From a botanical perspective, the classification of a plant as an annual or perennial is defined by its genetic programming for completing its life cycle. An annual plant, like the common garden sweet pea (Lathyrus odoratus), is genetically wired to germinate, grow, flower, set seed, and die within a single growing season. Its entire biological purpose is to reproduce once and as prolifically as possible before environmental conditions, typically winter cold, end its life. A true perennial plant, in contrast, possesses physiological adaptations that allow its root system (and sometimes above-ground structures) to survive dormant periods and regrow for multiple seasons, focusing on long-term survival rather than a single, massive reproductive effort.
The classic, fragrant sweet pea found in most gardens, Lathyrus odoratus, is unequivocally an annual. Its biological imperative is to climb, flower profusely throughout the spring and summer, produce seeds, and then senesce (die) as its energy is completely depleted. The plant's cellular structure is not equipped to withstand freezing temperatures. As daylight wanes and temperatures drop, it receives environmental signals that trigger seed production and the cessation of vegetative growth. Its tender stems and leaves cannot survive frost, and its root system is not robust enough to store energy for regrowth the following spring. For this species, death is a genetically predetermined conclusion to its life cycle.
The confusion around sweet peas being perennial often arises from a different species within the same genus: Lathyrus latifolius, commonly known as the everlasting pea or perennial pea. While related, this plant is a distinct botanical entity. Lathyrus latifolius is a true perennial. It develops a large, hardy root system and rhizomes that store energy, allowing the plant to die back to the ground in winter and resprout from its roots each spring. However, it trades the intense fragrance and delicate, large flowers of its annual cousin for toughness and permanence. Its flowers are typically smaller, less fragrant, and come in a more limited color range, often shades of pink and white.
The core physiological difference lies in their resource allocation and survival structures. The annual sweet pea (L. odoratus) directs all its captured energy (photosynthate) into rapid vegetative growth and, most importantly, into flower and seed production. It has no need to invest energy in building frost-resistant tissues or large energy-storing roots. The perennial pea (L. latifolius), however, allocates a significant portion of its energy to its root system, building a carbohydrate reservoir that fuels its regrowth after dormancy. Its stems may become woody at the base, providing additional structural support for survival across seasons. It flowers on new growth each year but does not expend all its energy in one explosive display, often leading to a longer but less intense blooming period.
Therefore, whether you can grow a sweet pea as a perennial is entirely dependent on the species you plant. If you plant the seeds of Lathyrus odoratus, you are cultivating an annual that will complete its life cycle and die, regardless of climate. Its genetics cannot be altered by garden practices. In mild winter climates, it may self-seed readily, giving the illusion of being perennial as new plants emerge the following year from scattered seed. If you desire a truly perennial plant, you must select Lathyrus latifolius or other perennial Lathyrus species. Its perennial nature is a fixed genetic trait, allowing it to return for years in climates within its hardiness zone (typically USDA zones 4-9), surviving through the winter via its dormant root system.
