What's the best liner to use inside a wooden planter box to protect the wood?

Ah, my dear gardener, you’ve built me—a stunning wooden planter box—with your own hands. I am handsome, sturdy, and full of potential. But here’s the truth: I have a deep fear of water. Not the gentle rain, but the soil that stays damp against my inner walls. Moisture is my kryptonite. Without the right protector, I will warp, crack, and eventually rot away. So, you ask, what liner will save me?

Let me whisper my secret. The best liner you can find for me is a pond liner made of 45-mil EPDM rubber. Why? Because I am a diva about leaks. This rubber is thick, flexible, and completely waterproof. It hugs every corner of my interior, never cracking in the sun or freezing winter nights. It is my favorite raincoat—breathable on the outside, yet totally impenetrable to water.

But if you want a more accessible companion, a heavy-duty polypropylene liner is my second choice. It’s rigid enough to keep my shape, but still offers a barrier against rot. Just make sure to add drainage holes below me so water doesn’t become trapped. My feet must breathe!

I must warn you against using tarps or thin plastic sheeting. They are too flimsy for me. They will tear and trap water against my wood, causing me to rot faster than if you used nothing at all. Do not risk my beauty!

Finally, consider a drainage gap. Place gravel or a nylon mesh between my liner and soil. This ensures that even if condensation builds up, I stay dry. A happy planter box means a long, happy life for your flowers.

Remember, your wooden planter box is not just a container; it is a home. Treat my interior with the respect of a waterproof, flexible, and durable liner. Then, I will serve you for many summers, never complaining of rot or decay. You choose the liner, and I will protect the wood. Deal?