I had almost the exact same situation in my garden. 18" high up on a cinder block wall that I shared with my neighbor.

We didn't see so much white discoloration because that is usually associated with leeching of salt from below the soil lines. Since I imported a garden mix from a local gravel yard, there wasn't a salt problem like you'll find in native soil.

There aren't too many plants that would do well against a cinder wall in the summer, but with the fall/winter season approaching you might be able to plant right up on the wall.
Also, a watering system that drips water to the base of the plant will help reduce this issue. I don't know what you're planting in this bed, but chances are that you'll need room away from the wall for the plant to grow up and spread out.

A dripper near the base of your plant, or a deep soak less regularly, may help your problems too.

I am thinking of also building raised beds next to my block walls. For years we had a wood fence with Oleandors. All gone now. Couldn't I paint the block wall up to the height of a raised bed? Seems like there should be a masonry paint that would resist moisture.

Well I will be the voice of dissention and suggest you go with your original instinct and install the interior 18" wall. I say this because you can never tell just how strong or weak those block walls are around here. You need to be absolutely sure the wall can take the extra weight before you put it there or else you are looking at a very costly repair. Reinforce the interior and exterior walls with rebar.

As far as the water stains, I thank you for thinking about your neighbor (perhaps you can discuss this with my neighbor, they apparently have no regard...) Anyway, it is easy enough to put an emulsion on the block wall that will water seal the thing, then you dont have to worry about it. Home Depot will have the stuff, just ask someone there to show you.

I suggest tar for a lining. Definately line both walls with it. Inside of the walls of course where it will be covered by dirt. Nothing else will be permanent. It comes in a big bucket from home depot. It is cheap and it works like nothing else. If you don't use something you will have leaching from water and salt. I even had some water draining out through the cracks. We used the tar our second time around and it is a dream. It is messy to do but you won't regret it. Water leaching into the wall can also weaken it.

I have had two raised beds of this kind and have been extremly happy with them. My only other suggestion is to consider more than 18". About 3' can give you more depth to layer your plantings. Put big plants in back and small ones up front. You would even have room for a dwarf tree maybe.

Good luck.

I am planning exactly the same thing and have been entertaining bids from local contractors (it is entertaining). Yes you need to have the interior retaining wall if you current cinder block wall is not a retaining wall. Most of these walls are built more like fences and only have a 6 inch foundation, this is not strong enought to hold several tons of wet soil. A good way to check and see if your wall may be a retaining wall is to see whather your neighbors yard is on the same level as yours. If yours is say higher then it is likely that your current wall may be a retaining wall already i.e. it is already built to stop your yard from falling into their yard.

Definately use the tar on the back walls and you might even consider drainage pipes coming out of the front face so that should your planter bed flood during heavy rain etc there is an escape for the excess water other than over the top of your bed.

I did the same thing you are considering during the summer, and my wall between myself and the neigbor is exactly like yours, but textured and painted with 2 heavy coats of exterior paint. The bed portion against that wall is about 40 feet long, with 16" block front (4 high by 4"), and the dirt mounds to about 20" in the back. In between the sturdier support columns and the large drip system cement enclosure I made an interior retaining support system in 2 parts, made up of pressure treated 2x6's nailed to 8' by 2' 5'8 plywood, shaved at the top, and reinforced every 2 feet with 3/4 x 3 foot rebar. It is not visible above the mounding dirt, and it is 6" away from the wall itself. I placed dirt in between but did not pack, and being unamended clay, does not really allow water to soak in thus remaining dry and semi loose underneath. My main dirt area is ammended with peat moss, sand, and mulch, and my drip hoses occupy the area between the retaining system and the wall. I do not believe much weight, if any is against the dividing wall, as I filled it after the main area was filled, and have run a 36" metal spike in there well after and it ws not compacted at all. I finished in July, and the subsequent heavy rains of the summer did not impact the wall at all. If you want some pics, email me @ pheitmeyer@imagetag.com

> I made an interior retaining support system in 2 parts,
> made up of pressure treated 2x6's nailed to 8' by 2' 5'8
> plywood, shaved at the top, and reinforced every 2 feet
> with 3/4 x 3 foot rebar.

So you're using some kind of treated plywood as well?

No, the plywood isn't treated, just the main support of the thicker lumber. I figured that once the 5/8 thick plywood started to fail, that the dirt would be so settled that the sideways forces would be almost nil.

Your ideas sound good, and also seem to add a bit of insulation for the soil. Would enjoy seeing the pictures, pheitmeyer.

What would you do for up against your house? I can't make that part very deep, so cinder blocks wouldn't be very practical. I was thinking maybe the same type of frame as my free standing raised bed- with lumber on all 4 sides to protect the house. Is this necessary or too much?

Personally, if you live in a house like I do (cookie cutter stucco and chickenwire), you'll want more than a piece of wood between wet dirt and the house. Here is what I did:
block wall (fence block) for the bed
reinforced with rebar and block wire (those little ladder looking things :) )
coated the inside of the planter with waterproof stucco
tarred the waterproof stucco
No leaks what so ever, and it has been four years. Might want to try that.

Gene.

I have flowerbeds against my wall and I haven't had much problem with the water issue (use drip lines), my problem has been my dogs running it to death trying to get the lizards that get on the wall. I've tried fencing it but they jump the fence. Now I'm trying a new approach. I'm still going to grow my vines on the wall with some protection around their base from the dogs. I'm going out about 6 ft and building 4'x8' raised beds. I'm going to make 3 to start and leave enough room to mow between them.
Hopefully the vines will help shade and color the wall.

gemfire