Redroot Pigweed
Amaranthus retroflexus

Family: Amaranth (Amaranthaceae)
AKA: Green Amaranth, Rough Pigweed, Redroot Amaranth
Photo taken on: August 29, 2008
Location: Wet Mountains, Eastern CO
Life Zones: Plains to montane
Habitat: Waste ground, roadsides

Grows to 4 feet tall with a stout, smooth, reddish stem. Tiny flowers with spiny bracts grow in spikes. It is a weed producing over 100,000 seeds per plant.

Flora of North America reference.

Greenstripe
Amaranthus acanthochiton

Family: Amaranth (Amaranthaceae)
AKA: Greenstripe Amaranth
Photo taken on: August 3, 2018 - Photo Credit
Location: Apache Trail, Medanales, NM
Life Zones: Plains and foothills
Habitat: Sandy, disturbed ground

Grows to 32" branching from the base. Leaves are narrow and long with crinkled edges. Tiny green flowers hidden within large, prickly bracts with green veins grow in dense, stiff spikes.

Flora of North America reference.

I observed the plant below on November 4, 2018 in the same area. The leaves had fallen and the bracts dried but still distinctive with a hint of the green veins.