Wild marigold [Tagetes minuta L.][TAGMI][CDFA list: A] Photographs Map of Distribution

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SYNONYMS:tagetes, stinking roger, khaki weed

GENERAL DESCRIPTION:An erect summer annual, to 1 m tall. Resembles, but is less showy than the garden marigold (Tagetes erecta L.). Wild marigold is a problematic weed of pastures and numerous crops in East and South Africa, South America, and Australia. The seed has an unpleasant odor and can reduce the value of grain harvests when it is a contaminant. Introduced from western South America, where it is used medicinally, as a condiment, and for herbal tea. Commercially grown in Brazil and other countries for the oil, which is used in perfumes and for flavoring numerous food products. Some evidence suggests that the secondary compounds are inhibitory to parasitic root nematodes and other microbes. Wild marigold is sometimes an alternative host for Sclerotinia sclerotiorum, a fungal pathogen that can infect a variety of crops.

SEEDLINGS:No description available, but probably resemble garden marigold seedlings.

MATURE PLANT:Foliage pungent, glabrous, dotted with embedded translucent glands. Leaves once-pinnately compound or divided, opposite below, alternate or opposite above, 5-15 cm long. Leaflets 11-17, narrowly lanceolate, sharply toothed (serrate), 2-4 cm long.

ROOTS and UNDERGROUND STRUCTURES:Taprooted. Forms mycorrhizal associations.

FLOWERS:Spring-summer. Inflorescences panicle-like (cymes), +/- flat-topped, consist of numerous narrowly cylindrical flower heads, 7-10 mm long and ~ 3-4 mm in diameter. Phyllaries 3-5, in a single series and fused into a tube, dotted with glands, not splitting apart at maturity. Ray and disk flowers barely extend beyond the phyllaries. Ray flowers 1-5, pale yellow. Disk flowers 3-5, yellow to orange-yellow. Receptacles lack chaffy bracts.

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FRUITS and SEEDS:Achenes cylindrical, 4.5-7 mm long, dark brown. Pappus consists of 1-2 (unequal) awn-like scales, 2-3 mm long, and 3-5 ovate to lanceolate scales, 0.5-1 mm long, +/- fused.

HABITAT:Colonizes disturbed sites, especially cultivated or previously cultivated sites and orchards where wild marigold was planted to control nematodes. Tolerates low to high rainfall and elevations.

DISTRIBUTION:Southern Sierra Nevada foothills, southern San Joaquin Valley (Tulare Co.). Previous infestations now eradicated in southern South Coast Ranges, western Transverse Ranges (sw Santa Barbara Co.), and San Francisco Bay region (e San Mateo, nw Santa Clara cos.). To 1000 m (3300 ft).

PROPAGATION/PHENOLOGY:Reproduces by seed. Seed germination requires light, and most seed germinates between 20 and 30º C (optimal 25º C). Fresh seed lacks a dormancy period and germinates within 7 days of imbibing water. Seed imbibed at high temperatures for several days or dried out and reimbibed can germinate at an accelerated rate when temperature becomes near optimal. Seed often germinates on the soil surface and most seedlings emerge from soil depths of less than 6 mm.

MANAGEMENT FAVORING/DISCOURAGING SURVIVAL:Frequent cultivation or manual removal before flowers mature discourages survival. Populations can increase significantly after a burn.

SIMILAR SPECIES:Unlike wild marigold, garden varieties of marigold have showy flower heads larger than 10 mm long and 4 mm in diameter. Fetid marigold [Dyssodia papposa (Vent.) A. Hitchcock][DYSPA], an unpleasant smelling annual introduced from central and eastern North America, has separate phyllaries in 2-3 series and flower heads 4-10 mm in diameter. It is a weed of roadsides and disturbed sites in the eastern South Coast region (sw San Bernardino Co.). To 350 m (1150 ft).

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Prevention: Wild marigold is a native annual of Central and South America that has become a major agricultural weed in 35 countries worldwide. It has been detected in Tulare County, California and is considered a CDFA class A noxious weed. Wild marigold is a prolific seed producer (>29,000 seeds per plant) that may aggressively colonize waste areas, neglected rangeland, or poorly managed agricultural fields. Plants may reach a height of 2 m and effectively outcompete many desirable plants for light, moisture, and nutrients. The plants are strongly aromatic and have been used for teas and other medicinal purposes. Root extracts are allelopathic to many vegetables, corn, and sunflowers. Wild marigold is found in numerous habitats over a range of moisture regimes and elevations. If uncontrolled, it may rapidly expand its range in California. In Africa, the primary mechanism of seed dispersal is contaminated crop seed. Prevent new infestations by planting only certified weed free crop seed, and cleaning tillage equipment between fields.

Mechanical: In agricultural fields, cultivation is very effective in controlling wild marigold. However, new seedlings will rapidly germinate where the crop canopy does not shade the soil surface. Seed buried below a depth of 6 mm remain dormant. However, the persistence of wild marigold seed in the soil has not been determined.

Biological: There are no biological control agents for wild marigold. Livestock will consume it, but generally only when no other forages are available. Seed dispersal through animals is likely, but has not been extensively studied.

Burning: There is little information evaluating fire as a control strategy for wild marigold. However, observations from East Africa have shown that wild marigold is apparently favored in areas following fires or other land-clearing operations. This supports other research that has demonstrated its ability to dominate in highly fertile environments. Known infestations should be closely monitored following fires, as proliferation is likely.

Chemical: In agricultural fields, most preemergent herbicides are effective in controlling wild marigold. However, late season control may be reduced if herbicides leach below the germination zone. This may occur more readily on sandy, light textured soils.
In rangeland areas, 2,4-D, 2,4-DB, MCPA, and dicamba are effective on seedlings. However, late season flushes will not be controlled, and reapplication may be required. Although data is lacking, glyphosate clopyralid, and triclopyr may also be effective.

Drewes, F. E. and Staden, J. van. 1991. Reserve mobilization during germination of Tagetes minuta. Annals of Botany 68:79-83.
Hanson, P. M. and Smith, L. M. 1992. Economics of chemical and manual weed control in hybrid maize in the Kenya highlands. Tropical Pest Management 38:210-213.
Holm, L., Doll, J., Holm, E., Pancho, J., and Herberger, J. 1997. World weeds. Natural histories and distribution. New York: John Wiley & Sons, Inc.
Ivens, G. W. 1967. East African weeds and their control. Nairobi: Oxford University Press.
Jooste, J. and van Biljon, J. J. 1976. Metolachlor + atrazine: a combination pre-emergence herbicide for broad spectrum weed control in maize. Journal of the South African Society of Crop Production 5:85-90.
Kiran, K., Bedi, Y. S., and Kaul, K. 1995. Allelopathic influence of Tagetes species on germination and seedling growth of radish (Raphanus sativus) and lettuce (Lactuca sativa). Indian Journal of Agricultural Sciences 65:599-601.
Meissner, R., Nel, P. C., and Beyers, E. A. 1986. Allelopathic influence of Tagetes and Bidens infested soils on seedling growth of certain crop species. South African Journal of Plant and Soil 3:176-180.
Rambakudzibga, A. M. 1988. Allelopathic effects of wheat (Triticum aestivum L.) straw residues on the emergence and dry matter accumulation of selected arable weed species. Zimbabwe Journal of Agricultural Research 26:169-175.
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