When I was a kid we planted a lot of marigolds in our vegetable garden. The smell of the plants was so strong that it was easy to believe that they would keep away insect pests and nematodes. The marigolds were acting as a companion plant to members of the Solanaceae—tomato, eggplant, potato—tobacco, and chile peppers. Something that I didn’t hear as a kid was that they should not be planted next to legumes, as their roots secrete thiophenes, which kills the nitrogen-fixing bacteria in the legume roots.
More recently I have been told that marigold is important in herbal medicine, mostly applied topically to reduce inflammation brought on by various causes, include radiation therapy for breast cancer. But this marigold is a different one from the one that is so useful in gardens. The important herbal marigold is Calendula, particularly C. officionalis, while the insect repeller is Tagetes, often T. patula (“French” marigolds) in our garden.
The name “marigold” is applied to many members of the Asteraceae—the genera Calendula and Tagetes are both members of this family—but it is also applied as a vernacular name to plants like the marsh marigold, which is a member of the Ranunculaceae (buttercup) family (but has flowers that resemble those of T. tenufolia). This vernacular name is an elision of “Mary’s gold,” with Mary of course referring to the Virgin Mary. The original connection to the mother of God is not recorded, but the name appears to date from the 12th century, when it was applied to Calendula, which is native to the Mediterranean region of Europe.
Both Calendula and Tagetes, many species of which are native to Central and South America, are annuals. At least the most frequently cultivated and hybridized species are annuals; perennial species exist and are gaining popularity of late in horticultural circles. While perennials tend to bloom during a short period of time each year, annuals tend to have a much longer flowering period. In the climate of western Europe Calendula flowers almost year round and therefore manages to be in bloom for nearly every festival to the Virgin Mary from the Feast of the Annunciation in February, when the last blossoms of the previous year cling to old plants in southern Europe, to the Immaculate Conception in December. This is also has the genus got its Latin name; it is in flower on nearly every calends (first day of the month) through the year.
C. officinalis is a typical looking aster with small disc flowers at the center of the blossom and prominent oval ray flowers radiating out from the central disc. This arrangement and the golden color of the petals evoked the common depiction of the Virgin Mary, with golden rays of light streaming out from her head.
It is perhaps not coincidental that this was a plant already prominent in herb lore because of its ability to speed the healing of wounds and reduce skin inflammations. This would appear to be part of a wider phenomenon of pagan culture being incorporated into the Christian tradition as the latter displaced the former in peasant culture throughout Europe during the Middle Ages.
The marigold flowers are pulverized and suspended in oils and made into a tincture and applied directly to the skin to treat wounds and rashes. This application is somewhat accepted by Western medicine, but taking C. officinalis internally to easy stomach cramps and constipation is greeted less enthusiastically but not entirely dismissed. The important ingredient would appear to be flavonoids, which are present in high concentration in Calendula. These anti-oxidants are known to protect the body from unstable molecules (free radicals), viruses, and bacteria.
Tagetes have oil glands in the leaves and the plants have a strong odor. The main components of this oil are limonene, ocimene, tagetone, and valeric acid. In addition to its use as a companion plant, Tagetes species are also used in herbal medicine as as an antibiotic, antimicrobial, anti-parasitic, antiseptic, anti-spasmodic, disinfectant, insecticide, and sedative substance. All parts of the plant are steam distilled to yield an essential oil called “tagetes oil.”