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:
- At the end of the growing season, remove as many roots as possible from the soil by pulling, plowing or tilling. Doing so will reduce the number of safe places where nematodes can survive during the winter.
- In winter, till the soil several times to expose nematodes to the sun and weather. You may also want to solarize your soil (see the May 21, 2003 Backyard Gardener).
- In spring, plant half of the garden with marigolds and half with root-knot-resistant vegetable cultivars (a few are available – look for the “N” on the label). Plantings in blocks or strips are easy to manage. Strips may comprise one or several rows of vegetables. You will need about 300 marigold plants per 100 sq. ft.
- Use a marigold variety listed above.
- Space marigold plants, or thin seedlings, so they are 7 inches apart.
- Fertilize as needed or according to soil test recommendations. Nutrient imbalances can make nematode problems worse.
- Keep weeds under control.
- Before the first frost, remove as many seed-bearing flower heads as possible. Then, you will have seed for next year’s marigold patches, and fewer volunteer marigolds will sprout among your vegetables.
- Till the remaining marigolds into the soil.
- The following spring, repeat the process with this exception: plant marigolds where you planted vegetables the previous year and vice versa.
Enjoy those marigolds! The University of Arizona Cooperative Extension has publications and information on gardening and pest management. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at firstname.lastname@example.org and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column .
Marigolds and Nematode Management – June 16, 2004
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County