The beds are filled with a free-draining potting soil, and the wood at the bottom of the planters is not touching the wood of the deck, it's boosted up on 2" steel frames. This allows water to drain out of the bottom of the beds and through the deck boards onto the ground below. Air circulation between the planter and the deck allows it to partially dry between waterings.

The wood at the bottom of the planters will eventually rot out, but the spacers slow the process.

Thanks so much for the reply and in-depth information. I really appreciate it!