In order to germinate, seeds of many temperate-zone species require a prolonged exposure to temperatures below some critical threshold. This prevents germination:

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Prolonged exposure to temperatures below a critical threshold prevents germination during warm days in autumn. This is a common adaptation in temperate-zone species to ensure that the seed doesn't germinate prematurely and get damaged by frost or other adverse conditions.

In Order to Germinate, Seeds of Many Temperate-Zone Species Require