Acer negundo var. interius

Native

Wet or moist. Rich soil. Stream banks.

March to May

Reddish

30′ to 50′

This is a fast-growing deciduous tree rising on a single trunk from a shallow, fibrous root system. It may form a taproot in deep soils. Large individuals can reach over 100′ in height and 59 ″ in diameter at breast height, though in Minnesota mature trees are usually 30′ to 50′ tall and up to 36″ in diameter. It is prone to disease (including leaf anthracnose, bark canker, and trunk heart rot) making it moderately short-lived, usually living about 60 years, though older individual may survive up to 100 years.

The trunk is usually crooked and often divided close to the ground into several major limbs. In forests the trunk is sometimes long and straight. The crown is broad and uneven. The branches are long, spreading, crooked, and irregularly branched. They are brittle and subject to breakage. Exposed trees usually have broken tops. Sprouts are often produced from damaged areas on the lower trunk.

The bark on young trees is thin, light grayish-brown, and finely scaly. It has raised, buff or orange lenticels. On mature trees the bark is thicker, light brown to gray, and cut with shallow, narrow furrows and narrow, rounded, intersecting ridges. The ridges intersect every 6 inches or less. Orange lenticels are visible in the furrows. On older trees the bark is deeply furrowed and the ridges break horizontally into squarish blocks.

The twigs are moderately stout and densely covered with short, soft, velvety hairs. They are round in cross section; have white, solid, homogenous pith; and have a strong, disagreeable odor when broken. The leaf scars are small and broadly U-shaped. They have 3 bundle scars. The leaf scar wraps around the stem, the raised tips almost meeting those of the opposite leaf scar. The sap of a freshly cut twig is clear.

Terminal buds are egg-shaped, ⅛″ to 5 ⁄16″ long, and bluntly pointed at the tip. They usually have 4, sometimes 2, reddish, distinctly overlapping scales. The scales are densely covered with fine, whitish, appressed hairs. Lateral buds are slightly smaller and are tightly pressed against the twig. They are concealed by the base of the leaf stalk, and are not visible until the leaf has fallen off.

The leaves are deciduous, opposite, and 2¾″ to 9″ long. They are on 1⅜″ to 4″ long leaf stalks. The leaf stalks are usually hairless, occasionally short-hairy.

Two types of leaves are produced: early leaves, also called preformed leaves; and late leaves, also called neoformed leaves. Early leaves are fully formed and overwinter in the bud. They are the first leaves to mature in the spring. They are produced on short (preformed) shoots and at the base of long (partially preformed, partially neoformed) shoots. They are divided into only 3 leaflets and are broadly triangular egg-shaped in outline. Late leaves are formed at the end of long shoots. They are oblong egg-shaped in outline and are pinnately divided into usually 5, sometimes 7, rarely 9, leaflets.

The leaflets are 2″ to 4″ long, ¾″ to 2⅜″ wide, and oblong to egg-shaped. They are tapered or rounded at the base and taper to a sharp point at the tip with straight or concave sides along the tip. The terminal leaflet is sometimes inversely egg-shaped and sometimes 3-lobed. The upper surface is light green and usually hairy when young, becoming hairless or almost hairless at maturity. The lower surface is pale grayish-green and usually has tufts of hairs in the vein axils but are otherwise hairless. The margins are usually coarsely and irregularly toothed, with 3 to 5 teeth per side, sometimes untoothed. The lateral leaves often have 1 or 2 shallow lobes. In autumn the leaves turn yellow.

Male and female flowers are borne on separate trees, rarely on the same tree. They appear before the leaves or during leaf development in mid-April to late May. The male inflorescence is a tight, umbrella-like bundle (fascicle) of 10 to 20 flowers produced from a lateral bud on the twig. The fascicle droops at the end of a greenish-yellow, hairy, thread-like, ⅜″ to 1¾″ long flower stalk. The male flower has a yellowish-green calyx with 5 minute lobes, no petals, and 3 to 6 stamens.

The female inflorescence is a short, unbranched cluster (raceme) produced from buds at or near the branch tips. The raceme droops at the end of a greenish-yellow, hairy, thread-like, about ⅜″ long flower stalk. It is subtended by a leaf. The female flower has a yellowish-green calyx with 3 to 5 minute lobes, no petals, and a single, deeply lobed style with 2 stigmas.

The fruit is a pair of dry seed cases with papery wings attached (double samara). They occur in clusters that droop downward from long stalks. Each double samara is in the shape of an inverted V, connected at the top with the wings spreading apart at less than a 90° angle. Individual samaras (or keys) are 1¼″ to 2″ long. The seed cases are elongated and wrinkled. They are slightly connected to each other when young and separated at maturity. The wings are typically ¾″ to 1½″ long. The keys are sometimes hairless, sometimes short-hairy, green initially, turning yellowish green when the seeds are mature, then brown in the fall. They mature late August to late September and fall throughout the winter.

Saplings usually have just 3 leaflets, which may cause them to be misidentified as western poison ivy (Toxicodendron rydbergii). That plant, however, has alternate leaves.

Boxelder (Acer negundo var. negundo) twigs are hairless, green, shiny, and sometimes glaucous. The leaf undersides are sparsely hairy along the veins or almost hairless. The fruits are hairless.