The garden sweet pea most people grow for its fragrant, ruffled blooms, Lathyrus odoratus, is an annual. It completes its entire life cycle in a single growing season, from seed to flower to seed pod, then dies. But the confusion behind this question is understandable, because a closely related plant commonly sold as “perennial sweet pea” (Lathyrus latifolius) does come back year after year. These are two distinct species with different strengths, and knowing which one you have changes how you plant, what you expect from it, and whether it will return next spring.
The Garden Sweet Pea Is a True Annual
Lathyrus odoratus, native to southern Italy and the Aegean islands, germinates, grows, blooms, sets seed, and dies within a single season. This is not a weakness of cultivation or a response to cold weather. The plant is genetically programmed to complete its life cycle quickly. Research into the evolutionary history of the legume tribe Fabeae, to which sweet peas belong, found that the ancestral life form for the group was annual, meaning the annual habit is the deep default for these plants rather than a secondary adaptation.1PubMed Central. Systematics, biogeography, and character evolution of the legume tribe Fabeae with special focus on the middle-Atlantic island lineages
In practice, this means that no matter how well you care for your Lathyrus odoratus, it will not overwinter and regrow from its roots. Once temperatures climb in summer and the plant finishes flowering, it yellows, dries out, and is done. In mild climates with cool summers, bloom time can stretch longer, but the endpoint is always the same. You start fresh from seed each year.
Perennial Sweet Pea Is a Different Species
The plant sold as “perennial sweet pea” or “everlasting sweet pea” is Lathyrus latifolius. It looks similar enough to cause confusion: it is a climbing vine with winged stems and pea-shaped flowers in shades of pink, magenta, and white. It is hardy through cold winters, dying back to the ground in autumn and resprouting from a tough root system in spring. In many regions it is hardy down to USDA Zone 4 or even Zone 3, making it a reliable long-lived perennial across most of the United States and similar temperate climates.
The tradeoff is fragrance. Lathyrus odoratus, the annual, is grown specifically for its perfume, which is one of the most celebrated scents in the flower world. Lathyrus latifolius has almost no scent. For gardeners who fell in love with sweet peas through their smell, the perennial version can feel like a letdown. It makes up for this with sheer persistence. Once established, a perennial sweet pea can live for many years, spreading vigorously by both roots and self-sowing. In some parts of North America and Europe, it has naturalized along roadsides and fencerows to the point where it is considered mildly invasive.
How to Tell Which One You Have
If you bought a packet of sweet pea seeds from a garden center and the flowers smell wonderful, you almost certainly have Lathyrus odoratus. If the plant came back the following spring without replanting, or if you inherited it from a previous gardener and it seems unkillable, it is likely Lathyrus latifolius. Beyond fragrance, there are a few visual cues:
- Flower size and variety: Annual sweet peas come in an enormous range of colors and forms, including bicolors, ruffled edges, and pastels. Perennial sweet peas have a narrower color range, mostly pink, magenta, and white, with simpler petal shapes.
- Stem wings: Both species have “winged” stems (flat flanges running along the stem), but the wings on Lathyrus latifolius are broader and more pronounced.
- Leaflets: Annual sweet peas usually have one pair of leaflets per leaf node plus tendrils. Perennial sweet peas also have one pair but the leaflets tend to be wider and more leathery.
- Root system: If you dig up a perennial sweet pea, the roots are thick, fleshy, and spreading. Annual sweet pea roots are thinner and more fibrous.
Seed packets and plant tags sometimes use the common name “sweet pea” without specifying which species, which is the main source of the annual-versus-perennial confusion. Always check for the Latin name if it matters to you.
