Cosmos Growing Guide
Cosmos (Cosmos spp.) is an annual wildflower native to Mexico and Southern United States. They should be planted in full sun and will self-seed and come back every year in your garden. In the extreme heat in the southern states it will tolerate some part sun. Remove or transplant any that come up where you do not want them. They are easy to grow and will bloom through the fall season.
The most common types are C. bipinnatus which is the tallest and most colorful with red, maroon, pink or orange blooms, while C. sulphureus is shorter and has yellow blooms.
Cosmos are deer resistant and attract butterflies and other pollinators.
Starting from Seed
Cosmos can be started indoors 4 to 5-weeks before your last expected frost date. For more guidance our seed starting page will be helpful.
In the fall you can direct sow the seeds in your flower garden after the first frost and the soil has cooled off. A good time is when you are planting your fall bulbs such as daffodils and tulips. A large mass planting makes an impressive display.
Soil Preparation
Plant in full sun in a nice loose well-drained garden soil. They will do great in an area where the soil is poor and drains well since wet conditions may lead to disease.
When to Plant
They should be planted after the last expected frost in the spring. You can also sow the seeds in the fall after frost occurs and the soil cools off, but before the ground freezes.
Planting Cosmos
Follow the seed packet instructions. In general, thinly sow the seeds ¼-inch deep directly in the garden after the last frost. Seedlings emerge in 7 to 10-days.
Common varieties are C. sulphureus which can be spaced 10 to 12-inches apart, while the taller C. bipinnatus can be spaced 12 to 24-inches apart.
Watering and Care
They are low maintenance and easy to care for. All you need to do is add a layer of mulch to help retain moisture and keep out competing weeds. They will tolerate drought and dry soil conditions. During very dry spells give some water to keep the soil moist.
They usually do not need any fertilizer, but a light side dressing of organic fertilizer can be placed around the plant in the spring. Fertilizer that is high in nitrogen will encourage foliage rather than blooms. Pick a balanced fertilizer where the 3-numbers are the same or the first number is lower than the others.
More information can be found on the fertilizer page.
Diseases and Pests
Fusarium wilt and powdery mildew are two common fungal diseases often caused by damp or wet conditions. Increase air circulation by thinning plants.
Organic controls can be used by applying Bonide Copper Fungicide or Bonide Neem Oil. Read and follow the label directions. For serious problems your local cooperative extension service can be of help.
Popular Varieties
C. sulphureus and C. bipinnatus
Sources: American Meadows, Burpee, Eden Brothers.
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