Waiu-atua is the sole New Zealand representative of a very large genus that is found in many parts of the world. Euphorbia is roughly characterised by two major growth forms; the succulent species that achieve such a fascinating diversity over Africa’s many landscapes (although they do occur elsewhere), and the more conventional1, mostly soft-stemmed herbs that we sometimes see as weeds in gardens. The latter are usually referred to as milkweeds, because of the milky sap that all Euphorbia possess.
The succulent species can become trees with intriguing forms that bear more than a passing resemblance to cacti. However, our Euphorbia glauca is a creeping herb that normally grows to 60cm or less (though it can reach a metre tall). Its name commemorates a Greek physician from c. 2000 years ago, Euphorbos, who was said to be the first person to adapt the sap to medicinal purposes2.
There are few native plants that are as useful for establishing contrast within plantings as Euphorbia glauca, an increasingly rare plant of our coastal ecologies. A major factor in waiu-atua’s appeal is signalled in its botanical name – the epithet ‘glauca‘ refers to the glaucous (greyish-blue) hue of the foliage. Older leaves can also assume a magenta colouration, especially under cold conditions.
Partnered with its upright habit and distinctive leaf arrangement, E. glauca‘s striking colour makes it a superb garden plant. This has been recognised in recent years, as it has emerged from horticultural obscurity to become more visible in our gardens and public plantings. Such attention is valuable, as cultivation has the capacity to make New Zealanders more aware of our threatened species (like waiu-atua).
Although Euphorbia glauca is easy to grow, it is the kind of plant that can be a little tricky to grow well. I have found that two major considerations need to be observed in its cultivation. Firstly, the longest stems should be trimmed back periodically to prevent plants getting an overly leggy appearance. The second is that, although it grows on cliffs and sand dunes within nature3, it should not be allowed to dry out excessively within cultivation. This tends to result in sparse growth, and can lead to plants eventually fading.
In nature, it is essentially a plant of open places. Accordingly, it does not enjoy close competition from other plants within cultivation, especially where it may be overshadowed. It is sufficiently frost-hardy to be grown in lowland areas throughout New Zealand.
The major threat to Euphorbia glauca is the impact of browsing mammals (such as sheep, pigs and possums), as proven where areas have been fenced to exclude them4. Interestingly, if it does not have to deal with the depredations of animals, it is able to survive within the dense coverage created by marram grass – an exotic sand-binder that was introduced to deal with coastal erosion, and is now a threat to our coastal ecologies5.
E. glauca is native to both of New Zealand’s main islands, Stewart Island and the Chatham Islands. Like so much of our flora, it is found nowhere else in the world. Depending on where it occurs, waiu-atua shows differences in form – particularly in the colouration of the stems (which are typically reddish, but remain distinctly greenish-blue in some forms), and the stoutness of the foliage (plants from Auckland seem to have a particularly robust appearance).