Western ragweed

Ambrosia psilostachya

This perennial plant is usually 1-2' tall, but sometimes it becomes considerably higher; this plant often branches in the upper half. The stems are are pale green, terete, and pubescent-hairy; they have vertical lines of fine hairs. Both alternate and opposite leaves occur along the length of the central stem. The leaves are up to 5" long and 2" across; they are lanceolate in outline, but their structure is primarily simple-pinnate (although some leaves may be double-pinnate with secondary lobes that are few in number and small in size). The primary lobes are narrowly lanceolate or lanceolate in shape and shallow to moderately deep, tapering to blunt narrow tips; the sinuses between the lobes are either flat or concave. Depending on the local ecotype, these lobes are widely spaced along the leaf margins, or they are more abundant and crowded. The leaf margins are toothless or nearly so, and they are either flat or somewhat elevated. Both the upper and lower leaf surfaces are grayish green or whitish green and more or less covered with short fine pubescence.

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