The exact temperature and duration each plant needs to spend below it depends on the species and even the variety, although most of the fruit trees we grow outdoors in the UK have this “chill requirement”. For example, apple trees need to be exposed to between 700 and 1,000 hours below 6C (43F) but above freezing. Interestingly, plant breeders are developing varieties that have a far lower winter chill requirement so that apples can be grown in warmer climates than the UK’s, which is naturally ideal for them.