Plant spacing varies from 6 to 9 inches for the French marigolds and up to 18 inches for the taller African marigold varieties. Summer care of marigolds is simple. Water occasionally during dry weather and pinch off faded flowers to encourage additional bloom. Tall African marigolds may require staking to prevent the plants from falling over or lodging during storms. While marigolds are seldom bothered by insects and diseases, they are not problem free.
Spider mites can devastate marigolds in hot, dry weather.
Grasshoppers can also cause considerable damage. Aster yellows is an occasionally disease problem. In a related matter, some gardeners plant marigolds in their vegetable gardens to repel harmful insects. While the marigolds are an attractive addition to the garden, research studies have concluded they aren't effective in reducing insect damage on vegetable crops. Iowa State University , Ames, Iowa.