Sweet peas have been a garden favorite for centuries. We love them for their scent, asymmetry, early season color, and romantic appeal in bouquets. There are so many types of sweet peas to choose from! If you were wondering how to grow sweet peas from seed, you've come to the right place!
Sweet pea cultivars may be vining, climbing to heights of 10', or short and bushy. Their fragrance also varies from very fragrant to scentless, depending on cultivar, moisture level, time of day, age of the flower, and temperature (fragrance intensifies under cooler weather). Fragrance can also vary by color; in general, cream, lavender, and light pink shades are most fragrant, while scarlet shades are the least.
Sweet peas are photoperiodic, which means they bloom based on day/night length-the amount of light in a day. That's why when choosing which sweet pea is right for your garden, understanding their response to day length is important. There are long-day, day-neutral, and short-day types. As the name implies, long-day sweet peas start producing flower buds when the days are longer than nights, and bloom between spring and summer, which is why they are often referred to as spring- or summer-flowering sweet peas. Day-neutral types, also called winter-flowering types, will bloom regardless of day length, flowering successfully during the shorter, cooler days of fall and winter in mild climates; and in the cool of spring and summer in cold climates. Short-day types are sown in the late summer and fall, blooming in the fall and winter. Because of their bloom time, short-day sweet peas are best for mild climates and winter greenhouse production. While day length is a key element, flower production in sweet peas is also influenced by temperature, light intensity, health of the plant, and growing conditions.
Perennial sweet peas (Lathyrus latifolius) are not fragrant but persist year to year in USDA zones 3–8, and flower in mild weather from spring to fall, as they do not require a specific day length to begin flowering. Ornamental sweet peas are not edible like their closely-named vegetable counterparts (Pisum sativum), but they do attract hummingbirds and other pollinators and are deer resistant.
When to sow outside: RECOMMENDED. Cold Climates: 4 to 6 weeks before your average last frost date, or as soon as the soil can be worked. Mild Climates: Late fall to early winter for spring bloom. Ideal soil temperature is 55°–65°F.
When to start inside: 4 to 6 weeks before your average last frost date. Sow in biodegradable pots that can be planted in the ground with seedlings as roots are sensitive to disturbance. Grow in cool conditions (55°F or below).
Special sowing instructions: Seeds with hard coats like sweet peas can benefit from scarification, a means of breaking through the hard, outer covering to allow moisture to penetrate. Either nick the seeds with a nail clipper, gently scratch them on sandpaper, or soak them in room temperature water for 12 to 24 hours prior to sowing. Darkness aids germination; sow at recommended depth.
Use a lightweight, seed-starting mix/medium (sterile, and lighter than potting mix), and sow seeds 1" deep. Sow 2 seeds per pot, thinning to the strongest plant once leaves appear (clip extra plants at the soil level using scissors). The strongest plant may not be the tallest; look for thick, strong stems and deep color. By thinning early, you minimize the negative impact of crowding, like stretching for light. Read about more indoor sowing tips.
Sow seeds in generous, 3"–4" biodegradable containers that can be transplanted with the seedlings, reducing root disturbance at transplanting. Cover sown and watered containers with plastic wrap to help keep soil evenly moist while germinating. Remove plastic after germination.
When 3" tall, thin to 1 every 12".
Transplant after hardening off, transplant seedlings around your average last frost date, taking care to avoid root disturbance. Botanical Interests' biodegradable pots have tear-away bottoms (score other peat or biodegradable pots), allowing the roots to grow freely; roots may grow faster than pots degrade, and constricted roots can stunt plant growth.
How to sow
Sow in a group of 3 seeds every 12" at a depth of 1".
Optimal Growing Conditions
Sweet peas do best in fertile, well-drained soil that is rich in organic matter. Add organic material as needed prior to sowing or transplant.
Keep well watered.
Choose an area with full sun (6 or more hours per day), but in the coolest spot possible (for example, away from a south-facing brick structure that absorbs and radiates heat.) Afternoon shade can be beneficial.
Fertilizer is not necessary when organic material has been added to the soil. If fertilizer is used, make sure it is well-balanced, or low in nitrogen; too much nitrogen will produce an abundance of greenery and few flowers.
When seedlings have 3–4 pairs of leaves, you may pinch or cut off the top of the seedling, leaving 2–3 pairs of leaves to promote a fuller plant. Extend bloom time, and encourage more blossoms by picking spent flowers frequently and removing any seed pods.
If growing vining types, install trellis or other supports prior to sowing or transplant to avoid disturbing roots later.
Mulch around plants to help keep soil cool and moist, which is ideal for sweet peas.
Keep sweet pea garden well-weeded as weeds compete for nutrients, sun, and moisture.
For longest vase life, harvest flowers in the morning, when a few flowers on the stem are still closed. Immediately place stems in water and use floral preservative if available.