Since you want a dense hedge, you should start pruning before the height you want is reached, so I would severely shorten back, by half or more, any long shoots. However, since you want to give any new growth subsequent to pruning time to harden off before winter, I might, depending on where you are located, and when your first hard freeze is usually (and on when your area stays frozen), wait to do my pruning until spring. You may get some winter die-back anyway, so better to do your shaping in the spring, when you can tell what is dead and what is growing. Next year, I would do any trimming of long shoots as I see them, keeping to your rough shape while allowing for growth. It may be "slower" to establish the shape roughly now, and keep it roughly trimmed, but it will grow up much more densely than if you let shoots grow to the height you want and then cut them. I would do a lightish clipping several times a season, to establish and keep the shape you want. Just remember that hedges need to be narrower at their tops than at their bases - shaded branches are MUCH more apt to die out on you. The ultimate shape can be with a square top or with a rounded top, but the sides should slant down to the ground, growing wider as they go - it can be a very slight taper, but it MUST taper.

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.

They bloom fairly early here (CT) and after the color fades, all that's left is gray/silver the rest of the season. I don't have them in front but even behind other things I'm not happy with them. They keep their form but I'm really not impressed with how they look once they pass their prime. Given my druthers, I'd plant something else, maybe stokesia, in the same spot. Just my opinion--others may weigh in in their favor.

I have two of the blue sea holly out front. So far they are "interesting"... but not impressive. I am not impatient enough to rip anything out right now (the weeds even have a reprieve) so I'm hoping they get a bit more exciting eventually. Right now they are more invisible than anything. And they appear to be attracting.... flies. Oh well. They don't seem to stink or anything. :)