The four annual species of Tagetes commonly cultivated throughout the world as ornamentals are T. erecta L., T. patula L., T. lunulata Ort. and T. tenuifolia Cav. These four species were brought into cultivation over two millennia ago in the region that is now western Mexico (Soule 1993a). These early marigolds were used as ornamentals, medicinal plants, and as ritual plants (Nuttall 1920). Shortly after their introduction to the Old World, Tagetes were included in well-known herbals (Fuchs 1542; Gerard 1596), in part due to their potential medicinal uses.
They were to be used with care, since they had a strong, "rank odor" (Gerard 1596). Over the last four centuries, much of the strongly scented secondary compounds have been bred out of these annuals, while at the same time, larger and more ornate heads were selected.
Two other annual species of Tagetes are currently used in horticulture, T. filifolia Lag. and T. minuta L. Tagetes filifolia is a very short plant, with lacy, filiform leaves and two or three tiny white ray florets and is occasionally found in nursery catalogs under the name "Irish Lace." Tagetes minuta is grown in Brazil and other nations for essential oil extraction (Lawrence 1985; Soule 1993b, c) but has escaped cultivation and is considered a noxious weed in parts of southern Africa (Wells et al. 1986).
Since there are 27 species of annual Tagetes (Soule 1993a), the six thus far cultivated represent just a beginning of the potential horticultural usefulness of the genus. The current paper discusses four novel annual Tagetes which can potentially increase the popularity of the genus with the gardening public.
Of the 56 species in the genus, 29 are perennial (Soule 1993a). Only three of the perennial species are currently cultivated in the horticulture industry: Tagetes lucida Cav., T. lemmonii A. Gray, and T. palmeri A. Gray. The history of the cultivated perennial taxa is tangled and has led to occasional publication of erroneous information on nomenclature, origins, and culture.
Tagetes lemmonii was originally collected in southeastern Arizona in the late 1800s by the Lemmons, a husband and wife collecting team, after which it is named who supplied seed to Asa Gray at Harvard University. The Lemmons proceeded to Oakland, California where plants were established. The progeny of those plants made their way into the nursery trade in southern California, and, surprisingly, England (Anon. 1900). Tagetes palmeri is originally from southern Sonora, Mexico. It is used medicinally by local peoples (Soule 1993a) and appears to have been cultivated by the early Spanish missionaries. Even today, healthy specimens of T. palmeri can be found growing in the old mission yards of southern California. T. lemmonii and T. palmeri are sister species (Soule 1993a), and the two are identical in most respects, except that the latter has larger, showier heads, displays greater heat tolerance, and tends to grow into taller, more robust shrubs. T. lemmonii tends to be more cold tolerant.
Tagetes lucida is a Mexican species which has been in cultivation for over a millennia (Nuttall 1920). T. lucida was used by the Aztec as medicinal plants, and in ritual (Ortiz de Montellano 1990). When T. lucida was brought to the Old World, it was used as a condiment in France and England (Sweet 1817). Herbarium sheets from the 1800s show that the Spanish and French re-introduced the plant to the New World, calling it "false tarragon" or "falso hypericon." The species is now offered generally as "winter tarragon," but sold in Florida as "Florida tarragon," and in Texas as "Texas tarragon." These last two are not to be confused by the connoisseur with the "Mexican mint-marigold" which is the exact same species, but has been imported from across the border in recent times (Soule 1993b).
Since there are 29 species of perennial marigolds, the three species thus far cultivated represent a bare beginning of the potential horticultural usefulness of the genus. This study was conducted to determine which species of perennial marigolds are the most adaptable to cultivation, and which might potentially prove to be marketable.
To determine overall suitability as an ornamental crop, plants were evaluated based on floral, vegetative, habit, and environmental characters. Floral characters included duration of flowering, color, size, and number of flowering heads. Vegetative characters evaluated were foliage form, color, and density. Habit characters include plant height, overall form, and general plant attractiveness. Environmental characters tested the general acceptability as an ornamental plant, and included cold hardiness, heat tolerance, water requirements, and adaptability to container cultivation. Habitat information is included, and covers habitat preferences and soil requirements.
