The leaves are pinnate with two leaflets and a terminal tendril, which twines around supporting plants and structures, helping the sweet pea to climb.

In the wild plant the flowers are purple, 2–3.5 cm (3⁄4–1+1⁄2 in) broad; they are larger and highly variable in color in the many cultivars.

His initial success and recognition came while serving as head gardener for the Earl of Radnor, raising new cultivars of pelargoniums and dahlias.

They are grown for their flower colour (usually in pastel shades of blue, pink, purple and white, including bi-colours), and for their intense unique fragrance.

Unlike the edible pea, there is evidence that seeds of members of the genus Lathyrus are toxic if ingested in quantity.

A related species, Lathyrus sativus, is grown for human consumption but when it forms a major part of the diet it causes symptoms of toxicity called lathyrism.

Symptoms are similar to those of scurvy and copper deficiency, which share the common feature of inhibiting proper formation of collagen fibrils.

Seeds of the sweet pea contain beta-aminopropionitrile that prevents the cross-linking of collagen by inhibiting lysyl oxidase and thus the formation of allysine, leading to loose skin.

The sweet pea is thus a model organism being used in early experimentations in genetics, particularly by the pioneer geneticist Reginald Punnett.

It is highly suitable as a genetic subject because of its ability to self-pollinate and its easily observed Mendelian traits such as colour, height and petal form.