Family: Dogbane Family (Apocynaceae)
Origin and Distribution: Hemp dogbane is a native of North America. Currently, its range extends throughout the continental U.S. including all of the counties in Ohio. Hemp dogbane is a weed of roadsides, thickets, open woods, wastelands, pastures, old fields, and cultivated fields. A recent survey of Ohio growers rated hemp dogbane second behind Canada thistle as troublesome perennial weeds in corn-soybean rotations. The species prefers gravelly soil and it often grows in moist habitats.
Plant Description: Hemp dogbane is a stout perennial with a woody stem that is undivided at the base but becomes much-branched in its upper half. All parts of the plant exude a milky sap when cut, broken, or crushed. Hemp dogbane can be identified by its leaves, which are 2 to 6 inches long, opposite (2 leaves per node), elliptical, and smooth edged, and its small flowers that are greenish-white and form in clusters located at the ends of main stems and primary branches. Fruits are distinctive 4- to 8-inch-long pods that generally occur in pairs. Hemp dogbane reproduces by seeds, but the primary mode of reproduction of plants in agronomic fields is vegetative by way of creeping roots.
- Root system - A characteristic of the root system is the ability to spread by way of horizontal creeping roots.
- Seedlings & Shoots - Young leaves appear yellowish green with a powdery bloom beneath.
- Stems - Stems are erect, slender, woody at the base, and marked with discontinuous purple lines. Stems are covered with a fibrous bark and exude a milky sap if cut or crushed. Although stems are undivided at the base, they become much-branched in the upper half of the plant. Plants are usually less than 4 feet tall.
- Leaves - Leaves are 2 to 6 inches long, opposite (2 leaves per node), and smooth-edged. Leaf shape resembles a slightly flattened egg. Leaves attach to stems by way of short leaf stalks (petioles). The underside of leaves may be finely hairy but the upper surface is usually smooth and pale or bluish green.
- Flowers - Hemp dogbane flowers are small, bell-shaped, greenish-white, 5-petaled, and arranged in dense clusters. Flower clusters are located at the ends of main stems and principal branches.
- Fruits & Seeds - Fruits are reddish-brown pods that are 4 to 8 inches long, sickle-shaped, smooth, narrow, and they generally occur in pairs. The brown seeds are thin, flat, and have a tuft of silky hairs at one end.
Similar Species: Spreading dogbane (Apocynum androsaemifolium) is a perennial that also spreads by underground creeping roots. It is distributed over the same general area as hemp dogbane but prefers drier habitat, is generally not as common, and is usually smaller. Spreading dogbane flowers are pinkish-white and form in clusters located on main stems, at the ends of principle branches, and at stem nodes where leaves attach to stems. They are usually larger than hemp dogbane flowers, which are greenish-white and form in clusters that are always terminally located at the ends of main stems or principle branches. Hybrids having various combinations of these traits as well as intermediate characteristics commonly occur. Common milkweed (Asclepias syriaca) and hemp dogbane share such characteristics as creeping roots, opposite leaves, and milky sap. However, young leaves of common milkweed are finely hairy while those of hemp dogbane are nearly hairless. Also, stems of hemp dogbane branch more than those of common milkweed, flowers differ in color, and pods of hemp dogbane are long and narrow while those of common milkweed and much broader.
Biology: Flowers are formed in June through August. Hemp dogbane shoots produced by creeping roots emerge from late May to mid-June. Creeping roots of a 2-year-old hemp dogbane plant were observed to grow 14 feet deep with a horizontal spread of 20 feet. Because plants spread by way of creeping horizontal roots, hemp dogbane plants often arise in patches.
Toxicity: There are no reported cases or direct experimental evidence of poisoning in humans, but hemp dogbane is considered by many to be poisonous to livestock. Several compounds possibly toxic to humans and other animals have been isolated but little is known about if or how they contribute to the toxic nature of the plant. There are reports of horses, cattle, and sheep poisoned from eating green or dried hemp dogbane. At one time, it was considered that 0.5 to 1 ounces of green or dry leaves could kill a horse or a cow. However, recent evidence suggests that this report was the result of an error.
Facts and Folklore:
- The fibrous stems and roots of hemp dogbane were used by Native Americans to make rope and clothing and they used the tough bark like hemp in basket making.
- Patches of hemp dogbane are capable of doubling in size during one growing season.
- Hemp dogbane is generally a problem weed in soybean (where data indicates that about 3 hemp dogbane shoots per square foot reduced yields up to 90%) compared with corn (where 10 to 15% yield losses have been reported).