Carpentry and Woodworking - Mitered top boards on planter box

I have made two planter boxes recently, one from pine and one from cedar. Both have coat(s) of the exterior polyurethane on them. The top boards are 1x3s on the pine box and 1x4s on the cedar one. The mitered top boards are perfect as these leave my shop but within a few days, following lots of sun and/or lots of rain, the boards seem to shrink and there are gaps at the mitered corners. I am very new to woodworking and need to know what I am doing wrong. Thanks!!

The wood may have had a high moisture content and when exposed to sun/heat it shrunk. Also it doesn't hurt ot use ext wood glue on the joints.

Wood movement in outdoor stuff, especially in a mitered joint, can be nearly impossible to prevent. Using an exterior wood glue, such as Titebond 3 (2 will do, but 3 is water*proof*) or a polyurethane glue may help.

In outdoor planters, I'd personally use a butt joint instead of a miter, much less likey to cause seperation problems, but not foolproof. You probably didn't do anything wrong other than putting it outdoors, lol.

Wood will expand and contract depending on the relative humidity, regardless of how much sealer (the poly) you slather on. Heat, especially direct sun, will do it as well. Exterior grade poly (spar urethane) is made to be flexible, so it doesn't crack and peel as the wood expands, so you made a good choice.

Cedar is a good choice for outdoor stuff (don't get too hooked on the color, it will fade). So is white oak and black locust, but one is 'spensive and the other's hard to find. They are all rot resistant.

Use joints that allow for expansion and contraction. This is inevitable in a situation such as you have described. Actually, it's inevitable in almost any pure woodworking situation. However you attach the miters, be it with dowels, biscuits or such, always glue the attachment fitting, and not the joint itself. (the buscuit or dowel). This will ensure that the fitting that is providing the strength for the joint continues to do so while the joint retains the ability to expand and contract, and move with changes in humidity.

You can use some sort of ironclad fastening system that will not let the joint flex, such as space age glue, cement coated staples and waffle connectors, and your miters will look great while the rest of the wood warps out of shape. Design your joints with some flexibility and they will look good and serve you well. It's not a bad idea, also, to use a polysulfide sealant to fill gaps for cosmetic purposes. This is not something you will likely find at a Big Box Home Improvement Warehouse, be it Blue or Orange. And if you do, the employees likely will not know what the heck you are talking about. It is the sealant that is used on oceangoing yachts and vessels to bed and seal the joints between teak deck boards, among other things. It's not that much more expensive than anything you will find in a home improvement warehouse, but well worth it! If it can stand up to the rigors of crossing oceans, being constantly exposed to flexing, Harsh UV sunlight, saltwater exposure as well as oxalic acid deck cleaners and sanding, I know from experience that it does just great in the joints of a backyard planter.

Just remember: The strength of the joint comes from the joinery, not the sealant or glue. let the wood move, while keeping its shape.

Thanks for all the great advice! I appreciate the help.

have you tried sealing the open grain at the ends of your miter before you put them together?

Most wood glues would have sealed it plenty, and if you sealed the end grain before glueing, the glue won't bond, seems to me.