Cosmos can be planted in full sun. However, in very hot regions, they can take afternoon shade. Give them protection from strong winds. Cosmos tolerates a wide range of soil types, including poor soil. Plants need even moisture to get started, but mature cosmos are drought tolerant; plants produce more and larger flowers, however, if they are watered regularly.
Move these heat-loving annuals to the garden after danger of frost has passed and the soil has warmed. Cosmos need light soil with average to poor fertility that has a neutral or slightly alkaline pH and is well-drained. Soil that is too rich yields weak-stemmed, sparsely flowered plants that bloom late and flop over, so avoid soil that has been heavily amended, and don’t feed the plants. Tall cosmos sometimes require staking to prevent their thick, hollow stems from breaking due to heavy rain or wind. Instead of staking, a gardener may also space plants closer than the recommended 2 feet and have the plants support each other. Deadheading is recommended since it lengthens the bloom season.