Tall and beautiful, cosmos flowers were named for their ability to remind the observer of the glory of the cosmos. Both the annual and perennial versions grow quickly and flower profusely in hot and dry conditions. This makes the cosmos one of the easiest flowers to grow in your garden, however you must be careful about the mistakes you may make.
Burying The Seeds
Annual varieties of the cosmos flower grow best when fresh seeds are planted every year. Some seeds from previous years may germinate, but providing fresh seeds ensures a good fill for your garden. When spreading the seeds, it is important not to bury them too deep in your soil bed. Rake the area lightly to ensure the seeds settle into the soil rather than sprinkling with any covering soil or mulch.
Cosmos grow best in poor to moderate soil. They developed in the deserts of Mexico and the American southwest and thrive in seemingly infertile regions. If you put cosmos in especially fertile soil or over feed your cosmos, then they will grow more greenery than anything else, becoming tall and spindly with few of the flowers that make the cosmos so appealing.
This doesn't mean you should not provide any fertilizer. Fertilize your cosmos with a generic, all purpose fertilizer once or twice a year.
As said above, these flowers developed in the deserts of Mexico and the American Southwest. These are dry areas, so the cosmos developed to survive in conditions with minimum water. The best way to water these flowers it to watch them. If the tips of the greenery begin to wilt, then give them some water. Otherwise, don't.
In addition, the soil you plant cosmos in should drain well. If you have soils heavy in clay, try adding sand or planting cosmos in a raised flower bed to improve drainage.
Too Little Sun
Cosmos grow best in full sun or with a little bit of partial shade. Don't plant these flowers in a shady garden location. Give them a spot with lots of sun, 8 to 10 hours at a minimum, unless you are willing to accept fewer flowers and spindly growth.
Too Much Wind
Most varieties of cosmos grow tall; some up to 7 feet. Many gardeners will suggest staking the taller versions, especially in windy areas as the wind will knock these flowers over. So if you live in a windy area, these flowers may not be the best option for your garden.
Too Little Pruning
Cosmos blossom best when trimmed. These flowers look great in vases or pressed and dried. You can leave the flowers on the plant, but then the plant will go to seed and produce fewer blossoms. To encourage your cosmos to keep flowering all season, pick the flowers in their prime or at least deadhead your plants.
Also, if you planted in the spring, you can cut your cosmos back to 12 to 18 inches after it stops blossoming. This will trigger growth all over again and the plants will be back blooming within a month.