Concrete blocks for garden beds

I am planning to build raised vegetable garden beds. One option is to use manufactured hollow concrete blocks. The beds will be 1.4 m (4 ft) by 10 m (30 ft). the standard blocks are nominally 200mm (8 ") high so I need two courses high. This will be built on a site that has a gentle slope, the area for the beds will be levelled beforehand. Drains will be built to prevent ground water from coursing through the vege garden. We don't have earthquakes, snow, floods, tornados or hurricanes.
The soil is dark and fertile but rather heavy with quite a high proportion of clay. The inside of the beds will be filled with this soil amended with compost etc. The blocks come in nominal 100mm (4") 150mm (6") and 200mm (8") widths. The price of the blocks is roughly proportionate to the width.
My question is what kind of construction do I need to make these beds stable while not spending time and money over-engineering them? I don't mind the odd small crack here or there but the walls must not fall over.
As I see it the walls would be stretcher bond but could be: 1) laid dry 2) mortared 3) reinforced 4) mortared and reinforced
Obviously the cheapest will be 100mm blocks laid dry but I wonder if they will be stable. Mortared and reinforced 200mm will be very strong but also rather expensive. What is by best compromise that is stable under these conditions for the least cost?
David
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Lay up the courses and pound 3/8" rebar "hairpins" or "candy canes" through the holes into the dirt every couple of feet. Pass the ends of the rebar over the intersection of two blocks, through the holes and into the dirt.
This will make it stable enough to keep the vegetables from escaping, but if you want to enlarge or remove, you just yank out the rebar and unstack the blocks.
Tsu
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I don't see any problem with you rplans so long as there are no invasive trees or shrubs within 200'. That minimal height should cause no instability problems. Reinforcing rod would help stabilize stress points, as at walkways. Also barring any locally invasive weeds or grasses, this should work ok. Otherwise you might need a cement barrier below ground for 12-18" to keep out the invaders. Nearby trees, look for another place.

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Dry stack, pound in 5/8 rebar down flush or lower then top, fill all holes w/ dirt. To make more stable, mix in 10% cement / the dirt. But just soil is good for two courses.
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Howdy folks, On the production side of the garden bed, run some weed cloth down the inside of the block to prevent some of your weeds like Bermuda grass that propagate underground from taking up residence under the blocks or in the cavities or else you'll be fighting them forever. On the outside of the walls holding a garden bed, run some weed cloth flat, and cover with a heavy mulch to keep the weeds away from the wall, and save on trimmer string. This is more of a gardening tip than a construction tip but you do want to do it when you build the wall so it is a consideration during construction.
Steve Coyle www.austingardencenter.com
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I made raised beds with cinder blocks a couple of years ago. I originally put them in sans mortar with the rebar as mentioned. They kept falling over whenever I worked the beds. I then put them in with mortar on a thin footing. I'm not very adept at this sort of thing, but the beds are still up. I filled the holes in the blocks with some gravel to within an inch or so of the top and topped it off with the left over mortar and cement. I garden in a cold climate and I was worried about frost heaves, but no problems so far. Only drawback is that I can't put my supports next to the bed walls like I could with the dried laid walls or wood walls. A small inconvience when compared with the savings of time on repairing the beds each year.
David Hare-Scott wrote:

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On Wed, 16 Jul 2003 17:53:13 -0600, AF

I made a raised bed at least five years ago with two courses of blocks. I merely leveled the ground they would rest on, as well as I could.
I used no mortar, rebar, or anything else.
I've had no trouble with them.
vince norris
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