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Giant Ragweed (Ambrosia trifida)

• Family: Aster (Asteraceae)
• Flowering: July-October.
• Field Marks: This coarse, rough-hairy plant is recognized by its large, 3-lobed, opposite leaves and its terminal spikes of greenish flowers.
• Habitat: Alluvial soil in fields, along streams, around ponds and lakes, low woods, roadside ditches, disturbed soil.
• Habit: Very robust annual with fibrous roots.
• Stems: Erect, up to 15 feet tall, much branched, rough-hairy.
• Leaves: Opposite, simple, deeply 3- (or 5-) lobed, the lobes toothed, rough-hairy on both sides, up to 10 inches long; leaf stalks hairy.
• Flowers: Crowded into small green heads in elongated spikes, the heads either composed of only male or female flowers, but both sexes found on the same plant.
• Male Flowers: Up to 1/4 inch across, 5- or 6-lobed; stamens 5.
• Female Flowers: Borne in ellipsoid heads up to 1/2 inch long, each with 4-6 sharp, spine-like projections; ovary inferior.
• Fruits: Nutlets ellipsoid, beaked, black, shiny, 1/4-1/2 inch long.
• Notes: This species is a primary cause of hay fever. The leaves are browsed by deer.