   Marigolds and Nematode Management - June 16, 2004 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Marigolds are one of the toughest summer annual flowers we can grow here in north central Arizona. They tolerate the heat and alkaline soils, but come in many shapes, colors, and heights. Shapes range from single to frilled to massive round balls. Colors include yellow, gold, orange, ivory, mahogany, and bicolor combinations. Heights range from 6 to 36 inches. With this kind of variability, you can hardly go wrong with marigolds. The African marigold (Tagetes erecta) is the tallest variety and has the pom-pom type flowers. Now, there are also shorter varieties of African marigolds available. French marigolds (Tagetes patula) are shorter and bushier usually reaching a height of 6 to 12 inches. Signet or dwarf marigolds (Tagetes tenuifolia) have small (1/2 inch) blooms and lacy, fragrant foliage. All marigolds do best in full sun and require a minimum of six hours of direct sunlight. Prepare soil as you would for a vegetable crop by adding composted organic matter, a little phosphorus, and some soil sulfur if you have highly alkaline soil. Nitrogen should be applied sparingly. Too much nitrogen will produce lots of foliage and few flowers. They can be grown easily from seed or nursery transplants. Do not over water marigolds and allow the soil to dry somewhat between watering. Seeds can easily be collected and planted in the following year. Marigolds are relatively pest free and many people interplant them in their vegetable gardens to deter insect pests. While the data is lacking as to whether marigolds actually deter insect pests, they definitely attract beneficial insects such as lacewings, ladybeetles, and parasitic wasps. A vegetable garden with some planted flowers is also more attractive and this makes it more enjoyable to work in. Recent research indicates that marigolds contain compounds toxic to root knot and other plant-parasitic nematodes (microscopic round worms that damage plant roots). Root knot nematodes are not native to our area, but can be brought in with infected plant materials (see the July 7, 1999 Backyard Gardener for more information). The research showed that marigolds, especially certain varieties of French marigolds, significantly reduced root knot nematode populations the following year. Varieties of French marigolds shown to have nematocidal properties are: Bolero, Bonita Mixed, Goldie, Gypsy Sunshine, Petite, Petite Harmony, Petite Gold, Scarlet Sophie, Single Gold, and Tangerine. If you would like to try using marigolds to manage root knot nematodes, here are some recommendations:
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