Cosmos are tall flowers that come in a showcase of colors, from deep purple to pale lilac to bright yellow. Their height makes them popular with homeowners and gardeners as a backdrop for flower beds or to line fences and walkways. There are several different varieties of cosmos, with some being perennials and some annuals. Cosmos are no-fuss plants that require very little maintenance. Taking too much care with them can be just as detrimental to their growth as taking too little. To enjoy cosmo blooms for as long as possible during the growing season, you need to find the happy medium.
Water lightly. Once the seedlings have begun to grow, a report published by Texas A&M University recommends watering your cosmos only if the foliage begins to show signs of wilting.
Fertilize cosmos when they begin to first emerge as seedlings with a 5-10-10 liquid fertilizer. After initial fertilization, cosmos do not require any. In fact, Washington State University Extension reports that using too much fertilizer can cause the cosmo plant to produce foliage at the expense of blooms. If you have cosmos in a container, fertilize them as seedlings with the same 5-10-10 fertilizer but repeat once a month. Refer to the package on your particular brand of fertilizer for the proper amount to use.
Stake your cosmos. Cosmos can grow to a height of 4 feet, and if they are not staked they can suffer damage from high winds and bad weather. Choose wooden or bamboo plant stakes, and put them into the ground approximately 6 to 10 inches away from the plants. Be careful to not inflict root damage when placing the stakes. Tie a coated wire to the stake and then tie the other end loosely around the cosmos. Bring the wire back to the stake in a figure-8 pattern and secure it. You can also use round tomato cages as a staking mechanism. Simply put the cages in place when the cosmos are young seedlings, and leave them be. They will grow up into the cages.
Trim your cosmos after they have bloomed to keep them flowering. Cut stems that have bloomed a third of the way down. This will encourage a second bloom within the same season. After the second bloom, you can let the flowers wilt on the stems and allow the flower seed pods to remain. When the seed pods are dry, they can be harvested for seeds to plant next spring, or you can let nature take its course and let the cosmos reseed themselves. Even the annual cosmo varieties will sometimes spread seeds that get buried deep enough to bloom the following spring.