Carelessweed or Palmer Amaranth (Amaranthus palmeri) is one of the most common weeds found here in southeastern Arizona.
Carelessweed grows up to 6 feet (1.8 m) tall and can be recognized by its lance-shaped to egg-shaped leaves which often have a variegated band of silvery green and by its unusually tall, slender, green flower spikes.
The central, terminal flower spike is always much longer than the lateral flower spikes. The flower spikes are stiff and prickly, especially when they are drying in the fall, so I wouldn't recommend walking through a dense stand of Carelessweed while wearing shorts.
Carelessweed seeds (amaranth) are edible and can be ground into a nutritious flour. The leaves of Carelessweed are also edible and can be used like spinach, however this plant can concentrate nitrates in the leaves when grown in soils high in nitrogen, such as those polluted by agricultural runoff or where nitrogen fertilizers have been used. Such high nitrates are unhealthy, so the leaves should only be used when the plants are found growing in low nitrogen soils and in nitrogen fertilizer-free areas.