Plants under trees . . . some of the issues. Where I live the native Oaks shouldn't be watered, hence don't plant under the Oaks if you plan on watering. The other issue I've run across is that trees create shade, often deep shade and not much will grow in deep shade. I wouldn't worry too much about robbing the tree of nutrients, they have deep roots and you can always feed them if they are fruit trees. I have a Meyer lemon that I have been growing under for years. The thing provides lemons for three families and I rarely feed it. Nastutiums are wonderful, they are pretty and you can eat them too! I agree it's a nice combination.