can sea holly plants be split and if they can when is the best time?

Ah - is this a new question, or did you just forget the word "sea" in your previous one!? If by sea holly you mean Eryngium, you can pull apart, carefully, a large plant, which hopefully will have more than one central tap root, in the Spring. Otherwise, take root cuttings.

Agree with Bamboo and if it's a species, not a named variety you can grow from seed.