I have some old Lily of the Valley plants around the northeast corner of my house, and they grow nice and thick around the foundation, but failed to spread into the four-five foot wide strip along the house which I intended them to. It has been three years now, and sometimes a new shoot comes up in the strip, but they do not last long. The strip is a mulched area that goes between four and five feet wide, and over 20' long, between the house and a walkway. Is there a way to to make them spread, or will I have to plant more in the area? It spreads quite well for me in nursery beds and under trees.
I had the exact same problem and I planted wild geraniums in the front. My guess was that the lilies grew towards the foundation because the stone of the foundation keeps the dirt moist and a little cooler. I noticed the lilies love to spread their roots/shoots under the pavers of a stone path as well. Good luck