Flower:

Flowers are small, ¼ inch or less across, yellowish green, 5 pointed and star-like, short stalked, in clusters of 1 to 3 in the leaf axils of new growth. There are no petals, the triangular star points are sepals set around a flattened, saucer like recepticle. Male and female flowers are on separate plants, the male flowers with 5 yellowish stamens surrounding a green center, females with a 3-parted style in the center.

Leaves and bark:

Leaves are simple and alternate, oval or oblong lance-elliptic, 2 to 4½ inches long and ¾ to 2¼ inches wide, the edges with rounded teeth, with or without glands in the serrations, and 5 to 8 veins per side. The upper surface is darker green, appearing shiny or with sparse hairs; the lower surface is lighter green with hairs typically present.

At the base of the leaf stalk of new growth is a pair of narrow, leafy appendages (stipules). Main branching stems are horizontally spreading, typically less than 1 inch in diameter at the base with thin, roughish red-brown to gray bark. Side branches are erect, spindly, and thornless. New growth is green or brownish with minute hairs turning grayish brown the second year.

Fruit:

Fruit is a short-stalked, shiny, black berry up to 1/3 inch in diameter, with 1 to 3 seeds inside. The berry is often irregularly slightly lobed with a faint groove on the surface between each seed.

Notes:

For all the negative press that buckthorns get, many people are surprised that Minnesota has a native buckthorn. While fairly widespread and common within its preferred habitats across the northern 2/3 of the state, as you'd expect with native species it is not invasive at all. It does produce a fruit with viable seeds that are likely eaten and spread by wildlife, but co-evolved factors limit its introduction and spread across the broader environment. A primary means of reproduction is by a process called “layering”, where the horizontal stems and branches produce roots, creating expanding colonies up to 50 feet across. The root/branch structures of these colonies may become disconnected over time producing genetically identical but independently functioning individuals within the colony. This native can be distinguished from the invasive non-native buckthorns by its short stature (3 feet or less at maturity), the obvious stipules at the base of the leaf stalk, 5-parted flowers, and leaves with rounded teeth and 5 to 8 veins per side. Glossy Buckthorn (Frangula alnus) has toothless leaves with 6 to 9 veins per side, and Common Buckthorn (Rhamnus cathartica) has 4-parted flowers and leaves with rounded teeth and only 3 or 4 veins per side. Both of the non-natives grow significantly taller and bushier than Alder-leaved Buckthorn, and their clusters have much more than 3 flowers and produce far more fruit.