Sweet peas (*Lathyrus odoratus*) are classified as annual flowering plants. From a botanical perspective, their life cycle is intrinsically linked to cooler temperatures. They are native to the Mediterranean region, specifically Sicily, southern Italy, and the Aegean Islands. This origin is crucial to understanding their cold tolerance. The Mediterranean climate is characterized by mild, wet winters and hot, dry summers. Consequently, sweet peas have evolved to complete the majority of their vigorous growth and flowering during the cool, moist periods of the year, making them inherently cool-season plants.
The term "frost" itself encompasses a range of conditions, and a plant's tolerance is not absolute. Sweet peas are considered hardy to light frosts. This means they can typically survive brief exposures to temperatures as low as 28 to 32°F (-2 to 0°C). At a cellular level, the plant achieves this through a process known as cold acclimation. When temperatures drop gradually, the plant increases sugar concentration within its cells, which acts as a natural antifreeze, lowering the freezing point of cell contents and preventing the formation of sharp ice crystals that would puncture and destroy cell membranes.
When temperatures drop below a sweet pea's tolerance level, the plant experiences frost damage. This is a physical process. The water inside and between the plant's cells freezes. As water freezes, it expands. This expansion causes the delicate cell membranes and walls to rupture. Once these tissues thaw, the damaged cells lose their structural integrity, leading to the characteristic symptoms of frost damage: leaves and stems become soft, water-soaked, and blackened. This is essentially a form of plant cell death, as the vital structures for transporting water and nutrients are compromised.
A sweet pea's ability to withstand cold is not static; it is heavily influenced by its stage of development and prior environmental exposure. Mature, established plants that have been "hardened off" possess significantly greater frost resistance than young, tender seedlings or freshly germinated plants. Hardening off is the process of gradually exposing a plant to outdoor conditions—wind, cooler temperatures, and full sun—over 7-10 days. This process triggers the aforementioned physiological changes, such as the thickening of cell walls and the accumulation of protective compounds, which prepare the plant's vascular and foliar systems for the stress of colder nighttime temperatures.
From the plant's viewpoint, the temperature at the root zone is often more critical than the air temperature during a frost event. While the foliage may suffer damage from a hard freeze, the plant's crown (the point where the stem meets the roots) and root system are better insulated below the soil surface. If the soil does not freeze, the roots remain alive and functional. A sweet pea with damaged foliage but a healthy root system can often regenerate new growth from buds at the crown once milder conditions return, especially if it is a perennial variety or in a mild climate. This highlights the importance of well-draining soil, as waterlogged soil will freeze more deeply and cause more severe root damage.
