Sweet peas (Lathyrus odoratus) are classified as annual flowering plants, meaning they complete their entire life cycle—from germination to seed production—within a single growing season. From a plant's perspective, their relationship with frost is dictated by their physiological makeup. Their tender, herbaceous tissues, including new shoots, leaves, and stems, have a high water content. When temperatures drop below the freezing point (0°C or 32°F), the water within these plant cells freezes, forming sharp ice crystals. These crystals physically puncture and rupture the cell membranes, a catastrophic event for the plant. Once these cells are damaged, they cannot function or be repaired, leading to the characteristic blackened, wilted, and mushy appearance of frost-killed foliage.
In horticultural terms, sweet peas are considered "half-hardy" or "frost-tender." This places them in a category between truly tender plants (like basil or tomatoes, which are killed by any frost) and fully hardy plants (like kale or pansies, which can survive hard freezes). The sweet pea plant's level of hardiness is not static; it changes with its stage of growth. Mature, established plants that have been properly acclimated (hardened off) to cooler autumn or early spring conditions can withstand a very light frost, perhaps down to about -2°C to -4°C (28°F to 25°F) for a short period. Their slightly more developed cellular structure and accumulated sugars, which act as a natural antifreeze, offer a small degree of protection. However, this tolerance is extremely limited.
The greatest risk of frost damage is to young sweet pea seedlings and newly transplanted plants. These juvenile plants are exceptionally succulent and lack the structural integrity and sugar reserves of a mature specimen. A frost event that might only nip the tips of an older plant will almost certainly be fatal to seedlings. Their entire vascular system is underdeveloped, making them highly susceptible to cellular damage from ice formation. Therefore, while a gardener might risk sowing seeds in late autumn for an early spring bloom, the emerging seedlings in early spring are in grave danger if an unexpected late frost occurs. The plant's sole mission at this stage is to establish itself, and a frost directly undermines this crucial process.
It is crucial to distinguish between frost and simply cool weather. Sweet peas are cool-season plants that thrive in the moderate temperatures of spring and autumn. They actively prefer growing conditions between 7°C and 18°C (45°F to 65°F). In fact, exposure to consistently cool (but not freezing) temperatures triggers a process called cold acclimation. During this process, the plant undergoes physiological changes, such as reducing its water content and increasing the concentration of soluble sugars and proteins in its cells. This biochemical adjustment effectively lowers the freezing point of the cell sap, providing a few degrees of protection against light frosts. This is why an established sweet pea plant in the garden in late fall may survive a mild frost that would kill an indoor-grown seedling suddenly placed outside.
