Beds on concrete pads

I am considering doing this for my mother in law. Her yard has the most fierce quake grass imaginable. It bores right up through her asphalt driveway, and not just at the edges. No weed cloth will hold it back. But right out side her back door is a patio with mostly morning sun. I could put boxes on that.

My question is what are the real life experiences of people with various depths of boxes on concrete? How well do they drain and how did you compensate to handle drainage?

I remember one lady who did 6" boxes and had awful drainage problems making for a terrible mess and she gave up. But as I did not get to really see her mix or boxes I can not tell what was going on. My instinct is to make the boxes 12" (2 cedar fence boards high) and pour any sort of good draining stuff in the bottom, like I do for flower pots. And then fill enough and leave a good lip for mulch etc. But maybe I am over thinking?

My other thought is to just get her some earth boxes. They would make watering easy as well.

I have moved a few of my boxes to a section of pavement in my driveway. They all have wooden bottoms and are lifted above the pavement with 1x2s. This works quite well for me and the peppers and eggplants like the extra heat in that location.

I'm glad Camp beat me to replying. I didn't think of really smooth concrete.

Some of my boxes are on beveled 12" x 12" concrete pavers or brick bases. They drain fine.

I put a shem (piece of bark) under one edge of each of the pepper pots set on a plastic sheet on top of press board, so water could escape and drain out of the holes on the bottoms. Anything to break the "seal."

If you are going to set boxes on a patio or driveway, set "shems" under the wood frame. Even a metal washers doubled up under the perimeter. Anything to break the "seal."

If you are going for air boxes (are they the same as table tops?) per Mel, drill a hole in the center of each square and one in each corner of the box. That would be 16 + 4 holes for a 4 x 4.

One of my FIRST SFG about 25 years ago?? was on straight concrete. No barrier on the bottom, just my MM to a depth of a little over 6". We used 2x6X8', nailed them into a rectangle, filled with MM and DONE!! Worked great! I didn't have a "sealing" problem. When the MM was "too wet" it just drained out the bottom. I didn't have a problem with the MM washing out of the bottom either. Good luck!!

Thanks for the feed back.
I have a couple months before I decide what to do.

I think all she wants to grow is a couple cherry tomatoes and some salad stuff. 3X4 would easily do that, or 2 Earthboxes I guess.