How to Grow Sweet Peas

Sow early: the one rule that matters

Sweet peas are cool-season annuals. WSU Clark County Extension says they grow best at 70 to 80°F and most varieties show stress at 85°F. Once temperatures reach the mid-80s, flowering slows and the vines decline. Every timing decision is about getting plants to bloom before that point.

Cold-winter regions: sow outdoors as soon as the ground can be worked, around the time you would plant edible peas. That is roughly 4 to 6 weeks before your last frost; the dates for your zone above show the window. Illinois Extension gives mid-March to mid-April. WSU says the seed needs steady soil temperatures of 55 to 65°F to germinate, so soil temperature, not the calendar, should settle the exact day. The seedlings can take a light frost. Cornell protects young plants under low tunnels only when temperatures drop below 25°F.

Mild-winter, hot-summer regions: sow in fall. LSU AgCenter tells Gulf South gardeners to plant in October or November. The plants grow slowly through winter and bloom from March until late April or early May. UF/IFAS Nassau County lists November for north Florida, and University of Arizona’s low-desert chart runs from October into early February. Riverside County Master Gardeners in California say to plant between early fall and earliest winter in the inland valleys and desert.

Mild, cool-summer regions: there is more leeway. In the Pacific Northwest, sweet peas can also be sown later in spring for late-summer bloom. In California’s Napa Valley, where winters are mild, Master Gardeners sow in late fall, and the vines finish by early to mid-June.

Starting indoors

Indoor starts help where soil warms slowly. WSU suggests sowing several seeds in individual 2- to 3-inch pots at 60 to 75°F, then thinning to the strongest seedling. Napa County Master Gardeners prefer deep 4-inch pots because sweet peas make abundant roots, and plant out when seedlings are 2 1/2 to 3 inches tall. Keep young plants in a cold frame until conditions outside allow planting, and set them no deeper than they grew in the pot.

Preparing the seed and the bed

Sweet pea seed has a hard coat. Nick it with nail clippers, soak it overnight, or both. Cornell soaks seed in cold water for 24 hours. Germination then takes 10 to 21 days at 60°F.

Prepare the bed before sowing. Fall is ideal, so you are not digging wet spring soil. WSU recommends digging in generous organic matter to 12 to 18 inches. Sweet peas prefer a neutral to slightly alkaline pH of 7.0 to 7.5, so add lime to acid soils based on a soil test. Drainage must be good.

Put the support up first. Vining types climb 5 to 8 feet or more using tendrils, and those tendrils can grip only thin supports. Netting, wire fencing or string works well. Wrap thick poles with twine. In San Antonio, Texas A&M AgriLife sows sweet peas at the base of tomato cages in fall, so they climb the cages after the tomatoes finish.

Sow seed 1 to 2 inches deep. When seedlings are a few inches tall, thin them to 6 to 8 inches apart for air circulation. Protect them from slugs, snails and birds until they are 4 to 5 inches tall; netting or strawberry baskets keep birds off.

Care through the season

Keep the roots cool and moist. Sweet peas like their tops in sun and their roots in cool, moist soil. Mulch after planting. Napa County Master Gardeners plant low annuals in front to shade the roots. Sweet peas dislike wide swings in soil moisture, and Texas A&M stresses mulch because the roots are shallow and do not tolerate drying out.

Feed for flowers, not leaves. As legumes, sweet peas fix some of their own nitrogen. WSU recommends fertilizers higher in phosphorus, such as a 10-20-10, and warns that high nitrogen produces vines with few flowers. Feed lightly through the season.

Train and pinch. Guide young shoots onto the support when they are 4 to 5 inches long, and they will climb on their own. WSU suggests pinching the growing tips of young plants to encourage branching.

Rotate. NC State Extension advises against planting sweet peas in the same spot in consecutive years. In hot, humid summers like North Carolina’s, they are a challenge, so early sowing matters even more there.

Choosing varieties

WSU describes three groups of vining sweet peas. Heirloom or old-fashioned types have smaller flowers but the strongest scent and tolerate more heat. Early-flowering or winter types bloom with about 10 hours of daylight, which suits winter flowers in mild climates. Spring- and summer-flowering types need 12 to 15 hours of daylight and have the largest blooms. Bush types grow 6 to 36 inches and suit beds and pots, though they are less useful for cutting.

NC State Extension lists ‘Mars’, ‘Memories’, ‘Oban Bay’ and ‘Old Spice’, noting that ‘Old Spice’ is heat tolerant. Riverside County Master Gardeners suggest ‘Old Spice Mix’ and ‘Royal Family’ for fences and trellises, and the dwarf Supersnoop types for a low hedge in Southern California’s inland valleys. In Cornell’s New York high-tunnel trial, ‘Mammoth Choice Mix’ gave the most stems, at 34 per plant.

Cutting and deadheading

Pick often. Cut stems as soon as the sprays open, either as bare flower stems, generally 10 to 14 inches long, or as leafy shoots with flowers attached. Regular cutting keeps new flowers coming. Any pods left to form signal the plant to stop. Cut sweet peas last about five days in a vase.

When the vines fade in heat, WSU suggests cutting them off at ground level and leaving the roots in place for the next planting.

Common problems

• Aphids: they suck sap and spread viruses, including pea enation mosaic, which WSU notes is common west of the Cascades. Hose them off early.
• Powdery mildew: a white coating late in the season, worse when plants are crowded. Space plants properly. WSU notes control is rarely needed because it arrives as the plants are fading anyway.
• Slugs, snails and birds: they can wipe out seedlings. Bait and net until plants are 4 to 5 inches tall.
• Heat stall: plants stop flowering above about 85°F. Sow earlier next year.
• Freeze losses in the South: Texas A&M AgriLife reports sometimes resowing several times between November and February in San Antonio, because seedlings fail in both hard freezes and heat.