It’s difficult to photograph tall, narrow plants, but I have done my best to represent Eryngium agavifolium. This impressive stalk stands a bit taller than me, about five and a half feet. I added E. agavifolium to the garden in August of last year and it survived the long, cold winter in good shape. It is native to Argentina, where it can be found growing on stony hills and river banks.
The eryngiums are a diverse genus with a wide distribution around the world. They are often referred to as Sea Holly, after E. maritimum, a salt tolerant species native to the coast of western Europe, the Mediterranean and Black Sea, where it grows on sand dunes. All the eryngiums are generally happy in full sun, with some preferring poor, well-drained soil.
Here’s a closer look at the thick basal leaves of E. agavifolium. The leaves are lined with impressive spiny teeth. Having reached in to remove a weed, I can attest to the fact that the spines are fearsomely sharp.
And here are the thistle-like flower heads. The flowers were attracting small insects, including this colourful leaf beetle, a Western Corn Rootworm (Diabrotica virgifera).
This spring, I added a North American native eryngium, E. yuccifolium. It makes claim to one of the best common names in the garden, Rattlesnake Master, which is apparently derived from a belief that the plant could cure rattlesnake bites. It was once a plant of the tallgrass prairie (back when there was plenty of tallgrass prairie.)
Yuccifolium is similar to agavifolium, but is smaller in all its parts, with shorter, narrower basal leaves and a slightly shorter stalk. At the moment, my yuccifolium is just a baby and only a foot tall, but growing well.
The eryngium genus offers both giants and dwarves. Here is E. planum ‘Blue Hobbit’, a compact plant under a foot tall in full flower. It has a headful of small silver-blue flowers that make more of a splash in the garden than the smooth, green basal leaves. E. planum is native to Eastern Europe, where it grows in dry places, along roadsides and rocky slopes. So far as I could ascertain, ‘Blue Hobbit’ was developed by Oregon breeder Log House Plants.
Willow House Chronicles 
I always LOVE the botany lesson. I do not know this plant. Interesting. At first glance it looked a bit like common teasel
Dipsacus fullonum, but a closer look, they are surely not in the same family. Thanks again for new things to learn.
You’re right, especially the thistly flower heads are a bit reminiscent of teasel. Fortuantly, we don’t have much of that weed around here.
You have a nice collection of them. I like best your agavifolium. I only have Miss Wilmot’s ghost (giganteum) which self-seeds a lot. Iexpected it to be quite tall (giganteum) but it is not (Perhaps what I have is not Miss Wilmot’s.
I have always meant to give Miss Wilmot a try, if only for the name! But for no good reason, have never got around to it. Blue Hobbit would likely do well in your rock garden.