I would like to create a rock garden of lilies and irises. The garden size is approximately 10 ft x 10 ft. I would like to plant iris and lily bulbs, leaving a large enough hole for future growth, and then sheet mulch (using cardboard) the unplanted areas and then cover the sheet mulch with rocks. Since irises and lilies usually spread, my guess is that the sheet mulch and weight of the rocks would prevent spreading. Would this harm any of my original irises and lilies? I haven't started the project yet. Thank you. Jane