I had one until last year. It produced thousands of seeds and every one was viable and the seedlings hard to remove. I totally dug it out last fall and had to chase its unwanted offspring all summer. It may be that some cultivars are less seedy but it's not welcome in my garden again.

Thanks for the info. That's what I was afraid of. Marg

'Blue Hobbit' Sea Holly is a very dwarf type that flowers and seeds like mad and has spread and jumped around very quickly, it's now on my you're not welcome in my garden list!

Do you think deadheading the plant would help keep it from spreading so much? Or is it like the thistle that it spreads by underground runners?

I'm sure it would help if you deadheaded, but the problem is sea holly is really hard to deadhead. The decorative bracts don't fall off after flowering. In fact the flowers are barely noticeable so it's not obvious when it has set seed. And it doesn't produce one flush of flowers that ripen all at the same time. It definitely doesn't spread underground. It forms one very large taproot which makes even tiny seedlings hard to pull.

A Globe Thistle might be a similar but better choice - I have never had one but I've been thinking I'd like to get one. Maybe someone could comment on the relative seediness of those?