Ocean Locoweed

Locoweed is the common name given to many species in the large genus Astragalus that have bilaterally symmetrical flowers like those of the pea, and inflated, blatter-like pods.

Species of locoweed have many personalities. Some cause bizarre behavior and death to livestock and are aggressively removed from range lands; ours is the host plant for the endangered butterfly, the Palos Verdes blue, and is planted in restoration projects. Some species are highly toxic to humans, others are used medicinally. The roots of some species habor nitrogen-fixing bacteria.

The species in the Reserve is ocean locoweed (Astragalus trichopodus), a species native to California, Baja California and northwestern Mexico. We know virtually nothing about its poisonous characteristics, but appreciate it for its pretty flowers and strange pods and for the lovely butterflies it attracts. Until more is known about its toxicity (or lack thereof) we do not suggest a taste test.

Description ^2,4,26,59

Ocean locoweed is a perennial herb that often dies back to the taproot in the summer. The plant is usually less than 32 inches (80 cm) high, and is composed of several upright to ascending branching stems covered with soft green foliage. Stems and foliage are made more or less silvery by a dense coating of short, white hairs. Parts of the plant may be lightly tinged with red.

The leaves are generally less than 10 inches (24 cm) long, pinnately compound with many sub-opposite leaflets. (As many as 48 have been reported.⁴)There are two small, triangular stipules at the base of each leaf. Leaflets are narrow to broadly ovate to obovate, sometimes notched at the end. Larger leaves are less than one inch long, decreasing in size toward the leaf tip.

The flowers of ocean locoweed are densely whirled on terminal spikes. The calyx is a five-pointed tube. The flower has the bilateral shape of a typical pea flower. It is crescent-shaped, arching down and out, about 1/2 inch (1.3 cm) long and greenish-white to cream in color. The petals are sometimes described with pinkish veins, but this is not evident in our plants. The upper petal is large and flares upward forming the “banner”. Two side petals (“wings”) are directed forward, enclosing the remaining two petals which are fused lengthwise into a “keel”. In turn, the keel encloses the male and female reproductive structures. There are ten stamens; one is free, and the filaments of nine are united into a ribbon and free only at the top; there is a single pistil. The stamens remain after the petals have dropped. Flowers begin blooming when the winter rains begin and continue through the spring usually January through June.¹

The fruit is a one-chambered, inflated bladder-like pod, beaked at the outer end. Loose clusters of fruit droop from the flower stalk. Developing fruit are bright green, aging to kraft-paper brown. Up to 30 seeds are attached along the upper suture and, when dry, the fruit splits open along the lower suture.

Classification ²

Ocean locoweed is a dicot angiosperm in the pea (legume) family (Fabaceae; previously called Leguminaceae). Members of this family are characterized by their fruit, which is an elongated pod with seeds attached along one seam and which usually opens along the opposite seam. Most members of this family have a bilaterally symmetrical flower similar to a pea or sweet pea flower. Many members have compound leaves and many are associated with nitrogen-fixing bacteria in root nodules.¹¹

The pea family is the third largest family of angiosperms in the world and one of the most economically important, often associated with developing societies.⁴⁴ In addition to peas and beans, the Fabaceae includes peanuts, licorice, acacia, and clover. Other members of this family found commonly in the Reserve include deerweed (Acmispon glabra), collar lupine (Lupinus truncata) and chaparral sweet pea (Lathyrus latiflorus).⁴⁸

The genus Astragalus is a large, widely distributed genus with nearly 100 species in California.² It is best known for its production of several toxins that often cause death in wildlife and livestock, but not all members of the genus produce toxins. Like many members of the pea family, some species of locoweed also harbor nitrogen-fixing bacteria in their roots. However, we could find no information for either toxicity or nitrogen fixation in our species.

There are three varieties of ocean locoweed reported from California, differentiated by small differences in the fruit morphology. Our variety is var. lonchus.⁴⁸

Ecology  

The common name ” locoweed” is generally applied to the entire Astragalus genus and comes from the fact that many species contain one or more toxins that can cause bizarre behavior and death in livestock. Locoweed has been called the most widespread poisonous plant problem in the Western United States.²³⁹

The primary toxin in Astragalus spp. is swainsonine, an indolizidine alkaloid.²³⁹ Like many toxins, this is a large molecule and its production takes energy from the plant – energy that would otherwise be used for growth and reproduction. Presumably, then, toxins confer some benefit to the plant that offsets the loss of growth. Research suggests that often these toxins have been evolved (and are evolving) to reduce herbivory,⁴¹ and that the driving forces are usually insects, not large mammals; although once a toxin has evolved, the protective benefits may be broad. Of course, the herbivores, be they large or small, continually evolve strategies for dealing with the toxin. We see only a small portion of a perpetual ever-changing merry-go-round with the plants keeping one adaptation ahead of the hungry herbivores. The relationships apparent today may have little to do with the driving relationships that were important thousands of years ago.

Human Uses  

The California Poison Control System²⁰⁹ classifies the toxicity of Astragalus spp. as “major” and writes: “Ingestion of these plants, especially in large amounts, is expected to cause serious effects to the heart, liver, kidneys or brain. If ingested in any amount, call the poison center immediately.”

However, not all species of Astragalus are toxic.^59,76 In Baja California, the natives chewed Astragalus to relieve sore throats, and they boiled the roots to soothe toothaches. The Aborigines may have believed the seedpod was a remedy for rattlesnake bites.⁷⁶ Astragalus propinquus (A. membranaceus) is one of the fundamental herbs of Chinese medicine,⁴¹ and a search of the internet will reveal modern uses of Astragalus in a variety of herbal medicines.^41,213

To emphasize the obvious, until the toxicity (or lack thereof) of our local species is determined, ingesting ocean locoweed, for whatever reason, is not advised.

Interesting Facts  

Ocean locoweed is the host plant for the endangered Palos Verdes blue butterfly.¹¹⁶ In 1983, Rancho Palos Verdes bulldozed the last known population of the Palos Verdes blue to make way for baseball fields. The species was thought extinct until 1994 when biologists found a tiny colony on Navy property. The Navy promptly ceased construction of a fuel line through the area, and a careful restoration was begun, replacing nonnative species with natives, including ocean locoweed. The Palos Verdes Blue has since made a comeback.¹¹⁶

More locally, ocean locoweed is the host plant for the Western tailed-blue. Caterpillars tunnel into the fruits where they feast on the developing seeds. Shining a light through a green seedpod will often reveal a caterpillar within, but Allen and Roberts caution that removing the seedpod will ultimately kill both the pod and caterpillar.⁵⁹