I agree that Lucifer can be a bit enthusiastic - both in height and babies (not sure if the bulb divides or if it re-seeds itself). I am ruthless about cutting back swords that start growing sideways, and it is good to plant something directly in front of it to give it some support. I've used lemon balm, masterwort, peony, daylilly, and shasta daisy with good results. As Springcolor notes, mine get well over 5' tall, perhaps 6', and tend to flop over if given the chance. I've seen some more well behaved shorter ones, but don't have any myself. Despite its foibles, I really like the late summer pop this gives and have transplanted it to several locations so I get a lot of color from this plant. I've also shared this with lots of folks, often simply referred to as 'that red flower'.
Here's kind of a cool picture of a plant that truly filled in the gap -- one of the rocks broke way from this wall and before we got around to re-mortaring it (low on the priority list), this happy little helianthemum just rooted itself in the gap and cascaded on down. I rather like the look and don't plan to fix the wall after all.
