Series Offered: Antigua, Moonsong, Lunacy, Moonstruck, Inca II, Perfection
1. Soil and pH Needs
African Marigolds are tolerant of many different soil types, but like most bedding plants prefer a well-draining soil such as a sandy loam. Heavy clay soils will retard root growth and starve the plant of oxygen, which leads root disease problems. Soil should be amended with fully composted organic matter prior to planting and in the case of clay soils the addition of pea gravel equal to at least 25% of the soil volume will aid in aeration. Organic matter that has not been fully composted will tie up nitrogen and sulfur leading to nutrient deficiencies and poor growth.
African Marigolds are not tolerant of low pH soils. Low pH causes iron and/or manganese toxicity, which exhibits itself as brown to black speckling of the foliage. See photos below. Soil pH should be 6.0-7.4 and should be tested prior to planting but after adding any ammendments. This can be done quite easily with a portable pH tester available for about $100. Mix one part soil with two parts distilled water, stir well and wait thirty minutes, then follow the instructions for using the pH meter. If pH is too low, the addition of lime is warranted. However, the rate will vary depending on how much you need to raise the pH and the type of soil you are dealing with. Clay soils or those with a lot of organic material have a high buffer capacity and require more lime than sandy soils, which have a low buffering capacity. A good soil testing lab can determine the lime requirement index (LRI) of your particular soil and can recommend types and rates of lime to use.
2. Fertility Needs
African Marigolds are average feeders in the landscape and have a crop life of about four months. An application of a balanced slow-release fertilizer such as 10-10-10 or 14-14-14 with a 3-4 month formulation applied to the bed at planting will keep the plants well fed, depending on the amount of irrigation required and the average daily temperature. In areas of high irrigation and high temperatures it may be better to use a formulation with a slower release rate since higher temperatures will cause the fertilizer to be released too soon. Fertilizers that are not slow-release are not recommended since the fertilizer will be leached out of the root zone long before the crop needs to be changed out in most cases.
3. Common Diseases
African Marigolds are not typically subject to many plant diseases. The primary concern is Botrytis or grey mold in the flowers. Make sure the plants are spaced correctly to provide good air circulation, avoid overhead irrigation or at least irrigate early in the morning so the flowers dry off before nightfall and if possible deadhead (remove) old flowers every two weeks. Root rot diseases can occur but are not common. If a root rot is suspected it is best to send samples to a qualified diagnostics lab for identification of the disease and control recommendations.
4. Common Insect Pests
Unfortunately African Marigolds are subject to infestations of several insect pests. Thrips can feed on the upper surface of leaves leaving them silvery looking, and like to feed on the pollen in the flowers which can distort flowers. Usually this is not a big problem in the landscape. Leaf Miners are attracted to African Marigolds and leave unsightly ‘mines’ or tunnels in the foliage but again are not often a severe issue in the landscape. The primary insect pests on African Marigolds are spider mites, which especially under hot and dry conditions can devastate a planting in a short period of time. Symptoms of spider mites are hard to spot in the beginning because the mites are very small and live on the undersides of the leaves. Their feeding will leave tiny spots on the upper surface of the leaves so this is an indication that you need to look closer and treat if necessary. When the infestation becomes severe the plants will start to get covered in webs, and it is easy to spot the mites crawling the webs at this stage. Unfortunately by then it is usually too late to save the planting, so keeping a close eye out during hot and dry conditions (especially Santa Ana winds) is the best preventative. There are excellent miticides available but will work best when the pest population is discovered early.
Old flowers on African Marigolds turn brown and are subject to Botrytis so deadheading (removing old flowers) every two-three weeks is very beneficial. If plants are really stretched at planting it is ok to bury the stems several inches. New roots will form along the stems and will help anchor the plant.