Chaparral Sweet Pea

Lathyrus vestitus

tree covered in purple flowers and green leaves

The chaparral sweet pea (Lathyrus vestitus) is a perennial vine native to the west coast of North America from Oregon to Baja California. Through spring into summer, the long twining stems weave their way through surrounding vegetation, decorating the normally drab shrubs and bare snags with unexpected pops of bright pink.

Although flowers and fruit are reminiscent of garden peas, many members of the sweet pea genus contain a potent neurotoxin in their peas and pods. If not properly treated, eating these cause paralysis. Until assays have been done on the chaparral sweet pea, eating the peas is not recommended.

Other Common Names:

San Diego Sweet Pea, Wild Sweet Pea, Bolander's pea, Canyon Sweet Pea, Common pacific pea, Pacific peavine, San Diego Pea

Description

Chaparral sweet pea is a branching, perennial vine that twines through shrubs and small trees. Stems are four-ridged and often reddish. The leaves are pinnately compound, with up to 12 leaflets arranged on either side of a central stem). Leaflets may be alternate along the axis or approximately opposite. At the base of each leaf is a pair of small stipules, one on each side of the leaf. These vary in shape; ours tend to be “V” shaped. Leaflets are narrowly oval to linear, less than two inches (5.3 cm) long; the outer ends are rounded with a tiny point; the leaflet margins are smooth. The terminal leaflet is typically modified into a branched, coiling tendril; occasionally one or more tendrils are found along the leaf axis, originating at the base of a leaflet or or replacing a leaflet altogether.

The flowers of chaparral sweet pea are similar to those of the ornamental sweet pea. A few to several (6 – 17) rose to lavender flowers occur along peduncles, opening sequentially from the bottom. In the Reserve, flowers are rose pink, although flowers of lavender to white, and two-toned are found elsewhere. Flowers are  “papilionaceous” (meaning butterfly-like; here, shaped like a pea flower), bilaterally symmetrical, with five sepals and five petals. Sepals are fused into two unequal lips, the upper with two teeth, the lower with three large lobes. There is one large petal, the banner, held upright, two lateral petals, the wings, directed forward and enclosing two smaller petals which are partially fused along the lower margin, into  the keel which, in turn, encloses the reproductive structures.  The banner is obovate, bent upward, often with faint, radiating darker nectar lines that help guide a pollinator to the nectar source deep in the throat of the flower. There are ten stamens, nine are united in a column around the pistil and one is free. The pistil has a superior ovary and one long, flattened style with a small, capitate stigma; the style bends upwards from the ovary and extends just beyond the anthers. The main bloom occurs between January and June.1

The fruit resemble a garden pea pod. The style persists as a beak. When dry, the two valves of the pod twist open from the outer end into two tightly coiled cylinders, releasing five to eight brownish, spherical seeds that are attached along the upper side of the pod. The peas of this genus often contain a paralyzing toxin (lathyrogen), but we could find no analysis of the toxins in our species.

close up of seed pod, similar to bean pod
purple flower petals
green long thin leaves

Distribution

Chaparral sweet pea is native to the west coast of North America from Washington to northern Baja California, generally below 5000 feet. It seems to prefer the damper areas in coastal sage scrub, chaparral and oak woodlands.

In the Reserve, vines are found in East Basin in the north facing chaparral-covered slopes. Occasional plants grow near the trail.

ariel view of the different parts of the conservancy

Classification

Chaparral sweet pea  is a dicot angiosperm in the pea family (Fabaceae, formerly Leguminosae), the third largest family of flowering plants in the world. Members of the pea family are characterized by their distinctive flower (papilionaceous) in which five petals have a bilateral arrangement of one upright banner, which envelopes the rest of the flower in bud stage, two wings and two lower petals, fused or not, that form a keel, enclosing the reproductive organs. Fruit is a one-chambered pod with seeds anchored along one side, that splits open on maturity. Leaves are often compound. Members of the pea family often harbor nitrogen-fixing bacteria in special root nodules. Some members of the pea family are toxic,143 including species in the sweet pea genus.209

Twenty-seven members of the pea family have been reported in the Reserve,48 many of them non-native species. Common native peas include ocean locoweed (Astragalus trichopodus), deerweed (Acmispon glaber) and collared lupine (Lupinus truncatus). Less easily recognized as members of the pea family, are the non-native acacia species that were planted before San Elijo was established as an ecological reserve.

Chaparral sweet pea is a variable species and several varieties have been named, and discarded again. Currently, there are three recognized varieties of chaparral sweet pea in California,2 The plants pictured here are San Diego sweet pea (var alefeldii), characterized by the strong pink (or wine red) color and a banner that is reflexed somewhat beyond the vertical.2 Variety vestitus is also listed in the Reserve,48

close up of pink/purple flower petals
close up of stem
purple flowers on a branch

Ecology

The unique shape of the classic pea flower is thought to be an adaptation to select for pollinators of a certain size, thereby increasing the efficiency of cross-pollination The flower’s reproductive organs are enclosed within the keel petals which in turn are enclosed by the wing petals. They are not readily accessible to pollinators. The nectar is secreted deep within the throat of the flower, forcing the insect to land. When a pollinator of sufficient weight lands on the platform formed by the wings, the wings and the keel are depressed, exposing the throat of the flower and bringing the stamens and pistils upward against the insect’s underparts, depositing pollen onto the insect from ripe anthers or receiving pollen onto the pistil flown in by the insect. This mechanism selects for larger pollinators over smaller. The chaparral sweet pea is primarily bee-pollinated,35,59 guided to the platform by the nectar lines on the banner.

In contrast, some species of peas, including the common garden pea (Pisum sativum) and garden sweet pea (Lathyrus ordoratus) are primarily self-pollinated, the pollen falling onto the pistil of the same flower without insect intervention.393 This characteristic enabled Mendel’s ground-breaking experiments into heredity, as well as the development of the vast numbers of sweet pea cultivars that perfume home gardens.

purple flower with different directions petals
purple flower petals
dried out purple flower and pollen

Human Uses

The Luisaño ate the leaves and stems for greens.17

purple flowers in bushes
green leafs
purple petaled flowers going in different directions

Interesting Facts

The charming chaparral pea has a dark side: the fruit (the pea) may be toxic59 and consumption may lead to paralysis. In his treatise “On Epidemics” Hippocrates wrote “At Ainos, all men and women who ate peas continuously became impotent in the legs and that state persisted.”395 The pea villain was not identified but is thought to have been grass pea (Lathyrus sativus), although other species in that genus have been implicated.

The toxin seems to be concentrated in the pea and can be deactivated by certain cooking methods,394 but until more is known, eating, or even tasting, the peas or pods of chaparral sweet pea is a poor idea.59

This neurological disease has become known as lathyrism or neurolathyrism. Lathyrus is the Greek word for “pea”.21 Whether the disease was named for the plant or the plant for the disease or whether both were named independently is unknown.

purple flowers in tree branch
purple flower with long petals extending opposite was
dried out flower petals hanging

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