Sandbar Willow
Salix interior
Willow family (Salicaceae)

Description: This shrub has two growth forms: a small tree up to 20' tall with a trunk up to 6" across, or a thicket of little-branched woody stems up to 8' tall. An older tree develops gray flaking bark on its trunk, while the trunk bark of younger trees is gray and more smooth. Woody branches and slender stems are variably colored, but often gray or yellowish-brown and smooth. The alternate leaves are up to 5" long and 2/5" (1 cm.) across; they are linear in shape and remotely denticulate along their margins (small distinct teeth that are widely spaced). Mature leaves are usually hairless; their upper surfaces are medium green, while their lower surfaces are pale green. At the base of each leaf, there is a short petiole that is hairless and light green. Sandbar Willow is dioecious: male florets and female florets are produced on separate plants in the form of catkins. Whether male or female, a catkin consists of a raceme of florets that are arranged in a spiraling pseudo-whorl.

Male catkins are –2" long and narrowly cylindrical in shape; they are erect, ascending, or lean sideways. Each male floret consists of a pair of stamens and a single oblong-oval bract that is deciduous; there are neither petals nor sepals. The lower half of the filaments of the stamens are densely covered with short silky hairs; the anthers of the stamens are bright yellow. The bract is yellow and covered with short silky hairs. Female catkins are 1–3" long and narrowly cylindrical; they are mostly green while immature, later becoming light brown. Each female floret consist of an ovoid-conic ovary about 5-9 mm. in length and a single oval-oblong bract that is deciduous. Both the ovary and bract are sparsely short-pubescent to hairless. At the apex of each ovary, there is a pair of tiny stigmas. The catkins usually develop at about the same time as the leaves from mid- to late spring. However, some shrubs bloom later during the summer. Each fertile ovary develops into a seed capsule that splits open into 2 parts to release numerous seeds that are minute and hairy. These seeds are distributed by the wind. The root system is branching and woody, often forming underground runners that develop into vegetative offshoots. Colonies of plants are often formed from these clonal offshoots.

Cultivation: The preference is full or partial sun and consistently moist to wet conditions. Different kinds of soil are tolerated, including those that are sandy, gravelly, silty, or loamy. This willow can spread aggressively from vegetative offshoots. Standing water is tolerated if it is temporary. A new willow shrub can be cultivated by breaking off a stem and sticking it into moist ground; new leaves will develop at the top while new roots develop below the ground surface. Willow seeds have a short period of viability; they must be scattered across moist ground within a week after the seed capsules split open.

Range & Habitat: The native Sandbar Willow has been collected from nearly all counties of Illinois (see Distribution Map); it is quite common and probably occurs in every county. Habitats include shrub swamps, borders of ponds and slow-moving rivers, gravel bars and sandbars, lake shore beaches, marshes, damp swales in prairies, and ditches. This weedy willow often appears in wet areas with a history of disturbance; it can reduce soil erosion.

Faunal Associations: The flowers are pollinated primarily by bees and flies, including cuckoo bees (Nomada spp.), Halictid bees, Andrenid bees, Syrphid flies, dance flies (Empis spp., Rhamphomyia spp.), thick-headed flies, Tachinid flies, and flesh flies. These insects seek nectar from the flowers, although some Halictid and Andrenid bees also collect pollen. The following oligolectic Andrenid bees are floral visitors of Sandbar Willow: Andrena andrenoides, Andrena erythrogaster, Andrena illinoiensis, Andrena mariae, and Andrena salictaria. A large number of insects feed on the leaves and other parts of willows. Such insect feeders include the butterfly caterpillars of Nymphalis antiopa (Mourning Cloak), Nymphalis vau-album j-album (Compton Tortoiseshell), Limenitis arthemis arthemis (White Admiral), Limenitis arthemis astyanax (Red-Spotted Purple), Satyrium acadicum (Acadian Hairstreak), and Satyrium liparops strigosum (Striped Hairstreak); also the skipper caterpillars of Erynnis icelus (Dreamy Duskywing). The caterpillars of several dozen, if not hundreds, of moth species feed on willows. Other insect feeders include numerous leaf and flea beetles (see Leaf & Flea Beetle Table), plant bugs (see Plant Bug Table), aphids (Cavariella aegopodii, Chaitophorus viminalis), leafhoppers (Idiocerus spp., Davisonia spp.), long-horned beetles (Oberea spp., Saperda spp.), sawfly larvae (Nematus ventralis, Trichiosoma viminalis), and thrips (Heterothrips salicis, Mycterothrips betulae, Pseudothrips inequalis). Many of these insect feeders are an important food source of insectivorous birds. Some birds feed on the buds or catkins of willows; these include the Mallard, Northern Pintail, Ruffed Grouse, and White-Crowned Sparrow. Fallen willow leaves are eaten by the Wood Turtle (Clemmys insculpta) and Snapping Turtle (Chelydra serpentina), while the seed capsules are sometimes eaten by the Fox Squirrel, Gray Squirrel, and Red Squirrel. Willow branches are a favorite food source of beavers; they also use the branches in the construction of their lodges and dams. White-Tailed Deer and Elk also browse on the leaves and twigs. In general, the value of willows to wildlife is high.

Photographic Location: The photograph of the male catkins was taken at a city park in Champaign, Illinois, while the remaining photographs were taken at Meadowbrook Park in Urbana, Illinois.

Comments: This is probably the most common willow in Illinois. Without the assistance of a beaver colony, it is difficult to control and its use is not recommended for wetland restorations. While willows are often difficult to identify, the Sandbar Willow can be readily recognized by its long slender leaves with widely spaced teeth. Other similar willows have teeth that are more densely spaced along the margins of their leaves, or their leaves lack distinct teeth altogether. Sometimes the Sandbar Willow is referred to as Salix exigua.