Arroyo Willow

Arroyo willows (Salix lasiolepis) grow in riparian areas (near permanent sources of fresh water). They lose their leaves in response, not to summer drought, but to the shorter days of winter. Willow catkins appear before the leaves with male and female flowers on different plants. The pollen-releasing male catkins are yellow (“yellow = fellow”), while the seed-producing catkins are green (“green = girl”). Unlike most catkin-bearing plants, willows are primarily insect pollinated.

Willows were extremely important to the Kumeyaay. A group of willows indicated a source of fresh water, and the branches, twigs, leaves and bark were used to make a number of items of daily life. Willow leaves and bark contain a compound (salicin) the active ingredient in aspirin; they were brewed into a tea to reduce pain and fever.

Description ^2,4,11

Arroyo willow is a large, sprawling, multi-trunked shrub or small tree, usually less than 30 feet (10 m) high. It is winter deciduous, but often retains some leaves throughout the year. Arroyo willows are riparian, needing a consistent source of fresh water. They often form dense groves where there is a shallow water table or seep.

Leaves are elongate, usually less than 6 inches (15 cm) long and 1 inch (2.5 cm) wide; mature leaves are often oblanceolate. Leaf tip is narrowly to broadly acute. Leaf margins may be smooth or shallowly, irregularly serrate or scalloped. Leaves are shiny green above, made pale below by waxy particles and short white hairs.

Flowers are dense, cylindrical clusters of tiny flowers, catkins, that appear February – May¹  before new leaves appear. Petals and sepals are absent. Male and female flowers occur on different plants. At the base of each male or female flower is a small, dark flower bract that is covered with long, shaggy, whitish hairs. Also, each flower has a small gland that secret nectar (nectaries) to attract pollinating insects.  Male catkins are 1-3 inches long (3-7 cm) colored yellow or greenish yellow by the abundant pollen (“yellow = fellow¹⁰⁰). Each flower consists of two stamens that are united at their bases.  Female catkins are slightly smaller and green in color. Female flowers consist of a green, bowling-pin-shaped pistil and a small stalk. The ovary + style tapers to two, small, two-lobed stigmas.

Seeds are tiny (<1/32 inch or 1 mm) and embedded in a  woolly mass of tangled hairs that lifts and carries the seeds away on the air currents.

Classification ^11,44

Arroyo willow is a dicot angiosperm in the willow family (Salicaceae). Members of this family, which includes poplars and cottonwoods, are important members of riparian communities. They are characterized by flowers that lack petals or sepals and which are densely clustered in catkins. Male and female catkins occur on different plants.⁴⁴

Four other species of willow are found in the reserve:⁴⁸ sandbar willow (S. exigua), black willow (S. gooddingii) red willow (S. laevigata) and Pacific willow (S. lasiandra). Arroyo willow is the most common and can be distinguished from other willows by characteristics of the mature leaf: the leaf length is less than 10X the leaf width, the leaf tends to be widest toward the tip, and the underside of the leaf is paler than the top side.⁵⁰ A large tree by the Rios Ave trailhead may be a hybrid or a non-native species.¹⁰⁰

Ecology ²⁰³

Although arroyo willow may survive short droughts, persistence requires a reliable source of fresh water. It is dependent upon flooding for regeneration. In addition to enhanced production of seedlings, large limbs which are dislodged by the force of flooding may root and produce a new plant.¹¹

Unlike the catkins of most species, willow catkins are primarily insect pollinated.⁴⁹⁹ Insects are attracted by nectaries, small glands within both male and female flowers that produce a sugary liquid.⁴ Seeds are wind-dispersed.

Human Uses  

Willows were important to the Kumeyaay.^18,37,282 A living willow indicated a source of fresh water. The flexible branches were bent to form support for their domed shaped homes (“ewaa”). Leaves formed the thatch on their ramadas. Twigs and leaves provided the support elements for baskets and repelled insects from the grains they held. Bark was pounded into flexible sheets for clothing and lashing material. Willow leaves and bark contain a compound (salicin) very much like aspirin; leaves were brewed into a tea to reduce pain and fever.

The pain-relieving properties of willows (and species in the unrelated genus Spirea) have been recognized since before the Greeks.^51,52,511 The pure form of the active ingredient, salicylic acid (which is derived from salicin), causes severe stomach pains, and it was not until 1915 that the Bayer Company in Germany developed a compound of salicylic acid that did not upset the stomach. Aspirin was the trade name; the word was derived from acetyl-chloride, the buffering salt, Spirea, the botanical source of the salicylic acid, and –in, a common German ending for a medicine. The Bayer Company lost the patent on this trademark name (and also the trademark name for Heroin) in the Treaty of Versailles following World War I.

Interesting Facts  

Many insects utilize willows, making them excellent habitat for insect-eating song birds. At the Nature Center willow grove, there are occasional outbreaks of the colorful Western Tussock Moth caterpillars.⁵³ Arroyo willow is a host plant for the Lorquin’s admiral, mourning cloak and western tiger swallowtail butterflies.¹¹⁶

Among the more interesting insects are several species of plant gall inducers.^22,54 Each insect stimulates a unique plant growth (a gall) around its eggs or larvae. These galls provide food and shelter for the developing insect. A variety of other insects have learned to invade galls and prey on, or compete with, the inducing species.