Annual bursage [Ambrosia acanthicarpa Hook.][FRSAC] Photographs

Giant ragweed [Ambrosia trifida L][AMBTR][CDFA list: B] Photographs Map of Distribution

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GENERAL DESCRIPTION:Erect summer annuals that typically colonize disturbed open sites. Pollen of Ambrosia species is a major cause of allergies in the summer/fall months.



ROOTS and UNDERGROUND STRUCTURES:Taproot short, thin, with many fibrous roots.

FLOWERS:Heads small, greenish, composed of staminate (male) or pistillate (female) disc flowers. Staminate and pistillate heads are separate on a single plant (monoecious). Terminal spikes consist of nodding staminate heads, 2-5 mm in diameter. Pistillate heads are clustered in the leaf axils below the spikes. Staminate head phyllaries fused, cup-like, with 3 longest lobes blackish along the midveins. Pistillate head phyllaries fused, persistent, enclose a single ovary, become a bur in fruit. Wind-pollinated.

FRUITS and SEEDS:Hardened phyllaries tightly enclose a single achene to form a bur. Burs +/- obovoid.

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POSTSENESCENCE CHARACTERISTICS:Rigid stems with fruits can persist into the winter months.




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MANAGEMENT FAVORING/DISCOURAGING SURVIVAL:giant ragweed cut in mid- to late summer can still recover and produce seed. Cultivation to prevent seed production can help control infestations.



Prevention: Giant ragweed is extremely competitive and is very difficult to control in many broadleaf crops. While relatively uncommon in California, it is important to prevent giant ragweed from increasing due to its competitive ability and important role as a human allergen.
One of the primary mechanisms of spread in crop areas is by harvesting equipment.

Mechanical: Tillage is effective for control of seedlings because of their early emergence in relation to many other summer annual weeds. Tillage becomes less effective as plants become larger. Under moist soil conditions, plants may be "transplanted" and begin growing in another area. Repeated mowing will effectively reduce seed production but will not eliminate giant ragweed.

Chemical: Populations of giant ragweed in other states have been found to be resistant to ALS herbicides. Pre-emergent herbicides include bromacil, prometrone, and tebuthiurun for non crop applications. Post-emergence herbicides which provide good control for giant ragweed are glyphosate, dicamba, and 2,4-D. Broadcast applications of glyphosate at 2.0 pt/A will control plants up to six inches in height. Tank mixing 2,4-D at 1 pint/A with glyphosate will also provide giant ragweed control. Imazethapyr is also recommended for post emergence control of giant ragweed.

Abul-Fatih, H.A. and F. A. Bazzaz1980. The biology of Ambrosia trifida L. IV. Demography of plants and leaves. New Phytologist 84 (1):107-111.
Abul-Fatih, H. A., F. A. Bazzaz, and R. Hunt. 1979. The biology of Ambrosia trifida L. III. Growth and biomass allocation. New Phytologist 83 (3):829-838.
Abul-Fatih, H. A. and F. A. Bazzaz. 1979. The biology of Ambrosia trifida L. II. Germination, emergence, growth and survival. New Phytologist 83 (3):817-827.
Ballard, T. O., M. E. Foley, and T. T. Bauman 1996. Response of common ragweed (Ambrosia artemisiifolia) and giant ragweed (Ambrosia trifida) to postemergence imazethapyr. Weed Science 44 (2):248-251.
Bassett, I. J. and W. C. Crompton. 1982. The biology of Canadian weeds. 55. Ambrosia trifida L. Canadian Journal of Plant Science 63:1003-1010.
Buhler, D. D.1997. Effects of tillage and light environment on emergence of 13 annual weeds.
Weed technology 11 (3):496-501.

Franey, R. J. and S. E. Hart. 1999. Time of application of cloransulam for giant ragweed (Ambrosia trifida) control in soybean (Glycine max). Weed Technology 13 (4):825-828.
Hartnett, D. C., B. B. Hartnett, and F. A. Bazzaz. 1987. Persistence of Ambrosia trifida populations in old fields and responses to successional changes. American Journal of Botany 74 (8):1239-1248.

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