I live by the ocean and have a large backyard area with River rock I'm looking to build a raised bed on top the rock and I've been doing my research. I do think 18 inches is fine because Gardeners Supply sells these super high bed kits and they are described as having a false 10 inch floor so it leads me to believe the plants only need 10 inches. The good news is that most county recycling centers have free compost so thats where I'll be going to shovel as much as I can into my husband's pick up the organic matter for my raised bed. Im still unsure if I should mix the compost with top soil since top soil is a pretty ambiguous term for "soil that came from the top layer somewhere".

Kelly

how to do this for REALLY low cost

Consider wire mesh fencing in circles, lined with landscape fabric to hold the soil in place.

Circles are the most stable when filled.

Yes, the compost from the landfill is a great way to go for very cheap fill. Just be really careful with your mix and *don't* compact it too much -- let it stay light and fluffy. I blew it on my first raised bed 1) Too much chicken poop in the mix and 2) packed it down too tightly. Resulted in an inferno of a raised bed -- the temperature was so high that it killed all of my first transplants and I literally had the red worms I'd added trying to jump out of the bed. (There I was, frantically scooping them up and tossing them out into the hedge muttering "Sorry, sorry, sorry!")

If compost is still producing heat, it is not ready for planting. Wait until the compost has cooled.

I'm not sure that I'd use landfill compost for edibles, you never know what people throw in there. Look around for compost that was previously used in the food chain, standards are much higher. Near me we have a mushroom farm that has loads of used soil that was used up growing mushrooms, but is perfect compost for use in veggies. You may be able to find something similar.

Definitely line the wood with plastic. It will last longer. I did that with my new raised beds last year even though I built them from 2x6 cedar.

If you get very wet weather, you may find the raised beds won’t drain well since they are sitting on concrete. You could elevate them ever so slightly so that the bottom boards are not flush with the concrete and use landscape cloth to contain the soil. You may have a mess on your patio though that you’ll need to hose off.

Unless you are growing carrots or daikon radish you may not need 18” - my beds are 12” deep (and nothing I grew last year had roots that went deeper, even my carrots). For the deep rooted veggies, just use some of the deep Rubbermaid totes (drill holes in the bottom) if esthetics are not that important. Or build some deeper large containers from lumber for those so they match your larger beds.

My daugher’s 2x4 bed.