Cosmos is a cheerful summer annual that's so easy to grow it almost takes care of itself. In fact, if you leave a few spent flowers on your plant, they will form seeds that drop to the ground when they are ready to grow into more plants. Available in a variety of colours and shapes, the daisy-like cosmos adds colour and joy to any border garden or flowerpot. The amount of pruning you'll need to do to keep this plant tidy and in constant summer bloom is limited to deadheading spent flowers.
Snip off individual flowers after the petals begin to drop. Cut the entire flower stem back to the branch from which it grew. This will encourage the plant to produce more blooms for continuous summer colour.
Cut all branches 1/3 of the way back to the main stem after the plant produces its first large array of flowers. This helps to keep the plant from becoming top heavy, and it also encourages the plant to develop a new flush of flowers soon thereafter.
Snip off individual ferny leaves or entire branches if they turn yellow or brown or look unattractive in any way. Cut all the way back to where the leaf or branch connects to the main plant.
Fertilise your cosmos after you prune it by giving it the recommended dose of a balanced plant food, such as one that has an N-P-K ratio of 10-10-10. This plant also responds well to natural fertilisers such as fish emulsion and worm castings.
Use cut plant material as mulch around your plant unless it appears to be diseased or infested with insect pests. Grow a variety of cosmos plants in different colours in the same area to add colour and interest to your garden.