Spiny pigweed (Amaranthus spinosus
) is an erect, often bushy, multi-branched summer annual with rigid, sharp spines. It's leaves are diamond-shaped to egg-shaped with the broader end closest to the stem, and many are variegated with a v-shaped watermark or lighter colored-area. Each leaf node along stems contains a pair of rigid, sharp spines. The stems are reddish, smooth, upright and branched. It's flowers a tiny green auxillary clusters of terminal spikes that appear slightly different depending on whether it is a male or female plant. A. spinosus
is a common weed found in pastures of horses living in tropical or subtropical climates. Like other pigweeds, A. spinosus
develops a strong taproot with a network of fibrous feeder roots. The taproot may or may not be distinctly reddish in color.
's stiff, sharp spines can cause damage to horses' skin when grazing. A. spinosus
is also a nitrate-accumulating plant, and is able to develop high levels of nitrates under certain environmental conditions which can cause nitrate toxicity