Marigolds thrive in full sun. They require well-drained soil and can suffer root rot when grown in wet soil or when over-irrigated. Marigolds with root rot can exhibit visible symptoms including wilt, lack of or less vigorous growth, diminished bloom, discolored foliage and tissue death. In the soil the roots will break down, becoming discolored and slimy and shedding their protective fleshy coating to expose the thin, wiry root cortex. You can save your marigolds by modifying cultural practices or by transplanting them to location with better drainage.
Water marigolds so that the soil around them remains evenly moist starting 1 inch down, but is not consistently soaking wet. Refrain from watering saturated soil until it dries out in the upper few inches. Monitor soil drainage carefully to ensure that the marigold roots are not suffocating in wet soil.
Water the soil when it appears dry. Drought stress will also hinder the marigold's ability to recover.
Fertilize marigolds with a slow-release complete and balanced fertilizer product with a guaranteed analysis of 10-10-10 or 20-20-20 to help the plant grow more tissues and compensate for the root rot. Apply according to the product label dosing directions and do not overfeed, as the stress of excess fertilizer can exacerbate problems on a compromised plant. Nestle the granules in the top 1 inch of soil and water lightly after application.
Relocate plants growing in a poorly drained site to a site where water drains readily through the soil to below the level of the root mass. Look for a spot where water does not pool on the surface of the soil. Raised beds, slopes, soil mounds and berms, sand-amended soil and containers can all help to achieve better drainage and rescue the plants.
Amend the existing soil to improve drainage and oxygenation if moving the marigolds is not an option. Dig up the plant to remove the root mass from the soil. Lift out some of the infected, poorly draining soil and replace it with a good-quality potting mix or a mixture of coarse sand and perlite or peat moss. Mix the soil amendments into the surrounding soil and replant the marigolds, ensuring good root to soil contact.