Eryngium planum is a herbaceous perennial that is hardy in the US in zones 5-9. It grows to a height of about 3-4 feet tall and spreads about 16-24 inches wide. Eryngium is native to the Alps, Jura and the Balkan mountains. It can be propagated by division, seed and root cuttings. Propagating it by division may be difficult because of the tap root.
I purchased mine (packet of 3) at a Wal-Mart garden center for $3.34 because of the striking blue flowers on the package. I made the mistake of planting them in partial sun instead of the full sun the package recommended. BIG mistake, they flopped over onto my Heucheras and spilled into the walkway reaching for more sun and I ended up having to stake them up.
Sea Holly seems to be a very easy to grow plant and is a good plant for a low water garden. I've not watered mine since the first two weeks when I planted the roots back March so they've survived on rain. The colors of the flowers changed from a silvery gray to a really nice blue, but it still wasn't as blue as the photo on the package nor do my flowers have as many bracts.
This spring a few relatives who were visiting commented on how "it smelled like feet" when they entered the house. It wasn't until the day that I went to stake them up that I discovered the source of the smell. This plant has a pretty bad odor and I don't know why any of the gardening websites that profile and recommend it fail to mention this. Maybe it isn't so bad if you have a large garden but in my tiny urban plot right up against the front steps it isn't a good quality to have in a plant because you can't escape it.
Once the origin of the smell was discovered it made sense to me why these plants was covered in flies during the middle of the day. On the plus side it does attract a lot of different bugs like a hover fly, webworm moth and the occasional red admiral and some bees. But they don't really make up for the swarms of flies in a small garden.
Perhaps I just have a bad cultivar and other Sea Hollies don't smell but if they do I don't understand why it is such a popular perennial to grow for cut flowers and why the Elizabethans considered this plant an aphrodisiac. It is a nice looking plant but it probably will end up being pulled before the summer is over.
(sorry if this goes out to you twice with this update but see Jodi's comment in the comments section)