There are no rules persay, but one thing you should keep in mind is what is going to be the
plants mature size, and will it crowd out, the surrounding
plants. Also keep in mind the mature size of any
trees that you may want to
plant. Remember the
tree will more then likely outlive you, so it will be there for quite some time. Think ahead about how close it is to a house, garden, powerlines, ect... Roots are also something to think about. I have seen lots of nice lawns ruined by people planting maple
trees. As the roots mature, they sap all the moisture out of the soil, and compact it as well, making it very hard to grow grass uner them.