Cosmos are charming flowers native to Mexico and other regions of Central and South America. These flowers add long-lasting color to the garden, have a range of symbolic meanings, and work brilliantly as cut flowers. As annuals, cosmos flowers appear relatively quickly and can grow up to 5 feet tall within a couple of months. In this article, we’ll run through everything you need to know about how long cosmos take to flower and discuss their growth expectations once fully mature.
How Long Do Cosmos Take to Flower? – The Essentials
Cosmos grow quickly and can flower within 7 to 12 weeks of sowing, depending on the variety. Within two months, cosmos flowers can reach their mature size. Most cosmos varieties grow between 1 and 5 feet high and up to 3 feet wide. Flowering can be delayed by soil that is too fertile or too much fertilizer.
|Scientific Name:||Cosmos spp.|
|Height & Spread:||text|
|Exposure:||6 to 8 hours of full sun or partial shade in hot, arid regions|
|Growing Zones:||Most Cosmos grow as annuals in USDA Zones 2 to 11. Chocolate cosmos is a tuberous perennial in Zones 7 to 11.|
|Flowering Season:||Most types of cosmos flower from early summer until the frosts arrive in the fall.|
|Colors:||Pink, purple, orange, red, reddish-brown, yellow, white, multi-colored|
The vast majority of cosmos flowers are grown from seed, making them easy to start off indoors. However, chocolate cosmos (Cosmos atrosanguineus) is a tuberous perennial similar to a dahlia. Chocolate cosmos are sensitive to cold, so take care when planting them.
When to Plant Cosmos
Most cosmos varieties are annuals and will only last for a year. As such, they develop quickly once seeds have been sown. Start cosmos seeds off indoors in early spring, approximately in March or April. Plant out young cosmos plants once the last frost has passed.
You can also sow cosmos seeds directly where you want them to grow. For this to work, the soil needs to be reasonably warm, so late spring is the best time. Once the seedlings emerge, thin them out to provide about 3 inches of space between each plant.
Chocolate cosmos tubers should be planted in late spring or early summer once all risk of frost has passed. May or June is usually the best time. In colder areas, chocolate cosmos tubers will need to be dug out of the ground and stored indoors over winter.
Where to Plant Cosmos
Cosmos need to be grown in warm, sheltered areas with full sun. Cosmos require 6 to 8 hours of full sun each day, so east or south-facing areas are ideal. Cosmos also need loose, well-draining soils that don’t contain too many nutrients, which can impair flower production.
How to Plant Cosmos
Start cosmos seeds off indoors in small pots in March or April and pot on as they grow. Once the risk of frost has passed in May, and the seedlings have several pairs of leaves, plant them out. Provide about 3 inches of space between each plant.
For chocolate cosmos, plant the tubers fairly deep and cover them with about 2 or 3 inches of soil. Choose a warm spot with good drainage and moist soil. Chocolate cosmos need slightly more fertile soils than annual cosmos varieties.
Plant cosmos in loose, well-draining soils that haven’t been amended with extra nutrients. Chalky or sandy soils are ideal because they provide good drainage and don’t contain too many nutrients. Soil that is too rich encourages cosmos plants to produce foliage at the cost of flowers.
Growth Expectations and Timings
Annual cosmos seeds develop quickly, with seedlings emerging in approximately 1 to 2 weeks. Annual cosmos can start flowering within 7 to 12 weeks of being sown. Most cosmos varieties will have reached their maximum size within two months of sowing.
Cosmos can grow between 1 and 5 feet tall, depending on the exact cultivar. Cosmos can also spread between 1 and 3 feet wide in bushy clumps. “Pinching” the buds when the cosmos is approximately 8 inches tall can help produce more flowers.
Chocolate cosmos require more patience as they are perennials, which tend to develop more slowly than annuals. Even if tubers remain in the ground over winter, chocolate cosmos can take a couple of years to establish. Chocolate cosmos plants can take years to reach their full size.
