Have you dug around them? I have to wonder if the ground underneath them is even suitable to grow anything else. It may well be so congested with roots and so dry and depleted of nutrients that it would be difficult to get anything established. And then, when you have the money to remove the trees (I don't blame you. That's exactly what I would want, too.), you'll have to worry about destroying the plantings underneath. If I were you, I would remove any dead limbs, especially if they are dangerous, and save every spare dime, including what you would spend on underplantings, till I can afford to remove the trees. And then say, "Good riddance". Good luck to you.