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 also can cause considerable damage. Aster yellows is occasionally a disease problem. Aster yellows is caused by microorganisms called phytoplasmas. The disease is transmitted from plant to plant by sap-sucking insects such as leafhoppers. Marigolds infected with aster yellows are stunted, yellow-green in color and their flower buds fail to open.