09/02/2026
When is a thistle not a thistle?
Have you ever met a plant nerd who wasnāt fascinated by thistles? I mean, who isnāt attracted in spring by ground-level rosettes of crinkly young leaves, their sculpted margins bristling with glistening spiny tips? Thatās in spring. In summer, as the stems elongate and the leaves arrange themselves to maximize light interception, the viewerās focus naturally moves upward to flower buds and ultimately to the thistleās spine-protected flowers. There is something satisfying in studying how this formidable architecture protects its stems so that it can safely produce flowers.
But wait! Are we talking about thistles, or something else? Properly, thistles are members of the sunflower family (Asteraceae), but to ward off feeding by herbivores all kinds of unrelated plants have adopted a similar prickly lifestyle. In fact, spiny leaf margins are exceptionally common in plants. Thistle-esque leaves is an adaption that shows up in a huge range of different families. The common denominator is generally aridity: with a limited water supply, plants are less common in the landscape and leaves are comparatively small (to reduce water loss). In these places, browse pressure is often intense. Both Acanthus sennii (Ethiopian acanthus) and several ungulate species including the Walia ibex, a wild mountain goat, are native to drier areas in the Ethiopian highlands. So, the pressure is on.
Acanthus are members of the bearās breeches family (Acanthaceae). In evolutionary terms, the branch that includes Acanthaceae diverged from that of Asteraceae (sunflowers, thistles, etc.) about 100 million years ago. Even so, for the Ethiopian acanthus you would have to look at leaf arrangement to see a difference (opposite vs. alternate in thistles). That is, before flowering, you would have to look to the leaves. In bloom right now, in the E. H. Lohbrunner Alpine Garden, in the African section (where else?), there is no question that this spectacular, bird-pollinated Ethiopian subshrub is no thistle.
