Deerweed

Much of the year, deerweed looks like a discarded bundle of sticks, but in spring, when the long slim branches are covered with small bright yellow and orange flowers, the plant is eye-catching.

Deerweed (Acmispon glaber) is one of the first species to colonize disturbed areas, especially post-burn regions. Like many members of the pea family, the roots of deerweed contain symbiotic nitrogen-fixing bacteria, which transform atmospheric nitrogen into nitrogen compounds that can be used by higher plants. Deerweed is the most abundant coastal sage scrub plant to support nitrogen fixation, which makes it an important post-fire colonizer.

Description ^2,4,11,26,59

Deerweed is a shrubby perennial with wiry, green branches arising from near the base of the plant. When not crowded, the plant assumes a hemispherical shape, usually less than three feet (1 m) high. Deerweed is drought deciduous, but the stems contain chlorophyll allowing the plant to continue to photosynthesize and grow.³⁵

The hairless or sparsely-haired leaves usually consist of three oval leaflets, 1/4-3/8 inch (6-15 mm) long; when more than three, leaflet arrangement is pinnate. Leaves may be shed during the dry summer.

Deerweed flowers are 1/4-3/8 inch (7-12mm) long, yellow, aging to orange. They have the bilateral shape of a typical pea flower. The upper petal is large and flares upward forming the “banner”. Two side petals (“wings”) are directed forward, enclosing or concealing the remaining two petals which are fused lengthwise into a “keel”. In turn, the keel encloses the male and female reproductive structures. Flowers are bisexual. There are ten stamens, nine united and one free, and a single pistil.^4,26 Clusters of two to seven flowers attach directly to stem at a single point, and many clusters along the stem open about the same time. The main bloom period is March-July, but some flowers may be found throughout the year.¹

Fruit is a two-seeded pod 5/8 inch (1-1.5 cm) long, somewhat curved upward and tapering to a long beak.  Seeds are an important food for many birds and small animals.¹¹

Classification ^2,59

Deerweed (Acmispon glaber) is a dicot angiosperm in the pea (legume) family (Fabaceae; previously called Leguminosae).⁴ 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 flower similar to a pea or sweet pea flower. Many members 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 ocean locoweed (Astragalus trichopodus), collared lupine (Lupinustruncata) and chaparral sweet pea (Lathyrus latiflorus).

There are six species of Acmispon reported from the Reserve⁴⁸ including the endangered Nuttall’s lotus (A. prostratus), and bishop’s lotus (A. strigosus), an early post-fire arrival after the Holmwood Canyon fire in 2019. Of the six, deerweed is the most common. For many years our species of Acmispon were placed in the old world genus Lotus and they are still found with this name in less recent publications.

There are two varieties of A. glaber, distinguished on basis of relative length of wings and keel. The variety in the Reserve is A. glaber var glaber.⁴⁸

Ecology

Although a deerweed plant rarely survives a fire, germination of seeds is stimulated by heat³⁴ leading to rapid post-fire colonization.¹⁴ Deerweed often becomes dominant the second year after a burn and may persist for several years,²⁶ being gradually displaced as the pre-fire vegetation recovers.³⁴ Like many members of the pea family, the roots of deerweed contain symbiotic nitrogen-fixing bacteria that transform atmospheric nitrogen into nitrogen compounds that can be used by other plants.^4,13 Thus deerweed aids soil recovery by replacing nitrogen lost during the fire.^34,35

The change in flower color from yellow to orange follows pollination. Most plants drop their petals immediately since the pollinator-attracting job of the petals is no longer needed. Nevertheless, a number of unrelated species have evolved a color change, leading a colleague of Charles Darwin, Fritz Müller, to speculate that retention of the petals enhances the color presence of the plant and hence the distance from which it can be recognized by a pollinator.⁶³ At the same time, Müller postulated that the color change directed the pollinator away from the fertilized flower, which no longer needed its services, to unfertilized flowers with ample rewards of nectar and pollen. In the intervening 200 years, ecologists have confirmed these speculations.⁶³ Furthermore, it has been shown that insects must learn the difference between colors. Naïve insects will visit the wrong flower before learning where the nectar and pollen can be found.⁶⁴

Human Uses

We have found no information about deerweed use by local Kumeyaay or Luiseño. The Chumash to the north used the branches as a broom for rough sweeping. The smoke from deerweed was used to blacken Juncus to be woven into baskets to provide design. Deerweed was also used for thatching sweathouses.¹⁵

Deerweed is highly recommended for butterfly gardens.⁶⁵

Interesting Facts

The common name, deerweed, appears to come from the fact that the plant is nutritious and readily eaten by deer and other grazers;²⁷ however not all references agree on the palatability.¹¹ The name broom derives from the shape of the plant, which resembles a broom buried handle down⁵⁹ and/or to its early use as a broom¹⁵.