Growing Annual Sweet Peas From Seed
Because Lathyrus odoratus must be started fresh each year, seed-starting technique matters more than it does for most flowers. Sweet pea seeds have a hard, smooth coat that can resist water uptake, which slows and reduces germination. A study on Lathyrus odoratus germination found that mechanically nicking the seed coat with a file or treating seeds with concentrated sulfuric acid greatly increased the permeability of the coat to water, improving germination under many conditions.2Scientia Horticulturae. Effects of temperature and seed coat treatments on germination of sweet pea For home gardeners, the practical takeaway is simple: before planting, nick the seed coat with a nail file or soak seeds overnight in water. This is one of the few flowers where this step genuinely makes a difference.
The same research noted something counterintuitive: if the seed coat was filed and then the seeds were exposed to excess free water, germination rates actually dropped, possibly because valuable solutes leached out of the seed during soaking.2Scientia Horticulturae. Effects of temperature and seed coat treatments on germination of sweet pea So if you nick the coat, plant the seeds directly into moist soil rather than also soaking them in water. One treatment or the other, not both.
Sweet peas are cool-season plants. In most climates, seeds go into the ground in late winter or very early spring, as soon as the soil can be worked. In mild-winter areas (roughly Zones 8 and above), you can sow in autumn for winter and early spring bloom. They do poorly once daytime temperatures regularly exceed about 80°F (27°C), which is why gardeners in hot-summer regions often find that their sweet peas bloom beautifully for a few weeks and then collapse seemingly overnight.
Why Annual Sweet Peas Stop Blooming and What to Do About It
The number one frustration with annual sweet peas is that they stop flowering too soon. This happens for two connected reasons: heat and seed-pod formation. As an annual, the plant’s biological goal is to produce seeds. Once it sets a few mature pods, chemical signals tell the plant its job is done and flowering slows or stops entirely. Hot weather accelerates this process.
Deadheading, or picking off spent flowers before they form pods, is the single most effective way to extend the bloom season. If you grow sweet peas for cut flowers and harvest them regularly, you are essentially deadheading every time you cut a stem, which is why cut-flower growers often get the longest bloom runs. Leaving even a handful of pods on the vine can trigger the shutdown.
Mulching around the base to keep roots cool, watering deeply and consistently, and choosing a planting spot that gets afternoon shade in hot climates all help, but none of these measures will override the annual life cycle once summer heat arrives in earnest. If you want sweet-pea-like flowers through a hot summer, the perennial species is a better bet since it is more heat-tolerant and reblooms without deadheading, though again, without the fragrance.
Growing Perennial Sweet Peas and Managing Spread
Lathyrus latifolius is forgiving to grow. Plant it once and it will likely outlast your interest in it. The roots spread underground and the plant self-sows readily, which means it can take over a fence, wall, or bank within a few seasons. For gardeners who want low-maintenance coverage on a slope or an unsightly chain-link fence, this is a virtue. For gardeners who want a tidy border, it can become a headache.
Managing perennial sweet pea involves cutting it back to the ground each winter after it dies back naturally, and in spring pulling or digging out any runners or seedlings that have strayed beyond where you want them. Because the root system grows robust and deep over time, established plants are hard to remove entirely. If you are planting in a shared garden or near a neighbor’s yard, it is worth knowing that this species is on invasive-species watch lists in some regions, particularly parts of the northern United States and Canada where it colonizes disturbed ground aggressively.
One advantage of perennial sweet peas that gets overlooked is their toughness in poor soils. They fix atmospheric nitrogen through a symbiotic relationship with Rhizobium bacteria in their root nodules, as legumes generally do. This means they can thrive in lean soils where other ornamental vines would struggle, and they actually improve the soil they grow in over time.
Other Perennial Lathyrus Species Worth Knowing
Lathyrus odoratus and Lathyrus latifolius are the two species most gardeners encounter, but the genus Lathyrus contains roughly 160 species, and several others are perennial and occasionally grown ornamentally. Lathyrus vernus, the spring vetchling, is a compact, non-climbing perennial that grows about a foot tall and blooms in early spring with purple flowers that age to blue. It is popular in European shade gardens. Lathyrus sylvestris, the flat pea, is a vigorous perennial climber native to Europe that has been used for erosion control. Lathyrus tuberosus, the tuberous pea, produces small but intensely fragrant pink flowers and spreads by underground tubers; it was historically eaten as a root vegetable in parts of Europe.
