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Male (staminate) and female (pistillate) flowers are physically separated on Western ragweed plants with the male flowers on spike-like racemes 1-6" long that arise from the upper stems, while female flowers occur more basally in small clusters at the base of the spike, along stems, or from leaf axils. Individual staminate flowerheads occur along the entire length of the spike, facing in all directions. They are 2-5 mm across, drooping from 1-4 mm long peduncles and developing a short, bell-like shape as they mature; they are covered by 3-5 oval, fuzzy phyllaries (bracts) in a single series where the lower half of each phyllary is fused to the other phyllaries. Beneath the phyllaries are approximately four staminate florets, each bearing five stamens with pale yellow anthers. The pistillate flowerheads are egg shaped and occur in clusters with a basal bract up to 1.25" long; pairs of smaller bracts lie just below each individual pistillate flowerhead. Pistillate flowerheads each sit in a cup of fused phyllaries 8 mm long and 3 mm across that has several tooth-like projections; each cup holds a single pistillate floret with a bifurcated style protruding from the apex of the floret. The florets are wind pollinated. Like other Ambrosia species in Jackson Park (common ragweed and giant ragweed), Western ragweed may contribute to hay fever symptoms. Western and common ragweed are similar to each other, but Western ragweed (almost always) has simple-pinnate leaves while common ragweed has double-pinnate leaves that are more deeply lobed and fern-like; Western ragweed is the hairier of the two.

Ambrosia

Ambrosia psilostachya

Western ragweed [Blooms: ?-Nov]