You might, over time if you have to move them or lift them. Are these trees and planters sitting on the ground?

Eventually water, rain, dirt, etc, will take a toll on the wood. How do you plan to build the planters? What are you using to connect the wood pieces? Screws, nails, staples. I've had planters fall apart after several years because the staples or screws became loose.

What I have done recently was to use a deck sealer on the wood. I put a few coats and added wire supports to hold the wood stable. So far, two years, all is well and they get moved frequently.

Wood will eventually rot or warp. But you could get enough time out of the planters until they need repotting which they will eventually.

Jane

So, in general, and this is somewhat reliant on the medium you use, most of the pressure is going to be pushed down toward the ground, as opposed to the sides. This is more true with softer / more compactable soils, as grittier/sandier ones will push slightly more to the sides. I used pressure-treated 4x4 lumber. There's some controversy out there about pressure-treated, and the old stuff (CCA) had arsenic. But for the last few years, it's been replaced by many safer alternatives, Copper Azole being one. Anywho, I attached the cedar planks by means of a dado cut (groove) in the 4x4s. Because I'm a nerd, I ran simulations in Solidworks to see what the effects on the structure would be over time, and it looked pretty good. But, I'd go for deck screws over nails / brads. As jane alluded to, you want to build this thing much how you'd build a deck, although I didn't use any sealer because I'm growing edibles, and there's some controversy about it.. and I'm a bit lazy ^_^ Sorry, I know this is more info than you asked for, but my last piece of advice is to now cheap out on the cedar, I've used dogear wood planks in the past (intended for a fence) and got a pretty severe amount of warp. I like the wire supports idea, jane!

The design is a primary factor. You can build a box that will fall apart the first time you try to move it, or you can build one that can be moved regularly from place to place and hold a half ton of soil for many years.

Al