Hardening off: why you stop feeding nitrogen now

Nitrogen tells a tree to keep growing; cold tells it to stop. Feed heavy nitrogen too late into the season and the two signals fight — the tree keeps pushing soft new growth right up to the first frost, and that growth is what a hard freeze damages first. Hardening off is the deliberate switch to a low- or no-nitrogen feed (or none at all) once the season turns, so the current year's wood has time to firm up before winter finds it soft. Watering and light stay the same; only the fertiliser changes.