Why are these new 3x5 timbers so incredibly heavy and wet?

Those 3x5 landscape timbers are notorious for being sold 'wet from the plant' because they're treated with a heavy saturation of CCA or ACQ and shipped out immediately to keep costs down. If you build with them while they're that heavy, you're guaranteed to have issues. As that water evaporates, the wood fibers shrink rapidly and unevenly. Since these are usally cut from the center of the tree (the pith), they have a massive amount of internal tension. If you pin them down now, they will likely check, split, or pull your fasteners right through the wood as they warp. Don't bother stacking them in a garage without some serious prep work. If you just throw them in a pile, the ones in the middle will stay wet and probably start growing mold, while the outer ones will twist. You need to **sticker** them. Lay down some scrap 1x1 strips of wood every two feet between every single layer of timbers. This allows airflow to hit all four sides of every piece. Even then, it might take a month or more for them to shed that initial ten or fifteen pounds of chemical juice. For a retaining wall, I'd actually suggest checking a dedicated lumber yard instead of the big box stores. The stuff you see at the home centers is often 'peeler cores'—the leftover center of a log after it's been spun to make plywood veneer. They are essentially the lowest grade of wood available. If you're stuck with the ones you have, definitely pre-drill every single hole once they've dried out a bit. If you try to drive a spike into a bone-dry timber without a pilot hole, you'll split the end wide open. But honestly, if you haven't bought them yet, wait for a different pallet or go elsewhere. Life is too short to fight wood that's trying to turn into a corkscrew while you're still leveling the first course. If they're literally dripping, they haven't had any time to stabilize at the treatment facility, and you're just paying for the weight of the water.

Back in the day, we wouldn't touch a peeler core with a ten-foot pole unless we were building a temporary fence for a goat. If you’re dead set on using those 'wet-from-the-well' timbers, skip the garage and find a shady spot out back. I used to coat the ends with some old latex paint or paraffin wax to slow down the drying. If the ends dry faster than the middle—wich they will—you’re going to see cracks you coud lose a finger in. Personally, I’d take 'em back and find some heartwood cedar or even old-school locust if you can find a guy with a portable mill. Much less fussing.

Oh wow, I'm so glad I found this thread before I headed out to the store tomorrow! I'm just getting started with my first DIY backyard projects and I had no idea wood could even be that heavy from chemicals. It sounds like a total nightmare to deal with if they start warping mid-build. Speaking of the chemicals, since they are literally dripping, does anyone know if that green stuff is safe if you have a vegetable garden nearby? I was thinking of using some for a raised bed but now I'm worried about the soil!

Quick update — I decided to take the advice about using a shady spot instead of the garage. I even coated the ends with some leftover paint like suggested. After two weeks, they’ve lightened up a ton and haven't started twisting yet. Glad I didn't just rush into it while they were dripping, or my retaining wall woud have been a disaster!