Monarch host plants

Have you found many garden centres and seed sources in NZ get the botanical, common names and descriptions of the swan plant family confused – perpetuating the misunderstanding as to which is which?

At first (1970’s) I was told the two varieties of milkweed on which the monarch caterpillars thrive best were the swan plant, Asclepias fruticosa and that the larger variety (most commonly referred to as giant swan plant) was A. physocarpa.

Imagine my surprise when emails to Kew Gardens and herbariums in Africa (where many Asclepias species originated from) advised that the names of these two plants had been revised in 2001 – (Goyder, D.J & Nicholas, A. 2001. A revision of Gomphocarpus R. Br. (Apocynaceae: Asclepiadeae). Kew Bulletin 56: 769-836). Dr Siro Masinde, Botanist in Charge of the East African Herbarium in Nairobi, Kenya, confirmed the names and descriptions for me concisely, and I quote:


“The current name for Asclepias fruticosa L. is Gomphocarpus fruticosus (L.) W.T. Aiton. It has five subspecies. This is the one with fruit/follicle that is ovoid, measuring 1.5-2.5 cm in diameter, tapering gradually or abruptly into an attenuate beak.” (i.e. the true Swan Plant).


“The current name for Asclepias physocarpa (E.Mey) Schltr. is Gomphocarpus physocarpus E. Mey. This is the plant with the larger fruit/follicle that is globose/rounded or subglobose, slightly depressed on one side, not beaked but occasionally somewhat angled at the apex, and very much inflated.

Similar emails were received from the Herbarium at Kew Gardens, UK, and the Botany Department, Rhodes University, South Africa – the confirmation we required.

Additional resources:



Feeding monarch larvae