Vernalisation
A period of cold that some plants need before they are able to flower.
Also: vernalization, chilling requirement
Many plants from places with cold winters will not flower until they have been through weeks of low temperatures. The cold tells them winter has come and gone. Tulips, hyacinths, most biennials and many fruit trees work this way. It is why tulips flower poorly a second year in warm climates, and why gardeners in such places buy pre-chilled bulbs or keep bulbs in a refrigerator for some weeks before planting.