Hi Magcurtsi,
Grass won't grow in shade.* If you have it planted in the root zone of the trees and continue watering frequently you risk killing your trees.* You will either have to cut your trees down or plant a groundcover.* You might find these helpful.
http://www.treesaregood.com/treecare/trees_turf.aspx
http://edis.ifas.ufl.edu/WO017
http://edis.ifas.ufl.edu/MG089
http://www.ext.colostate.edu/pubs/Garden/02926.html
Newt
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.