Janos, I have had L. candidum plenum , there may still be a small piece of it left in the garden. It is a slow grower for me, not vigorous at all, and has not bloomed. I know it prefers a lime/ clay soil, I have seen the single blooming and thriving in clayish soil, planted not very deep. I'll try to recall the source for the double, and email it to you. Perhaps you would have better luck with it than me. Rick