Here in Southern California, we have two tiers of mortared concrete block retaining walls with footers built about 15 years ago. They are only two foot high, with what looks like loamy soil behind them and soil beds that are three foot wide. Upper tier has a slight slope upwards connecting to back neighbor's yard. The tiers are about 53 feet long but divided by some steps into two 30 foot long beds and two 20 foot long beds.

The original goal was to put plants in them and irrigate (Irrigation lines were dug in about 6 years ago). However they were never planted. So walls still look good with a little mortar missing and a little efflorescence.

Soil was dug out to see what the drainage system was doing. There was a 3 inch french drain pipe sitting on top of the footer with no slope or wrong slope. However there was gravel and fabric around it with what looked like a thin hot mop type of paint on back of walls as well as a plastic covering it. Pipe ran under steps to connect both sides of tiers and only one outlet at one end, so pretty long run. Pipe from upper tier connected to lower tier pipe at the outlet site.

Now would like to plant fruiting and flowering trees, shrubs, perennials and annuals. As this is southern California, we get on average about 16 inches of rain a year. The irrigation plan is to have drip irrigation, with occasional deep watering with hose as needed. There is no groundwater here as it is inland from the coast about 15 miles.

However the plantings and their root systems seem at cross purposes with the principles of longevity and integrity of retaining walls. The plants will need a decent soil depth, preferably 18 inches or more, from one -three feet of width depending on plant type, and a soil type and watering scheme that will provide good drainage but also good water retention in the soil. I wish the walls were taller and wider, but this is what I have to work with right now.

Trying to reach a compromise in my head, I have toyed with containerizing the trees, and choosing other plants with non-invasive roots, and various modifications of soil texture to get the right moisture (need help with this), as well as different drain/pipe/gravel/outlet depths, widths, etc. to accomodate the roots. Also how compacted does the soil have to be and how would this affect the plants. But this is a little above my head as the science and logic is complex or even lacking out there, especially in this very site-specific situation.

Can possibly core another drainage pipe outlet on other end but can't go too deep under lower tier on this end because of limited slope in yard, (unless do a leach pipe?). On the other hand don't anticipate a lot of water coming out of pipe but really don't know how to figure this out either. I will be re-waterproofing the walls also with accompanying protective membrane. So if there is no need for the french drain pipe, then what if any are the drainage alternatives?

There are so many variables so going a little nuts. Any ideas and help to how to make this all work together or strategize would be greatly appreciated.