Amaranth

A plant grows with warmth, not with days. A growing degree day is one day's worth of useful warmth: how many degrees the day's average temperature is above the lowest temperature the crop grows at. Add them up from planting and you know how far along the crop is, whatever the calendar says.

Say that lowest temperature is 10°C. A day averaging 17°C gives 7 growing degree days. A day averaging 8°C gives none, as it is too cold for the crop to grow. That is why the same lettuce takes about 50 days in a summer and 90 in an autumn: it needs the same total warmth either way.

For amaranth, the FAO EcoCrop database gives a lowest growing temperature of 7°C and a shortest growing cycle of 30 days at its best temperature, about 525 growing degree days to a first harvest.

Where you've told us where your garden is, we use this and the weather there to predict when your amaranth will be ready to harvest. We don't use it for how long a plant lives or when it finishes: we go by what members have logged for that.