Sweet peas (Lathyrus odoratus) are cherished for their fragrant, colorful blossoms and are suited for cool-season growth. Starting them indoors is an effective strategy to get a head start on the growing season, typically six to eight weeks before the last expected frost date. This practice is solely for nurturing robust seedlings that will later be transplanted outside, as sweet peas are not adapted to be long-term houseplants. Controlling the initial environment produces strong, stocky plants that establish quickly and bloom earlier once moved to the garden.
Preparing Seeds and Containers
The thick seed coat of sweet peas often requires preparation to ensure uniform germination. This hard exterior can be physically weakened through scarification, where a small nick is made in the seed coat with a nail clipper or sandpaper, avoiding the ‘eye’ where the embryo is located. Alternatively, soaking the seeds overnight in room-temperature water softens the coat, allowing moisture to penetrate and initiate germination. Seeds that swell visibly after soaking are ready for planting; those that remain hard should be gently nicked and soaked again.
Sweet peas develop a substantial taproot early, necessitating deep, narrow containers to prevent the roots from becoming restricted or circling. Specialized root trainers or deep pots, at least five inches deep, are excellent choices. Common items like toilet paper tubes or cut-down milk cartons can also provide the required depth. Fill these containers with a sterile, well-draining seed-starting mix. This is important because garden soil compacts easily and harbors pathogens that cause fungal issues like damping off. Sow the prepared seeds about a half-inch deep into the pre-moistened medium.
Optimizing the Indoor Environment
Once planted, sweet peas require specific environmental conditions. For germination, a soil temperature range between 50 and 55°F is ideal, though conditions up to 70°F can speed up the process, with sprouts appearing within seven to fifteen days. Promptly move them to a cooler location immediately after sprouting to prevent them from becoming thin and weak. The seedlings thrive in a cool environment, with an ideal growing temperature range between 50 and 65°F during the day, and even cooler at night, sometimes as low as 35°F.
Maintaining cooler temperatures encourages the development of strong root systems and compact, sturdy stems. Excessively warm conditions lead to rapid, weak, or “leggy” growth. Since natural winter light is often insufficient, supplemental lighting is necessary to produce quality transplants. Position grow lights, such as full-spectrum LEDs or fluorescent tubes, six to twelve inches above the emerging seedlings. Provide illumination for twelve to sixteen hours daily. Ensuring good air circulation with a small fan helps strengthen the stems and reduces the risk of fungal diseases by preventing stagnant, humid air.
Training, Pinching, and Hardening Off
As the seedlings grow, they develop a vining habit, requiring structural attention. Once the plants reach three to four inches tall and have developed three to four pairs of true leaves, they should be “pinched.” Pinching involves removing the growing tip just above a set of leaves. This signals the plant to produce side shoots from the leaf nodes below the cut, encouraging a bushier growth habit. This results in a plant with multiple stems that will produce more flowers in the garden.
The small vines quickly start seeking support, so providing temporary assistance is helpful while they are indoors. Simple bamboo canes or twiggy sticks placed in the pots offer a structure for the tendrils to grasp, keeping the seedlings upright and preventing tangling. Before the young plants are permanently placed in the garden, they must undergo a gradual acclimation process called hardening off. This transition is necessary because the soft growth produced indoors is sensitive to the sun’s ultraviolet light, wind, and temperature fluctuations.
Begin the hardening off process about ten days before transplanting. Move the seedlings to a sheltered outdoor location for one to two hours on the first day. This initial spot should offer shade and protection from strong winds. Over the following days, gradually increase the duration of time outdoors, and slowly introduce them to direct morning sunlight. By the end of the ten-day period, the sweet peas should tolerate being outside day and night, with exposure to full sun and typical outdoor conditions, preparing them for transition into the garden.