I'm not sure this is Amaranthus retroflexus. The root isn't red and the very flat, mealy leaves look more like Fat Hen. See sighting 4260220 for another very red plant.
I would say it is Chenopodium album going by comparison with plants I saw today in Callum Brae. The leaf shape and colour matches current state of fat hen. The amaranth I saw today has brown limp dying leaves.