None of these match Lathyrus odoratus for flower size, color range, or fragrance intensity, which is why the annual species dominates the seed market despite needing to be replanted every year. Breeders have tried for decades to cross annual and perennial Lathyrus species to get a perennial plant with the annual’s fragrance and color range, but reproductive barriers between species in this genus are significant, and no commercially successful perennial-fragrant hybrid has yet reached the market.
A Note on Toxicity
Both annual and perennial sweet peas produce seeds that should not be eaten. The seeds of Lathyrus odoratus contain a compound called gamma-glutamyl-beta-aminopropionitrile, which interferes with normal collagen formation and can cause skeletal deformities and aortic rupture in animal studies.3PubMed Central. Naturally occurring toxic factors in plants and animals used as food This is distinct from lathyrism caused by Lathyrus sativus (the grass pea or chickling pea), a different species grown as a food crop in South Asia and parts of Africa, where prolonged consumption of the seeds as a dietary staple can cause irreversible paralysis of the lower limbs.
For gardeners, the risk is mostly theoretical. You would need to eat a substantial quantity of sweet pea seeds to cause harm, and the seeds are not appetizing. The concern is mainly around young children or pets who might chew on the pods. The flowers themselves are not considered toxic and are sometimes used as edible garnishes, though they have no culinary value beyond appearance. If you have small children or curious dogs, grow your sweet peas on a trellis out of easy reach and remove pods before they mature and split open.
Can You Treat an Annual Sweet Pea Like a Perennial in Mild Climates?
Gardeners in frost-free or nearly frost-free climates sometimes wonder if they can keep Lathyrus odoratus going through winter. The answer is a qualified maybe, but it is rarely worth the effort. In coastal Mediterranean or mild maritime climates where winter lows stay above freezing and summers stay cool, a sweet pea plant sown in autumn can technically persist for more than one calendar year, sometimes blooming from winter into late spring. But even in these ideal conditions, the plant deteriorates after its first major flush of bloom. Stems become woody, flowers get smaller and fewer, and disease pressure from mildew increases on older tissue.
In practice, even gardeners in these climates get better results by pulling out old plants and starting fresh from seed each season. The vigor and flower quality of a young sweet pea plant far exceeds that of one you have tried to nurse through a second season. The annual life cycle is not something you are fighting against; it is the reason the plant puts so much energy into those spectacular flowers. A perennial sweet pea distributes its energy differently: it invests heavily in root reserves and structural growth, which is exactly why its flowers, while pretty, are less showy and less fragrant.
Companion Planting and Succession
Because annual sweet peas finish early in the season, many gardeners plan for succession. A common approach is to grow sweet peas on a trellis or obelisk in a bed where a warm-season climber, like a morning glory, hyacinth bean vine, or small-fruited gourd, can take over the same support structure once the sweet peas are pulled out in early summer. This gives you vertical interest for the full growing season without the trellis sitting empty for months.
Sweet peas also make good companions for spring bulbs and cool-season vegetables. They can share a bed with lettuce, spinach, and peas (the edible kind), all of which prefer the same cool conditions. As legumes, sweet peas contribute nitrogen to the soil through their root nodules, which benefits neighboring plants modestly and leaves the bed slightly richer for whatever follows. The nitrogen contribution from a single season of an annual legume is not dramatic, but over several years of planting sweet peas in the same bed, the cumulative effect is measurable.
If you want the sweet pea look without the annual hassle and you can live without fragrance, consider planting Lathyrus latifolius as a permanent backdrop and growing a small patch of Lathyrus odoratus in a container or at the front of the bed each spring for scent. The perennial provides the vine coverage, and the annual provides the perfume, giving you the best of both species without relying entirely on either.