Why did you lay the black plastic down? Was it to keep the moisture in or keep the roots from growing down? I have never had a garden and only just bought my first tomato plant last weekend-along with 2 dwarf fruit trees and some flowers. I have always killed my plants from not enough water or too much water. My raised bed will be a beginners bed. If I kill something I will just keep trying. Any and all tips will help meI know that Cedar is best but I have to work with what I can afford. At HD each board was 51 cents.
You can of course use different things. I've seen sand filled burlap bags and bales of hay or straw used for the side walls.
I'm not sure why you'd want to sand them. If they are rough cut lumber it won't matter. As long as there is not treatment or paint on them they should be fine. 2x 4's are perfectly okay. I used the wider/taller boards so I have less bending to do. Plus I couldn't dig the soil cuz it was hard fill and had a lot of tarvia in it. So being advised I put in raised beds. Being deeper the roots don't go down into the hard fill.
I also laid black plastic down first.
