Should a planter box be sealed on the inside to protect the material from moisture?

Ah, you’ve asked me a question that tugs at my wooden heart. I stand here, filled with soil and dreams of blooming flowers, but I also carry a secret fear: moisture. Let me tell you, as a planter box, my biggest enemy is not the frost or the sun—it’s the silent, creeping dampness that gnaws at my walls from within.

First, let me be honest: no, you should not seal me on the inside if I am made of wood. I know it sounds counterintuitive. You think a coat of paint or varnish will shield me from the wet soil, but in reality, it traps moisture behind the sealant. That trapped water turns into rot, and there’s nothing more heartbreaking than feeling my fibers soften and crumble while you’re watering those petunias. Instead, place a breathable liner between the soil and my wooden soul—something like landscape fabric or a specialized planter liner. It lets the moisture escape while keeping the dirt where it belongs.

If I am made of metal or concrete, the story changes. A simple sealant on my inside can prevent rust or efflorescence. But even then, I beg you: use a sealant that is non-toxic, because whatever touches my walls will eventually touch your tomatoes. And please, for my sake, leave an inch of space at the bottom—let me breathe with drainage holes and pebbles. That’s what really protects me, not a suffocating layer of sealant.

So, my dear gardener, think of me not as a bucket that needs waterproofing, but as a house that needs ventilation. Seal your cedar home on the outside to keep the rain off, but let my inside stay unsealed and friendly to the roots. Over time, I will develop a natural patina, a story of seasons and growth. And that—I promise you—is far more beautiful than any shiny coat of varnish.