From a plant's perspective, vernalization is not a requirement but a sophisticated survival strategy. Sweet peas (Lathyrus odoratus) are annual plants, meaning their life cycle—from germination to seed production—is designed to be completed within a single growing season. The evolutionary advantage of sensing prolonged cold is to avoid flowering during a brief warm spell in late autumn or winter, which would be fatal to the delicate flowers and developing seed pods. By using winter's cold as a cue, the plant ensures that its flowering is perfectly timed for the stable, favorable conditions of spring, thereby maximizing its chances of successful pollination, seed set, and propagation of the next generation.
When a sweet pea seed or young plant experiences a sustained period of cold temperatures, typically between 35°F and 50°F (2°C and 10°C) for several weeks, a fundamental internal change is triggered. On a molecular level, the cold exposure leads to the epigenetic suppression of genes that repress flowering. Essentially, the cold signal "switches off" the inhibitors that prevent the plant from transitioning from its vegetative state (producing leaves and stems) to its reproductive state (producing flowers). This process prepares the plant's apical meristem (the growing tip) to develop flower buds instead of more leaves once warmer temperatures and longer days arrive in spring.
For a sweet pea plant grown from a fall-sown seed, this vernalization process occurs naturally as the young seedling endures the winter cold in the soil. This gives it a significant head start. When spring arrives, this vernalized plant is primed and ready to flower early and profusely, often producing longer stems and more blooms over a more extended period compared to a spring-sown plant. A spring-sown seed, planted into already-warm soil, misses this crucial cold signal. It must first grow vegetatively for a longer period before it internally decides conditions are suitable to begin flowering, which results in a later and often shorter bloom period, especially as summer heat arrives and can halt flowering.
The "need" for cold is not absolute across all sweet peas and depends heavily on the genetic makeup of the specific cultivar. Modern plant breeding has selected for varieties with varying vernalization requirements. Many heirloom and grandiflora types possess a strong vernalization requirement and perform dramatically better with a cold treatment. In contrast, some modern dwarf and spring-sown varieties have been bred for a reduced or negligible vernalization requirement. These cultivars have been selected to flower reliably without cold, making them more adaptable for gardeners in climates with mild winters or for those who miss the autumn planting window. Therefore, from the plant's viewpoint, the need is not universal but is a key genetic trait that dictates its ideal growing cycle.
