Answer: The roots will grow in both directions. They will probably reach the rocks. This is not necessarily a bad thing. Rocky soil usually drains well which is important for butterfly bush -- in too wet a soil it tends to suffer root problems and is also less winter hardy in a poorly drained location. If the soil is really well drained in addition to being somewhat shallow, you may need to water occasionally (water slowly and deeply) during dry spells in the heat of summer. I would also suggest a deep mulch in the winter, apply it in late fall. Good luck with your butterfly bush!
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