Learn the difference between varieties of marigold flowers.
While some marigolds are effective at keeping away some nematodes, there is no marigold that keeps away all nematodes.
Look up marigolds online, or visit a few garden forums and you’ll soon see there is an enormous amount of confusion and misinformation about this ancient plant. You’ll discover a lot of people confuse “pot marigold” (Calendula officinalis) and French or African marigolds (Tagetes sp.). Unfortunately you will find the attributes of one, particularly the use in repelling insects, mistakenly attributed to the other. Since they are not the same plant, and don’t have the same qualities, it’s worth separating fact from fiction.
Calendula (Calendula officinalis), called “pot marigold” in Europe, is not related to French or African marigolds. While both are in the overall Compositae/Asteraceae (daisy/aster family), they share few if any of the same uses or effects. Calendula flowers are proven useful for skin ailments, are edible and valuable in their own right, but pot marigold is not the garden marigold with the purported insect-repelling benefits.
Tagetes, with about 50 varieties, includes French marigold (T. petula) that we grow in our flower beds, African marigold (T. erecta), its taller cousin with larger flowers that we plant at the back of flower beds, and Mexican mint marigold (T. lucida) which is known for its tarragon-like flavor in cooking. All three are native to Central and South America.
In The Aztec Herbal Pharmacopoeia, Bernal Diaz, one of the Spanish Conquistadors, recorded in his diary that the Aztec Emperor, Montezuma, had marigolds growing in his gardens. He recorded that the plants were used both in food and medicine, as they still are today. You will find marigold flowers as ingredients in foods, medicines and even chicken feed, to make the egg yolks yellow.
It is recorded in several ancient sources including the Aztec Herbal, the Badianus Manuscript and other post-conquest documents, that marigolds, specifically T. petula and T. erecta, were an important medicine for treating fevers. T. lucida, the Mexican mint marigold we grow in our herb gardens today, was used for reducing phlegm, treating gout, stiffness of joints and digestive ailments.
But what about any of these plants' current uses in repelling insects? Are marigolds truly useful for companion planting or controlling root nematodes? Fortunately, there’s been a great deal of research into those questions. The University of California, Davis, North Carolina State University, University of Florida, and Louisiana State University, have all done extensive research into the effects, or lack of them, on marigold planting; following are some of their results.
Are marigolds effective as companion plants? According to the University of California, Davis, “When grown along with annual vegetables, nematode control usually isn’t very good.” Researchers at the University of Florida verified root-knot nematodes will find and reproduce on roots of susceptible crops or weeds, so inter-planting marigolds and susceptible crops together, is very risky and may increase the damage to the susceptible crops by increasing nematode populations because some varieties of marigolds attract root nematodes, while other varieties repel them.
All soils contain nematodes, some of which are beneficial and some that are detrimental to vegetable crops, provided they are in large enough numbers. Only soil tests specifically for nematodes can determine the soil population and kind of nematodes in any given garden.
If inter-planting or companion planting isn’t useful to repel insects, do marigolds have other beneficial uses in the garden? According to Cornell University Department of Agriculture, destructive above-ground pests often locate their food by smell (cabbage moths, for instance). Marigolds, as well as many culinary herbs such as garlic, chives, catnip, horehound, wormwood, basil, mints, and tansy, all give off scents which seem to repel pests, or at the very least, confuse the unwanted insects. A small amount of insect protection can be achieved by inter-planting some of these as companion plants — herbs and marigolds — in large enough numbers.
Marigolds are useful for repelling or eliminating large numbers of root nematodes, (provided a nematode assay shows you have them), by crop rotation. Researchers at Cornell University have demonstrated significantly beneficial effects in controlling nematodes by growing a thick cover crop of marigolds for one season prior to planting the vegetable crop. Researchers insist that in order to ensure nematode control, plantings should be dense, at the rate of one marigold plant every 7 inches in all directions. Further, to be effective, weeds must be removed since some nematodes are attracted to native weeds and will reproduce on them.
No marigold variety controls all types of nematodes. ‘Cracker Jack’ marigolds show good control of southern root-knot nematode but is a host plant for other nematodes such as the stubby-root and reniform nematodes. Growers need to determine which nematodes they have before choosing a marigold variety to use as a cover crop.
Planting a cover crop of marigolds after the first spring crop is finished, can be useful in controlling nematode populations. The spring crop of spinach, radishes, cucumbers or lettuce, for example, can be followed by densely planting marigolds for the remainder of the season. If populations of root nematodes are high, the crop rotation with marigolds will need to be done annually to keep the populations under control.
So the proven facts in all this? Companion planting of marigolds have little value other than being pretty and attracting some butterflies, and possibly masking the attractive smell (to pests) of the crop. To actually control nematodes, dense plantings of marigolds, spacing 7 inches apart, works best. Some marigolds repel nematodes, while other varieties can actually create a home for them to reproduce.
Nematodes aren’t the same in every part of the country. Unless you know you have severe problems with root nematodes, the best advice by the experts is to plant some marigolds and enjoy their beauty and avoid planting the ones that might help establish nematodes in your garden.
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