Since they grow 1 - 2 feet in width assume 2 ft. if you want space left between them. So plant leaving 1 ft. or more apart between each. I will often use a stick or the toe of my shoe to make rough circles, in this case 2 ft. circles. Place the plants in the centre of each circle.

I'm reading a book (Planting in a Post-Wild World) that recommends 36" with smaller ground cover type plants in between. The reasoning is that the plants' roots will compete directly when spacing them closer, and they'll grow shorter.

Oh whoops I just looked at that plant a little closer and it is a short plant and looks like it could be spaced a lot closer together than 3', like 1-2' as peren.all said.