We plant our Amaranth in beds at 30cm diagonal spacing’s which achieves 8 plants per square metre.
To achieve a heavy crop of seed the plants must go in at the same time as your main crop corn. and cucurbits (i.e. when the soil reaches 15°C). This will ensure that the plants grow to their full size before going to seed and it will also mean that you might get away with being able to harvest the mature seed before the birds find it. If the birds do find it, it is easy enough to cover with netting, as long as you are growing it the Bio intensive way, in raised beds.
Seed ripens from the bottom of the stem up. It’s easy to tell when to harvest the heads, they change colour from bright and showy to dull, and the seeds come out easily when you rub the head in the palms of your hands.