Annual bursage [Ambrosia
acanthicarpa Hook.][FRSAC] Photographs
Giant ragweed [Ambrosia
trifida L][AMBTR][CDFA list: B] Photographs Map of Distribution
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.
and UNDERGROUND STRUCTURES:Taproot short, thin, with many fibrous roots.
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.
and SEEDS:Hardened phyllaries tightly enclose a single achene
to form a bur. Burs +/- obovoid.
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CHARACTERISTICS:Rigid stems with fruits can persist into the winter
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
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.
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