Tips on designing with extra large pots
Over the years I've designed many gardens with extra large pots as the finishing touches to expansive pool decks or patios/decks, and have come to some conclusions about best practices to use when you don't intend to ever have to repot a containerized tree or palm. This advice applies equally to large pots that may be set within a landscape planted area as an accent, and perhaps set a bit below grade level to make it look more "nested". The typical small drain hole(s) on even the largest pots will over time get clogged by roots impeding drainage, affecting the health of the plantings or even cause them to cease to drain and drown if they get automatic irrigation and you're not paying attention. I've taken to cutting out an 8 inch square opening at all such pots to allow for the trees/palms to root into the ground below and facilitate drainage, as I've explained on the write-up for the photo below.
Here is a link that might be useful: Large pots and best practices