Well, we just had the big tree (actually, HUGE, not big) cut down that was closest to the leach field. Although, not because of the leach field, but because of concerns it might damage the house if something broke loose. We had noticed some rot, and the tree surgeon concurred that taking it out was probably the best course. Hated doing it, but as you say, more room for gardening! And more sunlight for my roses!
