I bought one at the nursery, haven't started any from seed. Actually, mine doesn't seem to have any seeds that are fertile. I have never had any seedlings, though it is one I don't deadhead.

Sorry, I can't remember the name of the variety, but it has a lovely, rich blue cast to the foliage. It often looks frosted on the leaf tips if the light is right. Very striking near the green plants. I grow mine in a bed that gets good sun after 12P, fairly good soil, with extra watering of all the plants when it gets dry too long. Ground is a bit on the low side of the driveway for drainage, but not moist soil. Plant seems happy, doesn't multiply, but always is there, has nice appearance each year. It is in with bulbs and other perennials that come and go all season. That garden always has a good deep layer of oak leaves each winter, because of fence behind.

I plan to check and find another this year. I only got one, in case it was a rampant spreader. I am leery of thistle type plants here on the farm anyway. Sure don't need more pest thistles growing! In this case, I will keep it and add more, since there has been no invasion from the seeds or roots. I do like it's looks and well behaved manner in this garden spot.

I am probably not much help to you, but I did try Sea Holly once a number of years ago. All I read about it, said they like hot, dry conditions with perfect drainage. I don't think they liked my humidity (or my amended clay soil either). I would be willing to bet that your seedlings' problem is too much water in a pot. Back off the water and let them go dry between (or at least experiemtn with one to see if this does the trick). Then get them into the ground ASAP. The variety I got was nothing to brag about, but pictures of some of the new ones in Garden Magazines are breathtaking.

I have the very common type of sea holly, started from seed many years ago. I just direct seeded them in the spring. It does sound like too much water. Once they adapt, they are very easy, indestructable plants, I love the steel blue stems & flowers. They do reseed, but not aggressively for me at least, & easy to identify seedlings & pull out early if you don't like them. I've heard they don't transplant well, thankfully no one told me that before I transplanted some because they did well. I imagine once you put these in ground they will do fine. They do better in very lean soil, they can flop in better conditions.

I do and it's hard to get rid of...I've tried moving mine and the roots are VERY deep and seems I can't get rid of them all...Even though I thought I got all the roots, more came up...I sprayed with Roundup and they still came back.

I guess the old saying "can't beat 'em, join 'em" has to go here, and so they stay :)