Answer: The rocks are not the ideal mulch in that they do reflect the heat (especially hard on evergreens in winter as well as during the summer) and do not rot down over time to help feed the soil. It is probably not going to kill them outright but it is just not really the best thing to use. If you could rake them off and re-use them elsewhere in the garden, perhaps as a path or patio area, then use organic mulch on your plants it would be preferable.
Too, it is surprising but insects such as termites like damp moist conditions so they are as likely to be attracted to stone mulches as to other mulches. Here is some information about deterring termites you may find interesting.
I hope this answers your question.
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