How to build a cinder block raised bed

Build it

1. Choose a sunny, level spot and mark a rectangle 9 ft 1⅜ in by 5 ft 2⅛ in. Clear or smother the grass first: the site guide shows how.
2. Level a strip of ground under each wall, as wide as a block: scrape off the high spots and tamp the soil firm, checking with the level on a straight board.
3. Lay 7 blocks end to end for one long wall, cores facing up and the ends tight together. Lay the second long wall parallel to it, 3 ft 10⅞ in away measured between the inside faces at both ends.
4. Fit 3 blocks across each end, between the long walls. Measure both diagonals on the outside: each should be about 10 ft 5¾ in. Nudge the walls until they match.
5. Set the second course on the first in the same layout, each block straight on top of the one below. Keep the blocks tight: Ohio State University’s raised bed guide lists soil loss at the block joints among the concerns with block beds.
6. If gophers or voles dig in your garden, lay hardware cloth across the bottom before the soil goes in. UC IPM advises hardware cloth or ¾-inch mesh poultry wire under raised beds for gophers; Colorado State University Extension gives ¼-inch hardware cloth for voles and pocket gophers.
7. Fill the bed, and the cores along the top too if you want a row of herbs or flowers.

How much soil it takes

Filled 14 inches deep, an inch under the top of the blocks, the inside takes 35.7 cubic feet of soil: 1.32 cubic yards, or 24 bags of 1.5 cubic feet. The cores are not counted; each holds a little more.

Open this bed in the soil calculator to change the depth, split the soil into a mix or compare bags with a delivered load. At this size a delivery is worth a quote: the bags or bulk guide compares the two.

Reaching the middle

The blocks are 7⅝ inches wide, so the middle of the soil is 31 inches from the outside of either long wall, against 24 inches across the walls of a 4 × 8 board bed. Check it against your own comfortable reach (the width guide shows how to measure it). If it is too far, lay 2 blocks across each end instead of 3: the inside is then 31¼ inches wide, the middle 23¼ inches from the outside, the bed takes 36 blocks and 16 bags at the same depth.

Are concrete blocks safe for growing food?

University of Maryland Extension says cement, cinder and concrete blocks are all made with cement and fine aggregates, that fly ash is often included, and that labels do not say what aggregate a block contains, with little research on the question. It leaves the choice to you; if you are concerned, it suggests sealing the blocks with polymer paint or choosing another material.

Ohio State University’s raised bed guide lists the other concerns: the expense, the weight, soil lost at the block joints, and lime leaching from the blocks into the soil, which affects its pH. A materials comparison sets blocks beside wood, metal and composite.

What this plan does not cover

This is a plan for a two-course bed of dry-laid blocks on open ground. It is not an engineered wall and makes no load rating: for a taller block bed, or one that retains a slope, follow a mason’s or the block maker’s guidance. For a bed on a patio or deck, read the hard-surface guide first.

References

• Concrete masonry unit, Wikipedia.
• The Safety of Materials Used for Building Raised Beds, University of Maryland Extension.
• Module 10: Raised Bed Basics, Ohio State University Farm to School.
• Pocket Gophers (Pest Notes), UC Statewide IPM Program.
• Vegetable Gardening in the Mountains, Colorado State University Extension.