Weed of the Month: Hemp Dogbane

Hemp dogbane is poisonous to horses, with the leaves being toxic at all times. Dried leaves in hay are also toxic. The toxic substance is a glycoside that might cause digestive disturbances, diarrhea, and overall weakness.

Photo: University of Kentucky's College of Agriculture, Food, and Environment

Common name: Hemp dogbane
Scientific name: Apocynum cannabinum L.

Life cycle: Perennial
Origin: North America
Poisonous: Yes

Hemp dogbane, sometimes called Indian hemp, grows throughout most of North America. This creeping perennial broadleaf weed can reach 5 feet tall. Mature plants are woody at the base. This species is frequently found in pastures and rangeland. It reproduces from seeds and buds on creeping, horizontal roots and from crown buds of the parent plant. Hemp dogbane forms colonies of plants from the creeping roots.

Hemp dogbane has small white flowers in its terminal. Leaves and stems contain a white, milky sap. This plant has more branching than other milkweed plants, and the leaves contain few, if any, hairs.

Hemp dogbane is poisonous to horses, with the leaves being toxic at all times. Dried leaves in hay are also toxic. The toxic substance is a glycoside and might cause digestive disturbances, diarrhea, and overall weakness.

Controlling hemp dogbane in pastures is very difficult. Mowing is generally ineffective and herbicidal treatment might require multiple applications. Consult your local Cooperative Extension Service personnel for herbicidal control in your area.

William W. Witt, PhD, professor emeritus in Plant and Soil Sciences, provided this information.


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