Western Ragweed

Western ragweed (Ambrosia psilostachya) is a tall, rather weedy plant native to much of North America and best known for its contribution to fall hay fever. The late summer flower heads are not conspicuous, but, allergies permitting, they are worth a close look. Pollen-bearing florets (male) hang from terminal spikes, resembling columns of little bells. The tiny seed-producing florets (female) occur in small clusters, just below the spikes of male flowers.

In spite of its sneeze-inducing qualities, many native American tribes used the plant medicinally, often for digestive problems. In the Santa Barbara area, the Chumash used western ragweed to reduce a fever. In Southern California, the Kumeyaay made a mash of stems and leaves to treat dandruff.

Description ^{4,11,23,59,306}

Western ragweed is a perennial herb, usually less than 4.5 feet tall, from an underground stem (rhizome) that produces numerous vertical shoots. The plant is covered with long, soft hairs. Petioles are short or absent.  Leaves are elliptical in outline, less than five inches (13 cm) long and deeply, pinnately divided into thin lobes. The larger lobes may have a few coarse teeth. The margins of the leaf are often turned slightly upwards, giving the leaf a rimmed appearance.

A ragweed flower has the basic structure of a composite flower head. The male and female florets are small and separated on different flower heads on the same plant. Numerous nodding male flower heads are densely packed along a terminal stem and resemble a column of small bells. A few or several male florets develop surrounded by a cup-shaped involucre of fused green phyllaries. Each male floret has five inconspicuous pale cream petals, united at the base and with triangular lobes that are often curved inward. There is no pistil, and there are four or five stamens. A few inconspicuous female flower heads occur in small clusters from leaf-like bracts below the male flower heads. The female floret is quite unlike the male. There is one female floret per flower head, with no petals or stamens. The involucre hugs the ovary with several rounded, white tubercles ringing the top of the ovary.  The pistil consists of an inferior ovary and a style with two, long, thread-like branches. Flowers appear in late summer (primarily June – November).¹ They are wind pollinated and are a primary source of late summer hay fever.⁵⁶

The fruit is dry and one seeded, developing within the involucre which forms a hard, dark covering, about 1/8 inch (2-3.5 mm) long with a conical top with an abrupt tip.

Classification ^{2,11,44,49,143}

Western ragweed (Ambrosia psilostachya) is a dicot angiosperm in the sunflower family, the Asteraceae. This is the largest family of vascular plants in the Northern Hemisphere. “Flowers” of Asteraceae are made up of one or both of two types: symmetrical disk florets and strapped-shaped ray florets. These are crowded onto a common base (receptacle) surrounded by leaf-like phyllaries and together are often assumed to be a single flower, which we call a flower head. Other familiar Asteraceae that occur in the Reserve include bush sunflower (Encelia californica), goldenbush (Isocoma menziesii), coyote brush (Baccharis pilularis) and California sagebrush (Artemisia californica).⁴⁸

In the sunflower family, species in the genus Ambrosia are distinguished from all but one other genus (Xanthium) by having unisexual flower heads on the same plant.² There is one other Ambrosia species in the Reserve, bathers delight (A. chamissonis),⁴⁸which is found on the sand dunes.

Ecology ⁴⁴⁹

Most plants are pollinated by insects or birds, but a few rely instead on the wind (they are anemophilous).^41,449 These latter include most of the pines, grasses, oaks and the infamous ragweeds. Instead of attracting biological pollinators with showy flowers, sweet scents, and or nectar, wind-pollinated plants produce prodigious quantities of small light pollen grains to maximize the probability that one grain reaches a receptive female ragweed flower. They play the weather lottery.

Western ragweed has many of the characteristics typical of a wind-pollinated flower: flowers are small and inconspicuous, lacking the large colorful petals that attract biological pollinators; they produce neither fragrance nor nectar, but instead divert this energy into pollen production – one ragweed may shed up to a billion small pollen grains in one season;^450,451 stamens and pistils are separated in different flower heads; male flower heads are at the top of the plant, the better to catch the airstream; female flower heads are lower where they are more likely to catch the settling pollen.

On the other hand, western ragweed does not have the open structure that often typifies a wind-pollinated plant. Stamens lack the long filaments that hold the anthers away from the flower where they blow in the wind and shed their pollen, and pistils lack the feathery stigmas that effectively filter the pollen from the air. Why a species evolves some strategies and not others – indeed, why plants become wind-pollinated at all – is the subject of active research.

Human Uses  

There are scattered reports of Native Americans throughout the western United States using western ragweed for medicinal purposes, mostly for digestive problems.²⁸² The Chumash, from the vicinity of Santa Barbara and the Channel Islands, made a bitter decoction of western ragweed to relieve a fever³⁶⁰ and the local Kumeyaay used it as a treatment for dandruff.²¹⁹

In the modern world, pollen grains of the ragweed genus provide scientists with information about past climate and vegetation.⁴⁵²  Plant groups can often be recognized by the morphology of their pollen,⁴⁵⁰ and where wind-pollinated plants are present, the high volume of pollen often leaves a record in the nearby sediments. In a recent study on Santa Rosa Island,⁴⁵³ layers of ragweed pollen and pollen from species in the goosefoot family, contrasted with a layer high in pollen from pines, unidentified species in the sunflower family and fragments of charcoal. These patterns led paleoecologists to postulate the changing availability of fresh water dating back over 5,000 years. A rapid increase of grass pollen near 1800 AD, corresponded with the introduction of non-native grasses, while a sharp decline 40 years later marked the introduction of grazing livestock.

Interesting Facts ^22,454

Occasionally, the leaves of western ragweed appear to be covered with small blisters. These are a type of plant gall formed by a mite (Aceria boycei), a tiny, relative of the spider. A single gall is a small bead, about 1/8 inch (2-4 mm) in diameter. Often, several beads coalesce into irregular clusters and sometimes the entire leaf may be distorted. Gall abundance appears greatest in the fall.