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Common ragweed is a highly branched, weedy plant that gets about 3' tall. Numerous stems bear cylindrical flower spikes 1-6" long and 1/4" wide; the flowers are tiny (1/8"), all male (except near the base of the spike) in cup-like involucres that are initially green but turn yellowish-green with age, bead-like, hanging from short stalks. The actual male flowers are hidden in the involucre, lack petals, have numerous yellow anthers and (notoriously) produce copious yellow pollen that is carried by the wind; female flowers are enclosed in leafy bracts with protruding paired white styles, and occur in cryptic clusters on the stems below the flower spikes and in leaf axils. If you suffer from seasonal allergies, this plant and its cousin, giant ragweed, are the culprits, NOT the unfairly maligned goldenrod (which has sticky pollen that never gets airborne). Common ragweed has pinnatifid (pinnate-like) leaves; giant ragweed has deeply-lobed leaves that appear palmate.

Ambrosia

Ambrosia artemisiifolia

common ragweed [Blooms: Aug-?]

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]