The annual sweet pea, Lathyrus odoratus, is a beloved climbing flower, prized for its delicate, ruffled blossoms and intense fragrance. Originating in the Mediterranean, this vining plant offers a wide spectrum of colors, making it an excellent choice for garden beds and cutting gardens. Cultivating these flowers from seed is a satisfying process that ensures a robust display and an abundance of fragrant blooms. Understanding the plant’s preferences for cool weather and vertical support is the first step toward a successful harvest.
Preparing the Seeds and Determining Planting Time
Sweet pea seeds possess a naturally hard outer shell, which can delay or prevent germination by inhibiting water absorption. To overcome this barrier, growers often employ scarification, which involves lightly nicking the seed coat with a file or sandpaper, taking care not to damage the inner embryo. An easier method is to soak the seeds in lukewarm water for 12 to 24 hours just before planting. Properly prepared seeds will swell noticeably, indicating the seed coat has softened and is ready for sowing.
Timing the planting correctly is important because sweet peas are cool-weather annuals that thrive in cold soil and tolerate light frosts. In temperate regions where frost occurs, the best practice is to start seeds indoors six to eight weeks before the last expected spring frost date. For gardeners in milder climates that do not experience hard freezes, seeds can be sown directly outdoors in the late fall to establish a strong root system over winter for earlier spring blooms. Starting seeds early ensures the plants mature and flower before the heat of summer causes them to fade.
Sowing the Seeds Indoors or Directly Outdoors
When starting seeds indoors, select a deep container, such as a root trainer or a four-inch pot, to accommodate the sweet pea’s long taproot and minimize transplant shock. Fill the containers with a sterile, well-draining seed-starting mix to prevent fungal issues. The seeds should be planted about a half-inch deep. Placing two seeds per pot is common practice to account for potential non-germination.
The ideal temperature for germination is cool, around 50°F to 65°F, with cooler temperatures promoting stronger, stockier growth. A cool location, like a cold frame, unheated greenhouse, or garage, is preferable to a warm indoor windowsill, which can cause the seedlings to become weak and leggy. Germination typically takes between seven and 15 days. The growing medium must be kept consistently moist during this period.
Hardening Off and Transplanting Established Seedlings
Once the seedlings have developed two to three sets of true leaves, they are ready to be acclimated to outdoor conditions in a process known as hardening off. This involves gradually exposing the young plants to the outside environment—sunlight, wind, and temperature fluctuations—over a period of 10 to 14 days. This slow introduction toughens the plant’s cell structure and prevents shock when they are moved to the garden. Sweet peas are cold-tolerant and can be transplanted outside as soon as the soil is workable, even if there is still a risk of light frost.
Sweet peas thrive in a location that receives full sun but keeps the roots cool, often achieved with a layer of mulch after planting. Prepare the garden bed by incorporating well-rotted manure or compost; this improves soil structure and provides necessary nutrients. When transplanting, carefully remove the seedling from its pot to avoid disturbing the root system, as sweet peas are sensitive to this. Plants should be spaced eight to 12 inches apart to ensure good air circulation, which helps prevent fungal diseases like powdery mildew.
Ongoing Care Trellising and Encouraging Blooms
Because Lathyrus odoratus is a vining plant that can reach heights of six to eight feet, a vertical support structure must be established immediately upon transplanting. Trellises, netting, or bamboo teepees are excellent options, allowing the plant’s tendrils to grip and climb. Providing this support guides the plant skyward, preventing the vines from sprawling and ensuring the flower stems grow long and straight for cutting.
Consistent moisture is important, as sweet peas will not flower well if the soil is allowed to dry out completely. Once the plants reach four to six inches in height, the central growing tip can be pinched off. This encourages the development of strong side shoots, resulting in a bushier plant with more flowering stems. Fertilizing with a low-nitrogen, high-potassium or high-phosphorus formula is recommended to support flower production.
The most effective way to prolong the blooming season is through diligent deadheading or regular cutting of the flowers. This practice prevents the plant from setting seed, which signals the end of its flowering cycle.