Planter Bed Retaining Walls. 2

[Lisaceae]

In a southern California backyard have two foot high mortared concrete block retaining walls with footer built 15 years ago with the goal of using them as planters but were never planted or irrigated. They are in two tiers of 50 feet long divided by steps so that there are two beds that are 30 feet long and two that are 20 feet long. Irrigation lines and connecting drain outlet was dug in about 6 years ago. The bed widths are each 3 feet wide. The lower beds are flat and upper beds have a small slope which connects to neighbors backyard. Soil in beds seems to be a loam. This area of California gets at most about 20 inches of rainfall a year and there is no groundwater/water table anywhere near the surface. The walls still look good with only some mortar missing and some efflorescence.
Now homeowner wants to plant fruiting and flowering trees, shrubs, perennials and annuals. Irrigation plan is to have drip with occasional deep watering with hose as needed. The soils were dug out to see what the drainage system was like. The french drain was sitting on top of footer with no slope or the wrong slope toward drain but did have gravel and fabric. There was only one drain outlet for the two walls of 50 ft run.
So the question is: In order to have plants like this it seems to be at cross purposes with retaining wall integrity or longevity. The plants need a good rooting depth (preferably 18 inches or more), and soil that drains well but also has good soil moisture holding capacity.
Or is there some compromise for all of this that might work? I have thought about putting the bigger trees in containers, and having plants with non-invasive or shallow rooting depths, making sure soil texture is right in terms of moisture holding capacity and drainage (need help with this also), and toying with lower drain depth and depth of gravel and an extra drain outlet to get slope (but can't go too deep on lower tier because of limited slope in backyard). I wish the walls were taller and wider, but right now this is what I have to work with.
There are just so many variables so going a little crazy.
Will be re-waterproofing the backs of the walls but am still puzzling over the french drain/gravel/compaction/outlet variable, and of course how to make it all work together. Any ideas would be most welcome.
Thank you.

 

[cvg]

I would forget about planting trees in either pots or the narrow beds. the planters are far too narrow and the roots will destroy the walls. stick with smaller plants that can survive on drip only

your drain is probably adequate as a two foot high garden wall probably could survive without any drain at all. but you should try flooding it to see what the percolation rate is

 

[Lisaceae]

Can't I fit 20 gallon containers behind walls (they are about 20 inch wide by 20 each deep)?? They can survive on drip. What about shrubs or small trees with shallow/non-invasive roots and vegetables or perennials?
I will do percolation test, but seat of my pants, soil is a loam and drains pretty well. But will test and let you know. Thanks

 

[Lisaceae]

Also is there any extra protection mat or other material I can add to back of wall that might help protect it from roots?. Also just to be clear, most of the time plants will be drip irrigated but occasionally some plants may need some hose watering. If soil percs well, can I forego any sort of drain/gravel then?

 

[cvg]

tree roots extend to the drip line which is far beyond your planter. so you will have roots extending way outside of your planter bed, under the walls

 

[Lisaceae]

Yes, but doesn't this also vary with the tree species, the size of tree, and the way its pruned and watered?

 

[cvg]

actually, it extends beyond the drip line.

varies with the size of the tree and species, not so much with the way it is pruned

http://www.gfc.state.ga.us/community-forests/Where are my trees roots final.pdf

 

[Lisaceae]

Ok I understand your concern. So to be more specific, the only trees I plan to place directly in the soil behind the walls are one Cherimoya which will be espaliered, and is known to have a weak root system, and a plumeria which is a small flowering shrub without a deep or invasive root system. There might be a rose bush as well which is also not known to have a deep root system, a couple of dragon fruit vines , and asparagus. Any other trees will be containerized. Since its a 30 foot wall, there should be plenty of room width-wise within the row for the plants. These will also be on the upper tier . So the soil is much deeper here. To tell you the truth I am more worried about the 10-15 asparagus plants than the trees or shrubs as their roots are quite extensive and greedy and there are alot of plants. Don't know much about the dragon fruit so need to check into it. Also the plants mentioned above can grow in containers if you have to (except the asparagus and Cherimoya), which means the roots can be restricted or pruned.

 

[Lisaceae]

As you can see I am being a bit stubborn about the plantings. Trying to reach some sort of compromise.
Really appreciate your advice and will try to figure out percolation. Really need to know if I need a drain or not also.

 

[Lisaceae]

HELLO CVG!!!! PERCOLATION RATE IS 1-2 INCHES PER HOUR. DO YOU THINK I NEED A DRAIN?
ALSO WAS WONDERING IF YOU THINK ROOT BARRIERS WORK (E.G. THOSE MADE BY "DEEP ROOT" OR "CENTURY"?) I KNOW THEY WOULD HAVE TO GO AT LEAST 30 INCHES DEEP. THIS QUESTION IS REALLY ABOUT THE NEIGHBOR'S TREES WHICH ARE HUGE AND ONLY 5 FEET AWAY FROM THE RETAINING WALL. SO THIS IS ANOTHER HEADACHE I AM TRYING TO DEAL WITH.
THANK YOU AGAIN.

 

[cvg]

do whatever you can to stop the neighbors tree roots
perc rate is moderate, but it wont hurt to put in a drain. Not for wall stability, but to prevent the garden soil from remaining wet and soggy for a long period of time.