There is no published number, and you should doubt anyone who hands you one. No land-grant extension source located gives a raised bed lifespan in years. The 10 to 20 years you see on product pages traces back to the companies selling the beds. What the university literature does cover is wood decay, and which species hold up in wet soil. That tells you more than a year count somebody wrote for a catalog.
The south wall of my cedar bed went soft in the seventh season. I pushed a thumb in near the dirt and it gave like wet cardboard. So I pulled that plank, cut a new one from a spare, and had the wall back together before lunch. Every other board was fine. University of Maine Cooperative Extension describes the same pattern. Beds rot one board at a time rather than all at once, and once your bed is packed with settled soil, losing even a whole board rarely brings the frame down.
Decay is a fungus feeding on wet wood. It starts where the board meets the dirt, because that strip stays damp for weeks after rain while the rest of the wall dries in the sun. Species decides how fast that goes. Utah State University Extension puts untreated fir at a few years before it rots through, while cedar and redwood resist rot on their own. That gap is the whole case for cedar bed durability. The heartwood of those species resists that decay without any treatment, so you buy years of extra life out of the same build.
A year count would be the wrong answer even if a university published one. Your rain, your soil and your lumber set the pace. The same cedar plank rots faster in a shaded corner that never dries out. One baking in full sun lasts longer, and a thick board buys you more time than a thin one. So the raised bed lifespan that matters is the one your own boards give you.
One real data point exists, and it belongs to a gardener rather than a study. Jonathan Foster, who writes for that same Maine extension service, reports his untreated hemlock beds at at least 6 to 7 years old, with no boards replaced and the frames still sound. Hemlock is not on the Forest Products Lab list, though Foster names it among naturally rot-resistant woods. He gives you a count of his own beds, not a lifespan you can bank on, and you should read it that way.
You get plenty of warning before a board quits. I walk my own beds every spring and press a thumb into the outside face, down where the wood stays wet longest. Sound lumber stays firm. A board that dents or flakes is near the end, so you can cut its replacement in March instead of scrambling in June while dirt pours into the path.
Treat any year count on a product page as a marketing claim. No extension source located backs those numbers, and the wood in your own bed will settle the question anyway.
Buy your boards off the USDA Forest Products Lab decay-resistant list. It runs to cedar, redwood, oak, black cherry, black locust and Osage orange, and every one of them beats fir in the same weather. Then budget for planks instead of beds. Swapping one board is an afternoon of work, so the frame you build this spring keeps going as long as you feed it fresh lumber.
Read the full article: Raised Garden Bed Specs Backed by Research
