Failing Flower Boxes?

Why are these deck planters falling apart when they get outdoors?

I am a beginner woodworker and very handy, I think. I have been making flower boxes for our deck. They are 12 x 12 x 36 in. I make them from 1x6 white pine. My problem is my miter cuts. The cuts look good. I glue and nail the joints after sanding them. If needed, I use some wood putty, then I caulk the seams and spray paint the flower boxes. Without fail, after two to three weeks in the weather, the joints pull apart. Can you tell me what I am doing wrong? - Bill Kreitler

Chris Marshall: Without a picture of your project (the one shown here is a previous Woodworker's Journal design), I'm going to have to guess about which of the mitered joints are pulling apart. My hunch is that they are the ones that attach the long front and back pieces to the short end pieces at the vertical corners. Another guess is that you might have the long grain of those short pieces going vertically, and they are meeting up with the end grain of the front and back pieces, which are oriented horizontally. Lots of guesses, but if I'm right so far, your miter joints are creating a cross-grain gluing situation. It's inherently weak, because the end grain won't form a strong glue bond to the long grain, no matter what glue you use. And, unless a miter joint forms a tight wood-to-wood connection, glue won't add much strength or fill any gaps very well. (If you need to use wood putty for some of these joints, they aren't tight enough.) Now, put this project outside where it's receiving both the drying effects of sunlight and douses of water from rain or plant watering, and you can add significant wood movement to the equation. Both the end pieces and the long fronts and backs are going to expand and contract across the grain as water moves in and out of the wood. In the case of your planters, those could be two different directions. Paint will help reduce the degree of wood movement, but it won't entirely prevent it from happening. No wood finish can stop wood movement completely. Once things start moving, especially in a cross-grain gluing situation, the joints are going to pull apart to some degree. It's the nature of wood. You might want to consider a different joint style for future flower boxes that won't show gaps as a result of wood movement. Box joints, rabbet-and-dado joints, housed dadoes, or even basic nailed butt joints might be a better solution for you than miters. And try, whenever possible, to keep the grain oriented the same way. The wood is still going to move, but at least it will happen more uniformly within the project.