Full explanation
Vernalization is why winter varieties can't be sown in spring: without winter cold they stay in tillering or rosette until autumn. Spring varieties don't need vernalization and go straight into stem elongation. The vernalization requirement (VRN) is genetic, controlled by VRN1, VRN2, VRN3 genes in wheat. Breeding types: true winter (strong VRN need, 40–60 days cold), weak-winter / 'facultative' (20–30 days), facultative winter (10–20 days, can be sown autumn or early spring in southern regions), spring (0 days). Practical implications in Ukraine: (1) abnormally warm 2019/2020 winter caused incomplete vernalization in southern oblasts — winter wheat yield dropped 10–15% in Kherson; (2) climate change 2020–2026 is making mild winters more frequent — breeding is moving toward facultative varieties.