CARE
In order to prolong flowering, you should deadhead the plants (remove the dead/faded flowers). This speeds up flower development and aids branching.
Because some of these plants can grow really tall, staking may be necessary. Offer them protection from strong winds, encourage balanced branching by pinching out central shoots or stem tips, or stake them. Growing them against a fence can also provide the necessary support.
Water regularly, but make sure you don t over-water the plants. Over-watering and over-fertilization can lead to plants with fewer flowers. Cosmos can tolerate dry soil, even in a hot, arid, sunbaked spot.
Cosmos beds may become weedy due to the fact that they self-seed, so remember to check them.