Salix humilis (Prairie Willow)
|Also known as:||Upland Willow|
|Life cycle:||perennial woody|
|Habitat:||sun; average to dry sandy or gravelly soil; prairies, savannas, dunes, bluffs, rock outcrops|
|Bloom season:||April - May|
|Plant height:||1 to 10 feet|
|Wetland Indicator Status:||GP: FACU MW: FACU NCNE: FACU|
|MN county distribution (click map to enlarge):|
|National distribution (click map to enlarge):|
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Male and female flowers are on separate plants (dioecious) in spike-like clusters (catkins) from buds along 1 year old branches, emerging before the leaves. Male catkins are oval to nearly round, ¼ to 1¼+ inches long, the flowers densely to somewhat loosely packed, each flower with 2 stamens, the tips (anthers) orange, yellow, purple or brown.
Female catkins are 1/3 to 1¾+ inches long, the flowers crowded on the spike, bulbous at the base with a long beak, densely covered in woolly hairs, and on slender stalks 1 to 2.5 mm long. At the base of each male and female flower stalk is a tiny, dark brown, black or bi-color, scale-like bract densely covered in long straight or wavy hairs.
Leaves and stems:
Leaves are alternate, 1 to 3½ inches long, to 1+ inch wide, 2.5 to 6 times as long as wide, sometimes narrowly oblong-elliptic but mostly widest above the middle or near the tip, rounded to pointed at the tip, wedge-shaped to somewhat rounded at the base, mostly toothless, the edges often rolled under (revolute) and sometimes also a bit wavy. The upper surface is medium to dark green, hairless to variously hairy and at least somewhat glossy, the lower surface pale blue-green to nearly white from a dense covering of woolly hairs, sometimes becoming hairless or nearly so with age. At the base of the short leaf stalk is a pair of tiny leaf-like appendages (stipules) that are pointed at the tip, but are often absent.
New leaves are yellowish to green, covered in woolly white or gray hairs sometimes also with a few rust-colored hairs. A network of veins is fairly distinct, the main veins most prominent and curved upward from the midvein. New branchlets are densely short-hairy and greenish-brown, yellowish-brown or red-brown, becoming brown the second year and the hairs sometimes persisting to the third year.
Stems are multiple, slender with smooth to slightly rough, gray to greenish-brown bark. Small colonies may form by a process known as layering, where a branch that touches the ground takes root and forms a new plant, detaching itself from the parent plant.
Fruit is a capsule 5 to 12 mm long, yellowish to reddish when mature, covered in woolly hairs, pear-shaped to narrowly conical with a long, straight to slightly curved beak. The capsule splits into two halves when mature, releasing the cottony seed; this happens before leaves are fully mature.
There are over 20 species of Willows in Minnesota; Prairie Willow is the only native willow shrub that is commonly found in drier habitats such as prairies, savannas, bluffs, Jack pine stands and forest edges, often in sandy or rocky soil, though it is also sometimes found along shores or the edges of wetter habitats. There are 2 varieties of Salix humilis, both of which are found in Minnesota, though (to me) they do not look much alike. According to Welby Smith's “Trees and Shrubs of Minnesota”, their ranges overlap but the smaller var. tristis is largely absent from the north-central and northeastern counties. Most herbarium records do not specify a var (see the MN distribution map) but it is assumed most of those are the more common var. humilis.
S. humilis var. humilis is typically a medium to large, multi-stemmed shrub up to 10 feet tall, averaging 6 feet. Branchlets are red-brown to greenish, woolly hairy to nearly hairless. Leaves are up to 3½ inches long, the upper surface hairless to variously hairy sometimes with a few rust-colored hairs, the lower sparsely to densely hairy; edges are flat to revolute and sometimes minutely toothed or wavy. Stipules are present on later leaves. Male catkins are up to 1 1/3 inch long (34 mm), female to 1¾+ inches (47 mm); capsules 7 to 12 mm long and tapering from the base to the tip. S. humilis var. tristis is dwarfed, rarely more than 3 feet tall; branchlets yellow-brown and woolly hairy; leaves up to 2 inches long, woolly hairy on both surfaces, hairs gray throughout, edges revolute and toothless; stipules absent or obscure; male catkins to ½ inch long, female not much longer; capsules 5 to 9 mm long and more pear-shaped.
The smaller var. tristis vaguely resembles Sage-leaf Willow (Salix candida), which is also short statured with woolly-hairy leaves, but the leaves are more linear and up to 4 inches long, and it lives in wet habitats. The larger var. humilis is more similar to Pussy Willow (Salix discolor), which has mostly hairless leaves with less pronounced veins and larger flowering and fruiting catkins, and also lives in wetter habitats and can take the form of a small tree, which Prairie Willow does not. But like Pussy Willow, Prairie Willow is one of the earliest flowering shrubs and an important food source for emerging pollinators.
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- Prairie Willow plant, var. humilis
- Flowering Prairie Willow male (left) and female (right), var. tristis
- fruiting Prairie Willow, var. tristis
- leaf underside, var. humilis
- leaf underside, var. tristis
- Prairie Willow fully leafed out in August
- seeds are released starting in early May
- leaves are at least somewhat glossy, new leaves yellowish
- anther color is red-orange to yellow to purple to brown
- midge galls somewhat resemble pine cones
- pollinators in mid spring
Photos by K. Chayka taken in Anoka, Dakota and Ramsey counties. Photos by Peter M. Dziuk taken in Anoka, Dakota and Hubbard counties.
Have you seen this plant in Minnesota, or have any other comments about it?