Milkweed

Swan plant? Milkweed? What’s the difference?

Milkweed or the Asclepiadoideae is a large family of plants with only a handful available in NZ. It is the host plant for the Monarch butterfly, which lays its eggs on the plant, and then the caterpillars begin to eat the leaves.

It is named for its milky juice, which contains alkaloids, latex, and several other complex compounds including cardenolides. Some species are known to be toxic. However, do not get the plant family confused with plants such as dandelion, sow thistles, portulaca and Euphorbia species etc. – all of which also have a milky sap.

Milkweed originates on two continents – the African and the American continents. The species that come from Africa were renamed ‘Gomphocarpus’ by Kew Gardens botanists. The American species are known as ‘Asclepias’, but all are members of the Asclepiadoideae family.

Milkweed heirarchy

 

A US scientist told us that milkweed is in fact one of the oldest plants in the world, as such having a primitive reproductive system – pollinia rather than pollen.

Milkweeds are also an important nectar source for bees and other nectar-seeking insects. The flower petals are smooth and rigid, and the feet of visiting insects slip into notches in the flowers, where the sticky bases of the pollen sacs attach to the feet, pulling them free when the pollinator flies off. Bees only gather nectar from milkweed flowers, and are generally not effective pollinators despite the frequency of visitation.

Asclepias were named after Asclepius, the Greek god of healing, because of the many folk-medicinal uses for the plants.

Species in the milkweed family grow their seeds in pods which contain silk or floss, each filament attached to an individual seed. When the seed pod ripens, the seeds are scattered by the wind. These milkweed filaments or floss are coated with wax, and have good insulation qualities.

If you are interested in knowing more about the pollination of the flowers, and how the seeds develop and are dispersed, here is an interesting website:

http://www.microscopy-uk.org.uk/mag/artfeb05/bjmilkweed.html

Tests have shown the ‘floss’ in the pods to be superior to down feathers for insulation. During World War II, over 5,000 tonnes of milkweed floss were collected in the United States as a substitute for kapok.

In the past, the high dextrose content of the nectar led to milkweed’s use as a source of sweetener for Native Americans and travellers. The fibre has also been used for cordage.

It is also a common folk remedy used for removing warts. Milkweed sap is applied directly to the wart several times daily until the wart falls off. Dandelion sap is often used in the same manner.

Milkweed also contains cardiac glycoside poisons which inhibit animal cells from maintaining a proper K+, Ca+ concentration gradient. As a result many natives of South America and Africa used arrows poisoned with these glycosides to fight and hunt more effectively.

Milkweed is toxic; but it is unlikely that anyone would eat the plant because the taste is unpalatable. Fatality is possible when animal consumes one-tenth its body weight in any part of the plant. Milkweed also causes mild dermatitis in some who come in contact with it. As a result of this, the NZ National Poisons Centre has published helpful information for parents and educators so that they can be comfortable with its use.

Also, the following document is useful:

Poisonous Plants in NZ (Children)
http://www.landcareresearch.co.nz/publications/infosheets/poisonplants/index.asp