Hi, I assume you are thinking of using the rock as a mulch? If so, I would not do that. Not because the rocks will heat up, but because, once in place, rocks can be a nightmare to work around. They work fine in a permanent planting like with trees and shrubs, but if you are growing perennials and bulbs, the rocks will make it very difficult to dig in your garden, divide your perennials or plant new bulbs. I would use shredded bark mulch as a mulch because it works well and you can pull it aside easily if you need to dig or plant. You do have to replace some of it every year or two, but at least it's not difficult to remove if you have to.