Climbing Milkweed

Climbing milkweed is an evergreen vine with subtly colored but attractive flowers that can be showy en masse.

Climbing milkweed (Funastrum cynanchoides) shares many characteristics with other milkweeds even though it has been placed in a different genus. Both have unusual flowers with specific pollination requirements, with the unusual result that very few flowers are pollinated, but those that are pollinated produce hundreds of seeds. We have yet to find a mature fruit of climbing milkweed in the Reserve.

Like other members of the milkweed family, twining milkweed produces a milky sap that is toxic to most organisms and deters their grazing. Unlike many other milkweeds, however, climbing milkweed is not an important host for monarchs butterflies, being consumed by them only as a last resort. It is listed as a host for the related queen butterfly, which is more common in desert environments than in sage scrub.

Description ^{4,11,26,59,192,193}

Climbing milkweed is an evergreen perennial vine with wiry green stems that reach 6 feet (2 m) in length and twine around the stems of support plants. Cut plants bleed a milky sap. Medium green leaves are opposite, narrowly linear with pointed tips and obtuse or truncated bases; the literature reports ovate leaves as well. Leaf margins are smooth and the leaves often fold up along the mid-rib. There are sparse hairs on the leaf underside.

Several flowers are born in a rounded cluster on a peduncle arising from a leaf base. The inconspicuous calyx below each flower is green and five-parted. Five petals are fused at the base into a symmetrical corolla with five mauve colored lobes that spread outward and upward. The corolla margins are often pale and fringed with fine hairs. A ring of pale tissue circles the inside of the corolla base. There are two ovaries; their styles are free but their stigmas are fused into one large structure. Unlike most flowers, only five areas of the fused stigma are receptive to pollen; these “stigmatic chambers” are located below the surface, each accessed by a vertical slit. Five stamens form a column around the pistil with the anthers fused to the stigma. There is a stigmatic chamber between each pair of anthers. Milkweeds, like orchids, do not shed pollen grains individually, but in discrete packets (pollinia) each carrying hundreds of pollen grains. Each anther produces two pollinia.

Stamens and pistils are obscured by elaborate appendages arising from the filament column, which enclose the stamens and act as reservoirs for copious amounts of nectar. In climbing milkweed, these appendages are semi-spherical white “hoods”. Additional small pale appendages arise from the anthers and are appressed to the stigma; each has a purple-tinged V shaped slit opening outward. These appendages decorate the edge of the stigma like the corona of a small sun. The main bloom period is April – July, but July rain in 2015 extended the bloom through August.

When a flower is fertilized, one of the two ovaries develops into an erect, elliptical or tear-drop shaped fruit, 2-4 inches (5-10 cm)long; when dry, the fruit splits longitudinally releasing seeds.  Seeds are flattened ovals, less than 1/4 inch (5.3 mm) in length, attached to a tuft of long, silky hairs that aid dispersal in the wind. We have not found fruits on our plants. Fruit set of milkweeds is typically very low (1-5 %)¹⁹³ so this may not be unusual. Valois³ has an excellent photograph of a mature fruit dispersing seeds.

Classification ^2,11,44,59

Climbing milkweed is a dicot angiosperm in the dogbane family (Apocynaceae), a circumglobal family centered in the tropics and subtropics. The dogbane family includes herbs, shrubs, trees and vines. Plants are characterized by white milky sap that contains toxic secondary compounds. Leaves are undivided and opposite and flowers are symmetrical and five-parted with sepals and petals fused at their bases. There are two superior ovaries.

Milkweeds were previously considered to comprise their own family, Asclepiadaceae. Studies indicated a close relationship with members of the Apocyanceae and the two families have recently been combined. Milkweeds are distinguished from dogbanes by their often-elaborate appendages within the flower and by the fact that only one of the two ovaries typically develops.^2,59

The dogbane family includes many familiar ornamental flowers including periwinkle, oleander, Natal plum and plumaria. Other species have medicinal uses and some have been grown as a source of rubber.⁴¹

The variety of climbing milkweed in the Reserve is var. hartwegii,⁴⁸ which is the only variety known from California.^2,7

Ecology ^192,193

Note: the following information was derived from studies of milkweeds in the genus Asclepias. The similarity between flower morphology of Asclepias spp. and climbing milkweed suggests that generalization from Asclepias is justified here.

The unusual, almost fanciful structure of the milkweed flower closely regulates pollen transfer among flowers. Each packet of pollen is paired with a packet from the adjacent anther; they are attached by a bridging structure that serves to hook the bristles of nectaring insects which then pull the two packets from the flower and carry them away. For pollination to occur, however, a pollen packet must be deposited on another flower in just the right orientation to slip through the slit into a stigmatic chamber. (This has been likened to a lock-and-key mechanism.) When this occurs, the packet breaks away from its partner and releases its pollen grains, which grow down to the ovary to initiate fertilization.

This complicated but haphazard process has two outcomes. Successful fertilization involves a large element of chance and it is not common. This is thought to be one reason for the low fruit set in natural populations. On the other hand, when successful fertilization occurs, almost every ovule in the ovary is fertilized. Most mature fruits contain 200 or more seeds.

Milkweeds, including climbing milkweed, get their common name from the abundant milky sap that they produce. This sap contains cardinolides (a type of cardiac glycoside) that is harmful or toxic to many organisms and deters many herbivores.¹⁹⁴ There are, however, a few milkweed specialists that have evolved to cope with the normally toxic cardinolides. Most famous of these is the monarch butterfly. A discussion of the relationship between climbing milkweed and monarchs, is given under the Interesting Facts tab.

Human Uses ^15,17,75

We know of no use specifically of climbing milkweed. North of San Diego, both Luiseño and Chumash Indians used the fiber from related milkweeds and dogbanes for smaller cordage such as for nets and snares and some clothing. Luiseño removed fibers from the stems by boiling or irregating them to wash away the pulp.⁷⁵ The Chumash cut the stems while green. When dry, stems were rubbed to remove the fibers and the fibers were twisted into string.¹⁵

Interesting Facts  

A handful of insects, often called “milkweed specialists” have adapted to the presence of the toxic cardenolides in milkweed tissues. These specialists have evolved the ability to sequester and concentrate the toxic compounds in their own bodies, giving them protection against many of their own predators.¹⁹² Best known of the milkweed specialists are the monarch butterflies which depend upon milkweeds for their entire life cycle. Many species of milkweed serve as hosts to the monarch, providing food and chemical protection for the caterpillars and nectar for the adults.

Climbing milkweed does not appear to be an important host for monarchs. The only specific record we have found is from Texas where climbing milkweed may be an emergency resource: “when monarch caterpillars have stripped all your milkweed plants of their leaves and are still hungry, they will eat the leaves of Funastrum.”¹⁹⁵ Climbing milkweed is a host for the related queen butterfly,¹¹⁶ but the queen occurs more often in desert habitats than in sage scrub.

Although we have seen many different insects nectaring on the flowers of climbing milkweed, we have not found any grazing the foliage.