From scarifying seeds to mulching tips and more, professional gardeners share their insight.

By Caroline Biggs
March 31, 2021
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Adored for their profusion of beautiful flowers, sweet peas (Lathyrus odoratus) are typically cultivated as an annual plant—and not a perennial—for a good reason. "As an annual, the climbing variety can grow up to eight feet tall and produce months of flowers," says Benjamin Godfrey, garden manager at Cornerstone Sonoma. "It can also be grown as a short bush."

sweat pea flowers in garden
Credit: Andyd / Getty Images

Along with their bountiful blooms and striking colors, sweet peas are popular for their strong, sweet scent, says gardening expert Melinda Myers. "They bring fragrance to outdoor spaces," she explains. "Whether it's a sweet pea-covered trellis or a hanging basket on your front porch, they provide a colorful and fragrant way to welcome guests to your home." Interested in incorporating some sweet peas into your home garden? From how to scarify sweet pea seeds to planting tips and more, Godfrey and Myers share their advice, ahead.

Scarify sweet pea seeds before planting.

For as soft and fragrant as their blooms may be, sweet pea seeds have notoriously hard exteriors. To speed up the germination process, which can take up to three weeks, Myers recommends carefully scarifying each seed (this involves weakening the seed coating) before planting. "You can do this by using a nail file or sandpaper to gently scratch the seed coat," she explains. You can also soak sweet pea seeds in water for about eight hours to help soften their "shell." "Soak seeds in damp paper towels overnight and they should be ready to plant the next day," Godfrey says.

Plant sweet peas in full sun and well-drained soil.

Once your sweet pea seeds are scarified, Myers says to plant them in an outdoor spot that receives at least six hours of direct sunlight every day for optimal flowering. "If you're in a hotter climate, provide some afternoon shade to help extend bloom time," she advises. When planting sweet pea seeds, Godfrey says they should be sown about one-inch deep and spaced three-to-four-inches apart to ensure they have plenty of space to grow. "Plant in rich, well-draining soil," he adds. "Sweet peas won't perform well if the soil is too wet or dry."

Start them indoors in colder climates.

If you live in a colder climate where the ground tends to freeze, Myers says you can start sweet pea seeds indoors during the winter. "Sweet peas can be started indoors up to six to eight weeks before the last spring frost, and then transported outdoors in early spring," she explains. "Just remember to space the seeds at least two-inches apart, or twine each plant as they grow, so they're easy to separate when it's time to move them outside."

Provide support for growing sweet peas.

Since most sweet peas are climbing vines, Godfrey says they often need support to grow tall. "When supported with a trellis, against a fence, with mesh or twine, the plant will grasp and hold on with its tendrils (which are like tiny strong ropes)," he explains. To avoid damaging the roots of your growing sweet pea plants, Myers recommends installing a trellis or a similar staking system prior to planting seeds or transplants. "This will make it easier to direct young plants up the support as they grow—instead of forcing them to climb—so they can climb with minimal interference," she explains.

Mulch matters.

Sweet peas are heavy feeders, which is why Godfrey says they'll grow best in soil that has been amended with lots of compost. If adding compost isn't an option, "fertilizing can make a big difference," he says. "Granular time released fertilizers are easiest because they can last for months, but diluted liquid fertilizers can be given weekly." Godfrey recommends using a fertilizer with higher levels of phosphorus than nitrogen to help boost flower production. "Too much nitrogen will stimulate the plant to reach for the stars instead of creating stellar flowers," he explains.

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