Chocolate cosmos are more compact than most annual cosmos varieties. Chocolate cosmos can reach up to 30 inches tall and spread approximately 18 inches wide. Resist the urge to check the tuber, as disturbance could affect growth. If there are no apparent signs of growth within 2 or 3 months, the tuber has failed.
Cosmos require plenty of sunlight to grow, so position them somewhere that provides 6 to 8 hours of full sun. Plant cosmos in loose, well-draining soils that lack nutrients, such as chalky or sandy soils. Cosmos are relatively drought-tolerant, so water them about once a week during the spring and summer.
Cosmos flowers will bloom quite happily without the need for fertilizer. These annuals thrive in nutrient-deficient soils, so incorrectly fertilizing them may hamper growth. Cosmos should only need small amounts of fertilizer if they grow in containers.
Cutting back cosmos at the right time can help you get more flowers. Pinch young plants when they’re approximately 8 inches tall to promote branching. Deadhead spent flowers throughout the season to help your cosmos focus on unopened buds.
After the first round of flowers has finished, cut the cosmos back to approximately 12 to 18 inches tall. This stimulates the cosmos to produce a fresh round of flowers.
Chocolate cosmos have slightly different care requirements to annual cosmos. Chocolate cosmos still need 6 to 8 hours of sun per day and should be watered about once per week. Chocolate cosmos prefer slightly more fertile soils, with loam-based soil being ideal.
If you can leave your chocolate cosmos in the ground over winter, you’ll need to cut them back. Once the frosts have arrived and flowering has finished, cut chocolate cosmos back to the ground. Then apply a layer of mulch to preserve heat and moisture. In colder regions, you’ll have to dig up the tubers and store them inside during the winter.
Factors Affecting Cosmos Growth
Even though cosmos flowers are drought-tolerant, they still need the right amount of water to perform well. Maintain a regular watering schedule of approximately once per week during the spring and summer. Take care not to overwater, as this can lead to problems such as root rot or fungal diseases.
It can be tricky to get the soil right for cosmos flowers. These annuals need loose, well-draining soils that don’t contain too many nutrients. In fact, if the soil is too nutritious, your cosmos may focus on growing foliage rather than producing flowers.
Cosmos do best in chalky, sandy, or loamy soils. These provide the ideal amount of drainage without being too fertile. Well-draining soil is vital as this helps prevent problems like root rot caused by waterlogged soil.
Amendments and Fertilizer
Most cosmos flowers shouldn’t require any fertilizer. Too much nutrition, including fertilizers, can force cosmos to produce leaves rather than flowers. Only cosmos flowers that are growing in containers benefit from small doses of fertilizer once a month.
If you need to amend the soil to make it more suitable for cosmos flowers, add in some grit or horticultural sand to improve drainage. This helps prevent the soil from becoming waterlogged due to overwatering.
Diseases and Pests
Cosmos can suffer from a range of diseases and pests, including aphids and botrytis blight. Usually, these problems will severely impact growth, causing your cosmos to become stunted or weak. Maintaining sound care standards and removing leaves and other debris around your cosmos is usually enough to protect your plants.
Use horticultural oils, insecticidal soap, or organic fungicides or pesticides to tackle pests and diseases.
Not only are cosmos flowers beautiful, but they also develop quickly and vigorously. Cosmos seedlings can appear in as little as seven days. These flowers can also begin to bloom within 7 to 12 weeks of sowing. Most cosmos grow between 1 and 5 feet high and 1 to 3 feet wide. Chocolate cosmos grow as a tuberous perennial and develop more slowly than annual cosmos varieties. For more, see our in-depth guide on the uses and benefits of cosmos flowers.
Edd is a budding content writer and gardener living in the United Kingdom. He has a bachelor's degree in Creative and Professional Writing and has written for several gardening publications online. He is passionate about nature and sustainability with a focus on gardening and wildlife.
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