Fertilization Do not fertilize marigolds for 7 to 10 days after transplanting or until the roots reach the sides and bottom of the container. Thereafter, fertilize on a constant liquid fertilization basis at 100 to 150 ppm nitrogen, using a fertilizer about equal in nitrogen and potassium content. Growers should test medium pH and soluble salts in-house on a weekly basis and send samples for laboratory testing every 2 weeks. Recommended foliar analysis ranges for marigolds are given in Table 3. Many marigold cultivars are facultative short-day plants, meaning that plants flower more quickly under short day lengths but will eventually flower regardless of photoperiod.
African cultivars display more of a photoperiodic response than French or triploid types do.
The critical photoperiod for Tagetes erecta is between 12.5 and 13 hours, meaning that plants flower more quickly at day lengths shorter than this. Cultivars of the African type can achieve faster flowering if provided with artificial short days. Start applying black cloth in the first 2 to 3 weeks after seed germination, beginning in late February. Using the black cloth will reduce the flowering time by up to 2 weeks and will produce more compact plants. Not all cultivars require black cloth, however.