Four perennial and four annual species of Tagetes were selected as the best potential new introductions. The four perennial species are suffrutescent and can be grown to marketable (1 gallon) container size under greenhouse conditions within six months. They can be sown directly after the last frost, or transplanted from greenhouse seedlings. All transplant well, and flowered in the first year. The four annual species can be sown directly after the last frost, or transplanted from greenhouse-grown seedlings. All transplant well.
A brief review of Asteraceae terminology is needed for discussion of the floral characteristics. Many small florets cluster together in a head. The heads are surrounded by an involucre, a series of highly modified leaves or bracts. In the case of Tagetes, the involucre consists of a single series of fused bracts, and the involucre grades smoothly into the peduncle, two key characters to the genus. At the base of each floret, the ovary develops into a single seeded fruit called an achene. The outer ring of florets in a Tagetes head tend to have showy corollas and are called ray corollas or rays. The less showy central florets are disk florets.
This mounding shrub is 0.3 to 0.5 m tall. Leaves are medium green and slightly glossy on the upper surface, often silvery on the lower surface. The leaves are narrowly pinnately dissected, but the foliage is very dense, thus a full appearance is maintained.
The species is found in remote canyons of the highlands of northern Argentina. It grows on cliff faces and rocky canyon bottoms, generally in full sun to afternoon shade conditions. The plants tolerate acid to neutral soils, and will bloom throughout the summer if temperatures remain below 37°C. They are normally subject to mild freezing conditions in winter, and in harder freeze will die back to the underground shoots. Adaptability to cultivation is excellent. With its delicate foliage and brilliant yellow corollas, T. campanulata, the canyon marigold, would make a lovely rock wall plant.
It is a low erect shrub 0.2 to 0.3 m tall, with deep green leaves which are moderately pinnately dissected (similar to T. patula). In full shade the foliage is more diffuse, but in part shade it can be very dense.
This species is an exception to the rule of thumb that Tagetes needs full sun, as they occur in full shade under pines in the Sierra Madre Oriental near Monterrey, Nuevo Leon, Mexico. Soils are limestone, but not highly alkaline due to the pine duff. Plants responded well in acidic to neutral soils. They tolerate heat to around 37°C, after which they cease flowering, but respond as the days cool. Plants withstand mild freezing, but generally die back to perrinating rootstocks. Tagetes mulleri, the shade-loving marigold, adapts to cultivation readily, and prefers full shade for at least the afternoon, if not all day.
The plant is an erect shrub that can reach 2 m, but plants can be readily pruned to 1 m. The leaves are very broadly pinnately dissected, and are a deep green color. In some populations, the leaves are covered above and below with a velvety pubescence. Foliage tends to be very dense.
The species is from the Mayan Highlands of Chiapas, Mexico and Guatemala, where it grows in mesic to hydric conditions. In Austin, Texas, plants froze to the ground each year, but came back from underground rootstock three years in a row. Generally, loamy, acidic soils are best, but clay is tolerated. Full sun is preferred, and partial shade is acceptable.
Natives stated that their goats will not eat this species (Soule 1993a). This unpalatability may extend to other ungulates, notably deer which are detrimental to home landscapes in some areas. Tagetes nelsonii, the Mayan marigold, displays good adaptability to cultivation.
This low mounding shrub is 0.2 to 0.3 m tall. The leaves are moderately pinnately dissected, and are a medium green, not glossy or notably pubescent. With full sun and mild pruning a very dense compact form can be maintained.
The species is a limestone endemic and would make a wonderful addition to the nursery trade, especially in limestone rich areas of the southwest. Plants grow in full sun, mesic to xeric conditions and also grow well in rocky, acid to alkaline, porous to clay, soils. They tolerate light freezes, and produce showy flowers virtually year round.
This species has been collected in a pasture with cattle, horses, and goats, and was notably not grazed by these herbivores (Soule 1993a), therefore, T. parryi may be a potential "deer-proof" plant for landscape sites with deer problems.
Adaptability to cultivation is excellent in Tagetes parryi, the limestone-loving marigold. Peduncles do not extend the heads much beyond the foliage, a preferred characteristic for a border plant, thus in the landscape, T. parryi would best be used to create a low border of continuous color.
