In the raised bed threads, some people indicated they extended their
growing season by a couple of weeks at either end. We bought a house
with an unheated green house last year and I kept track of the
temperature. Generally speaking the lows in the green house were the
same (-5 to -10 C) as the outside temperature. I did have water in
the green house (~40 litres) to act as a heat sink.

The transition from winter to spring here is abrupt and variable.
Days warm up to 15 C but the overnight lows are well below freezing.

Do the people who have success extending the season live in milder
climates with more gradual springs?

Are there better techniques than using water as heat sink?

Thanks