This is a vigorous erect annual (ht 1 to 2 m) with dramatic purplish leaves, and hails from tropical North and South America, where it was an important food crop. The foliage really does add an element of tropical drama and colour contrast to a summer border. Unlike with the related Amaranthus caudatus, with its wonderfully decorative elongated pendulous tassels of flowers, I think that the blooms of this species - an upright cluster of rather stunted purple-crimson spikes (see photo below) - detract from the amazing foliage, and I pinch out the growing tips of the young plants, to encourage branching and delay flowering for as long as possible. In my experience, the foliage goes off quite a bit after the plant flowers, losing its brilliant colour, and that is usually when I pull them out. The plant does self-seed prodigiously if the plants are left in long enough.
The little purple-leaved seedlings are very identifiable from a young age and appear in spring. Remove most of them, leaving just a few, as they need plenty of space to develop their full potential. It is possible to dig them up and transplant them when they are small.