. They suck the juices of plants and attack flower petals, leaves and stems. The plant will have a stippling, discolored flecking or silvering of the leaf surface. Thrips can spread many diseases from plant to plant. Burpee Recommends: Many thrips may be repelled by sheets of aluminum foil spread between rows of plants. Remove weeds from the bed and remove debris from the bed after frost. Check with your Cooperative Extension Service for pest controls.

FAQ

Why hasn’t my cosmos bloomed? Be careful not to over fertilize as too much fertilizer will give you beautiful plants but no flowers. Many varieties are day length sensitive and will bloom in late summer when the days are shorter.

Can I start cosmos inside? Cosmos is very easy to start outside and does not need to be started inside, however you can start them inside 6-8 weeks before the last frost.

Can I use cosmos as a cut flower? Yes the best time to pick them is right after the flower opens.  Cosmos foliage can also be used as greens in an arrangement.

Does cosmos attract beneficial insects to my garden? Yes it attracts pollinators such as bees and butterflies, as well as other beneficial insects.

Where should I put cosmos in my flower bed? Since some cosmos varieties are so tall, place them in the back of the flower bed. It can also be used as a temporary screen to cover up unsightly elements. Shorter varieties work well in the mixed border or in containers.

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