Sweet peas (Lathyrus odoratus) are popular annuals known for their fragrant, delicate blooms and climbing habit. Successful cultivation relies on understanding and managing temperature. Optimal conditions promote vigorous growth and abundant flowering, while temperature extremes can hinder development and shorten their blooming period.
Sweet Pea’s Ideal Temperature Range
Sweet peas thrive in specific temperature conditions. For germination, a soil temperature between 50°F and 55°F is effective. Once germinated, seedlings prefer cooler temperatures for initial growth, ideally between 35°F and 50°F.
Optimal flowering temperatures range from 45°F to 70°F. An ideal scenario involves daytime temperatures around 65°F and nighttime temperatures cooling to about 45°F. These cooler conditions encourage strong root systems and sturdy stems, essential for a long blooming season.
Sweet Peas and Cold Tolerance
Sweet peas tolerate cool weather. Young seedlings can endure light frosts and temperatures into the 20s°F, with some tolerating as low as 23°F (-5°C). However, prolonged periods below freezing or hard frosts (below 25°F or -4°C) can damage or kill mature plants and young pods.
Signs of cold stress include stunted growth or purple discoloration of leaves. To protect sweet peas from severe cold, covering them with horticultural fleece or row covers provides insulation. For potted plants, moving them to an unheated greenhouse, garage, or wrapping the container shields roots from cold. Watering plants before a frost also helps, as moist soil retains heat better than dry soil.
Sweet Peas and Heat Tolerance
Sweet peas are cool-season annuals that struggle in high temperatures. Temperatures consistently above 75°F can cause sweet peas to slow or cease production. When temperatures reach 80°F or higher, sweet peas often shut down, ending their blooming period. High heat can trigger the plant to “bolt,” meaning it prematurely goes to seed, ending its flowering cycle.
Signs of heat stress include wilting, yellowing or browning leaves, and reduced or ceased blooms. The plant may also drop buds or flowers to conserve resources. To mitigate heat stress, provide afternoon shade. Consistent, deep watering, particularly in the morning, helps keep roots cool and the plant hydrated. Applying mulch around the plant base helps retain soil moisture and lower soil temperatures.
Optimizing Planting for Temperature
Successful sweet pea cultivation depends on aligning planting times with temperature. In regions with mild winters (USDA Zones 8 and above), fall planting from late September through early November allows sweet peas to establish strong root systems during the cool season. This head start results in earlier, more prolific blooms in spring before intense summer heat arrives.
For colder climates (USDA Zones 7 and below), sweet peas are best planted in late winter or early spring, as soon as the soil is workable. This can be as early as January or February in milder parts of these zones, or March to April in colder areas. Starting seeds indoors 6 to 8 weeks before the last expected frost gives plants an advantage, ensuring they are well-developed before transplanting outdoors into cool soil. Sweet peas tolerate light frosts once planted out, making early spring planting feasible without waiting until after the last frost date.