This species was originally native in southwestern regions (mostly northern Mexico and Cal. to Tex.), but it has become widespread elsewhere in North America. The species has spread rapidly across the state: the first recorded coll. was made in 1981 (Medley et al. 1983); by 2015, it had been reported from about half of the counties, with new records mostly from central regions but still absent from Appalachian region (Green & Martin 2015). Most of these recent records are from the Weed Science laboratory at Univ, of Ky. and remain unverified with colls., but they are mapped here with open circles. In Ky., palmeri might be native on or near sandy banks of the Mississippi and lower Ohio Rivers. Records during the 1980s were mostly from sandy soils near larger rivers, in somewhat natural riparian habitats and in artificial habitats nearby (fields, roadsides, railroads). The great majority of plants now occur in fields cropped for soybeans (especially) or corn, including uplands far from river valleys. Like some other species of subgenus Acnida, palmeri has developed resistance to diverse herbicides (Ward et al. 2013): ALS-inhibitors [imazapic etc.], dinitroanilines, triazines, glyphosate, and HPPD inhibitors [topramezone etc.]. This species is generally considered part of subgenus Acnida, which is dioecious (versus monoecious in the rest of the genus); inflorescence mostly terminal, usually with elongated to nodding spikate segments (versus terminal but with less elongated segments, or with largely axillary glomerules); tepals 0-5 (versus usually 3-5); utricles indehiscent or dehiscent (versus usually dehiscent). However, recent analysis of nuclear DNA (Waselkov et al. 2018) has revealed a close relationship with spinosus (subgenus Amaranthus), whereas it is chloroplast DNA that links palmeri with the tuberculatus complex (subgenus Acnida). The leaves of palmeri do resemble those of spinosus, and sometimes have whitish chevron like spinosus. A. palmeri differs from tuberculatus as follows (FNA 4): bracts ca. 4 mm long (versus 2 mm), equaling or exceeding the outer tepals (versus shorter); inflorescences uninterrupted or interrupted in proximal section with few axillary clusters (versus often largely interrupted-moniliform with remote globose glomerules); leaf blades generally with broader shape, 1.5-7 × 1-3.5 cm (versus 1.5-15 × 0.5-3 cm), with L/W mostly 1.5-2 (versus 3-5); leaf apex bluntly or often sharply pointed (versus bluntly pointed or notched).