Vernalization

Many species of henbane require vernalization before flowering.

Vernalization is something plants do when the weather is cold. Vernalization is when some plants only sprout from seeds or grow flower buds in the spring if they have spent enough days in the cold. Some plants need colder temperatures than others. For example, an apple tree that has not had a cold winter will not form good flowers the next year. Some plants need more days of cold than others. Biennial and perennial plants (including poaceae) and some trees do this.