The interlocking blocks are only truly functional if they interlock - if you don't interlock them, they don't have structural integrity. So that negates the ability to leave "gaps" between them. Using natural rock to retain the terraced beds can work, however, and does allow for some spaces or crevices between the rocks where pockets of plants can be sited. But they are more expensive, very heavy if of an adequate size and it looks like access is fairly limited. How 'bout just planting at the wall edge with plants that will drape and cascade a bit? With the lushness of your summer garden, it looks like this will be a sufficiently shady location for ferns, etc.

At least on the ones I've looked at, each block only interlocks with the blocks above and below, rather than the ones next to each other. This means they could be spaced out with small gaps between them within a row without compromising the integrity of the wall, as long as the gaps are not too wide or numerous. I guess my main concern is about putting just soil behind the wall without the gravel and landscape fabric that are standard for this kind of construction--will it cause the wall to deteriorate, crack, crumble, etc.?

They do interlock side to side as well because each course is staggered over the previous one. Gardengal is correct as usual.

If you do what you have described, it will be a jumble of blocks that will cetainly move about and become very messy. She also suggested using natural rocks instead. They will look fine.

If you were to build a retaining wall and not use drainage stone behind it, it will weap soil with the water between the cracks. Even if it does not move the wall, the face of the wall will always be dirty. I see these fairly often. They are very noticeable in an unpleasant way.

If budget and planting is your goal, I'd recommend following Gardengal's suggestion of making formations of rocks. The existing planting style will look great with rocks.

At one point somebody posted about a concrete block retaining wall system that had built in planting pockets. I tried a few searches but could not come up with any hits.

- Brent

I bid a job using Loffelstein blocks about 10 years ago.
http://www.kulucrete.co.za/body_loffelstein.html

Pavestone makes something along the same lines:
http://www.pavestone.com/contractor/wall_alpenstein.html

I'm not a fan of- well, anything about Pavestone, the product or the company- but I guess if you won't see the block behind the plants then it doesn't really matter.

Dave