Q: My husband and I argue so much about this subject that we might be headed to divorce court! We have grown a pin oak for 20 years, although it still has a small canopy. Should I irrigate further out than the drip line or not?
A: The answer is: You’re both right!
The fine feeder roots that absorb water extend at different distances from the drip line depending on the species of tree. In my experience, a tree with dense evergreen leaves, such as hemlock and Southern magnolia, shingles water out from the canopy. This forces their roots to extend much further than a deciduous tree like an oak or maple, which might have most of their feeder roots at the drip line or a little beyond. My general advice is to irrigate trees five feet inside the drip line and a similar distance outside it.
Another instance of marital discord cured by horticultural knowledge!