Your home and contents insurance policy will generally cover things within your property, whether they're kept indoors or outdoors.
If it's permanently fixed, like a water feature, then the buildings policy will kick in. If it's things like an outdoor dining set, barbeque or tools, the contents side will cover you. Of course, this is all within reason. If you leave your phone unattended in the front yard while gardening, you probably won't be covered for that.