Ohio Perennial and Biennial Weed Guide

Family:

Milkweed Family (Asclepiadaceae)

Other Names:

cottonweed, silkweed, Virginia silk, wild cotton.

Origin and Distribution:

Common milkweed is a native of North America. Currently, its range includes the eastern half of the United States. Also, it has become naturalized in much of central and southern Europe. In Ohio, common milkweed is distributed throughout every county. Plants grow in clumps or patches in meadows, fencerows, roadsides, railways, waste places, reduced-tillage fields, and other open habitats. Common milkweed prefers rich sandy or gravelly loam soils that are well drained. It grows best in full sunlight or light shade. There are reports that the species does not grow well where boron is limiting.

Plant Description:

Common milkweed is a robust, erect perennial. Its stems and leaves exude a white milky sap if cut or crushed, which is a common characteristic of species in the Milkweed Family. It can be distinguished from other milkweeds by its purplish flowers that form ball-shaped clusters and large teardrop-shaped seed pods covered with warty bumps. After pods mature in autumn, they split lengthwise releasing numerous tufted seeds. Seed pods usually remain atop the dead stems throughout winter. The plant reproduces by seeds and creeping underground roots.

• Root System:

  The root system includes a perennial crown and horizontal creeping roots.

• Seedlings and Shoots:

  Young plants are covered with downy hairs. Young leaves are dark green, waxy, oblong, and have a pointed tip. Hairs present on the upper surface of young leaves are usually lost as the plant matures. All parts of the plant exude a milky sap when cut or crushed.

• Stems:

  Stems emerge either alone or in clusters from a single root crown. Stems are round, hollow, stout, unbranched, covered with short downy hairs, and 2 to 6 feet tall.

• Leaves:

  Leaves are opposite (2 leaves per node), 3 to 8 inches long, broadly oval, smooth edged, and have rounded to somewhat pointed tips and bases. The lower leaf surface is pale green and hairy compared with the upper surface, which may or may not have hairs. Leaves have a prominent white midrib and veins that resemble rungs of a ladder. Leaves attach to the stem by way of short, thick leaf stalks (petioles).

• Flowers:

  Flowers consist of 5 downward-pointing petals and a 5-part central crown that are purplish, usually tinged with green, and have a sweet odor. Flowers are 1/2 inch wide and form 2-inch-wide, ball-shaped clusters located at the ends of stems or stalks attached to the stem at a node.

• Fruits and Seeds:

  Seed pods are 3 to 5 inches long, 1 inch wide, gray-green, warty, and shaped like a long teardrop. As they mature, pods turn brown and split open lengthwise revealing a shiny yellow inner surface and releasing numerous brown seeds that are about 1/4 inch wide, flattened, and have a tuft of silky hair attached to one end.

Similar Species:

Common milkweed is easily confused with hemp dogbane (Apocynum cannabinum), which is a related species. However, seedlings and young shoots of hemp dogbane are smooth whereas those of common milkweed are hairy. Also, the upper half of hemp dogbane plants is more branched and its leaves and flowers are smaller compared with those of common milkweed. In addition, hemp dogbane flowers are greenish-white while those of common milkweed are purplish. Butterfly milkweed (Asclepias tuberosa) is similar in appearance to common milkweed except it has orange flowers and its sap is watery rather than milky.

Biology:

Common milkweed blooms from June to August. Seedlings do not flower until the second year. Usually, 1 or 2 flowers in each cluster mature into a seed pod. It has been estimated that a single common milkweed plant can produce 25 fruits and each fruit contains as many as 450 seeds. Seeds can float and fly and have been reported to survive at least 3 years buried in soil. Roots can grow 13 feet deep and the length of horizontal roots can increase up to 10 feet in a single season. A piece of root about 1 inch in length can produce a new plant. A common milkweed density of just over one shoot per square foot reduced wheat yields 47%. In soybean and corn, common milkweed is reported to reduce yields 19 and 10%, respectively. Cultivation can fragment and spread underground roots, which generally increases shoot numbers and population size. However, frequent cultivation or mowing depletes food reserves and ultimately inhibits roots from sprouting. Seedlings can be controlled using pre-emergence herbicide applications and some post-emergence herbicides are effective on older plants.

Toxicity:

All parts of the plant contain small quantities of potentially toxic substances. Common milkweed is toxic to poultry. Other livestock including sheep, goats, cattle, and horses can be poisoned if sufficient amounts of green or dry milkweed are consumed. Livestock usually avoid the plant unless other forage is unavailable. In humans, the plant is slightly toxic and only if eaten in very large quantities. However, it can cause contact dermatitis in some people.

Facts and Folklore:

• Linneaus used 'syriaca' when naming this species because he believed it was from Syria.

• The floss, which is 5 times more buoyant than cork and 6 times lighter and warmer than wool, has had a variety of uses such as stuffing pillows, mattresses, life preservers, and flight jackets.

• Scientists experimented with using the milky sap as an alternative source for rubber.

• The fibrous stems have been suggested as an alternative for flax or hemp.

• Native Americans used common milkweed flowers in jam and, after careful preparation, young shoots were eaten like asparagus.

• Caterpillars of monarch butterflies often feed on leaves of common milkweed. As a result, predators find both caterpillars and adult butterflies distasteful and generally avoid eating them.