The plants tend to naturally form low mounding shrubs to 0.3 m tall. The foliage is generally dense, formed of pale green leaves which are narrowly pinnately dissected.
Found in central Argentina on granitic and rhyolitic soils, this species responds well to garden soils from acid to neutral. Sandy soils are preferred, but plants did tolerably well in the clay soils of central Texas. In conditions of full sunlight, rich soil, and ample moisture, it forms a compact, rounded plant. Flowering tapered off in the mid-summer as temperatures rose above 37°C, but resumed with cooler weather.
The overall adaptability to cultivation of Tagetes argentina, the Argentinian marigold, is excellent, and this beautiful low annual should quickly find its place in home landscapes.
This species can easily reach 2 m, but responds readily to pinching or pruning and can be kept below 1 m in height. The leaves are narrowly pinnately dissected, with notable glands on them. The scent of the crushed foliage is a musky, anise-like one (Soule 1993b). Foliage is green to yellowish-green, and can become dense with pruning and full sun.
From the seasonally dry grasslands of southern Brazil, T. minuta has the potential to be an excellent companion plant in gardens since it releases secondary compounds called thiophenes from its roots (Grainge and Ahmed 1988). Thiophenes have been proven to kill root knot nematodes (Winoto-Suatmadji 1969), a serious garden pest. In good soil and moisture conditions, the plants can quickly reach 2 m, thus it also provides an excellent backdrop in the landscape. Due to the bactericidal secondary compounds released from the roots, one should not plant this near legume vegetables or legume trees. The plants do well in acid to mildly alkaline, sandy to clay, soils. Full sun in mesic conditions are best, although the plants tolerate some drying.
Adaptability to cultivation is generally good, the lack of flowering in some areas is not detrimental in this species which is best planted for its nematocidal properties. Seed is currently offered by one company as the "nematocidal marigold."
The vegetative character of T. remotiflora is an additional reason for its recommendation here. Most wild populations are high in anthocyanins, resulting in a deep maroon coloration to stems and leaves, especially along the veins. Several populations have such high levels of anthocyanins that the plants are maroon throughout. This character persisted in greenhouse and common garden conditions. The leaves are moderately pinnately dissected (similar to T. patula), and range from green to maroon. Depending on the population, the plants range from 0.5 to 2 m in height. They respond very well to pinching, producing a lower, more compact plant with moderately dense foliage.
This species originates from the wild and remote canyons of Guerrero in western Mexico. This may be one of the progenitors of the cultivated T. erecta and T. patula, as it is native to the region where maize was first cultivated, over five millennia ago (Soule 1993a). Adaptability to cultivation of T. remotiflora, the Guerrero marigold, is excellent.
A low plant, not over 0.3 m tall. The leaves are fimbrilate, or very narrowly pinnately dissected, and are a pale green, becoming almost blue-green in intense light situations. Foliage tends to be dense, forming a full backdrop for the floral display.
The species occurs from central Mexico into Colombia, however the white rayed variety is found only in central Mexico. The plants generally occur in grasslands and seasonally dry forests, in alkaline to acid soils. They tolerate rocky clay soils very well, and grow in full sun to part shade conditions.
Adaptability to cultivation is good. Tagetes subulata, the lace-leaf marigold, could quickly replace T. filifolia in the trade, since the plants are taller, with showier flowering heads, and a denser yet more delicate foliage.
Based on an evaluation of floral, vegetative, habit, and environmental characters, four species of perennial Tagetes (Tagetes campanulata, Tagetes mulleri, Tagetes nelsonii, and Tagetes parryi) appear highly suitable as novel new horticultural taxa and include resistance to deer predation, limestone tolerance, and adaptability to shade. These novel Tagetes could potentially form the basis for new trade in perennial marigolds as landscaping plants.
In recent years, annual marigolds have lost their lead as the single most popular bedding plants in the United States, in part due to the attraction of novel new species in other genera. Four species of annual Tagetes (Tagetes argentina, Tagetes minuta, Tagetes remotiflora, and Tagetes subulata) including two white-flowered species, appear highly suitable as novel new horticultural taxa. These four novel Tagetes could potentially renovate the trade in annual marigolds as bedding plants.