I want something low and evergreen for a section measuring 150cm x 65cm (almost a square metre). They currently have loads of Armeria maritima at Morrisons for £1 a pot. How many would I need for the space?
According to a scaled plan I've done based on a 30cm spread (info from Google), it seems I'd need ~14 plants. (See 1st pic.)
But it'll presumably take a few years (up to 5 years?) to spread out that much. Would you put up with it looking a bit sparse at first (see 2nd pic) or would you get more plants and move some of them elsewhere once they grow large enough?
The short answer is: it depends how quickly you want it covered and how much cash you want to invest! A lot of garden designers plant quite a bit more densely than is recommended to get the right 'look' at an earlier point. If they are the tiny pots, you might want to go to a 20cm spacing if money is no object.
It will spread 30cm or so given half a chance, though in my experience it tends to form a kind of clump instead of a very continuous mat, so you'll get lots of clumps sort of bobbling together pleasingly.
As the name suggests, its coastal so it likes DRAINAGE in capitals. If you're not already free draining, this is going to need a lot of stuff added to get the soil right.
I'm growing loads of this from seed for my green roof! That could be an option if you don't mind waiting and want to save some cash!