OP Mike Arr, some information on other's use of dimpled membranes for garden beds. I'm a first time poster but long time listener; there was a lack of response to your query, so I registered
Short response: lots of people do this, but not in the North Americas. You had a great idea for lengthening the life of the wood, assuming a few other factors
Long Response:
Fall of 2016 I spent a month in Julbach, Austria helping a friend convert their historic family bakery into an owner apartment and 4 room tourist B&B. This is a place where the hills are alive with music - get out of the way because large farm tractors are pedal to the metal on main street doing 30 mph past the 2 banks in the village. The economy revolves around farming and medical care for the elderly (the town is dying, all the younger folks are leaving). While doing the building renovation, I got to know many of the local farmers and traditional tradesmen in construction. There's a ton of old Europe permie stories from that trip, such as hand chiseled granite blocks instead of brick for walls, but I'll get to the HDPE liner now.
Everyone - and I mean everyone - in the village had lined their raised bed gardens with dimpled membrane. It was the first time, and only time, I've ever seen dimpled membrane lining raised beds outside your photos and my beds since being there in fall 2016. I walked every street in the town, then I asked about it - because I was about to build some raised beds at my place in the US.
This farming community - inundated with snow for months - swears by it. They said it doubles to triples the life of the wood when properly done. They had much advice on how to do a garden bed as well. I'll summarize below:
Dig a foot below grade and put a rock foundation in for the wood of the garden bed. Smooth with concrete (note, they said rock because of the frost line there. I asked about only cement and the response was why 4 ft of cement? Rock weathers well. If you pour cement, dig as deep as you need to go for your climate)
Layer galvanized hardware cloth into the foundation itself to keep out rodents. I poured my cement over the cloth itself.
Put ties, plates, and braces in the cement to elevate the wooden walls of your garden bed above the foundation. Just 1/8 will do. The goal here is to create airflow and zero ground contact. Moisture and soil is the enemy.
Install best rot resistant wood available for your budget. It will rot, eventually. Its wood. The goal is to reduce the long term labor, right?
Install dimpled membrane down to the hardware cloth (1 ft below grade) and 1 inch below the top of the wood.
Do not tape off the top of the membrane. The membrane is there to evacuate moisture and create a barrier for drainage. Air flow is key. If you care about it looking ugly, put a board on top of the bed rail to hide the membrane. Its additional protection for the structure, looks nice, and you can sit on it while gardening.
Anywayshow, these steps create a raised bed wooden garden wherein the wood is never ever ever in contact with soil and minimal water aside from surface rain or snow. UV damage will probably be your worst worry.
Putting this together is work, but this is Permies
(Alternatives: brick beds, poured concrete beds, and general hardscaping alternatives.)
