I live in northern Minnesota. I did not think 6 mil plastic sheeting would survive our winters without cracking. I checked out the cost of heavy duty pond liner, and it was rather expensive. That's how I ended up going the hügelkultur raised bed method.
Yep, I mainly use them for shallow root veggies as well. Works great for me.
Recently I saw someone post a great idea on building a small frame above the soil in a raised bed for planting long root vegetables in the hügelkultur raised bed. Makes a lot of sense to me to increase the height of just a section of the raised bed instead of making an entire deep raised bed.
Well, I have to buy my topsoil, so it makes economic sense to add an extender frame on a raised bed just for any plants that might need more soil depth than my standard topsoil 8 inches in the hügelkultur raised beds.
I think a water reservoir underneath the heavy drinkers like tomatoes would be a great setup.
Thanks for posting your pictures and walking through your build. Much appreciated.