Answer: The only rules to follow are to add plants that have similar requirements for sunshine and water as the existing plants and 2) plant in layers, with the shrubs being the tallest, mid-sized perennials next to them and short annuals or perennials in front of them. This way your eye will see the small plants, move up to the mid-sized plants and then stop at the shrubs. It's a nice effect, especially if you can sit outdoors and admire the yard.
If it's a sunny spot, you might consider growing Echinacea (purple coneflower) next to the shrubs, then adding Thrift (grasslike foliage and purple flowers), and finishing up with snow on the mountain or basket of gold. There are lots and lots of possibilities for plant combinations.
Best wishes with your garden!
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