I have lots of Eryngium alpinus plants that I grew from seed, is it OK to divide them, some are 3 years old?

There are two good reasons to be careful in dividing Eryngium. One is that they are tap rooted and you might not manage to divide them. If you have several then lift just one and try it. The second reason is the spiny leaves - sea 'holly'!

As Bulbaholic says they are spiny but they do divide well, plant into a prepared palnting hole and water well. now is the time to do as they are just coming back to life. enjoy the lovely display later in the season.