Construction Process
Calculating the Floorplan
Interior Dimensions Table
Exterior Dimensions Table and Costs
Adding Depth
Suncover / Bird Netting / Frost Protection
Painting
Automated Watering and Multiple Gardens

Building Raised Garden Beds

My preferred way of building a vegetable garden is a raised bed. This is particularly true where the native soil is not particularly rich in nutrients or does not drain well. The construction described below also gives the user a nice place to sit while planting, picking, pruning, weeding, etc. and tips to make the bed look prettier. The technique also works well for accent flower gardens or planters. Even if you don't build yours this way, hopefully this page will give you some ideas for your own construction.

My preferred materials for construction is cinder blocks. One of the primary reasons is because they won't rot out like wood does. They won't warp like wood can. They don't leach chemicals into the ground like some boards will. Termintes won't get in the cinder blocks. They offer better insulation from the sun's heat. Finally, it costs less to build with the blocks than buying a pre-fabbed wood option. Only the simplest wooden gardens (4 boards screwed together) are cheaper, but I submit they are also definitely inferior. If you are going to expend the time to scalp the lawn, and consider the costs of weed fabric, dirt, plants, netting, etc. then the amount saved as a percentage of the overall expenditure is small. My cinder block gardens have last over 15 years and are still just fine.

Cinder blocks typically come advertised as 8"x8"x16" (but like lumber are 1/2 short on each dimension so they are really 7.6"x7.6"x15.6") so your width/length should be a multiple of 7.6". (More on how half block increments work later). I prefer to make my gardens between 4 and 6 feet wide so that I can easy reach into and work from either side without stepping onto the soil which can damage roots. This works well for larger plants (tomatos, peppers, egg plants, ...) since these typically want to be around 2 feet apart. So planting each row 1 foot from the edge still allows me to have two rows of larger plants. Near the edges (between the bigger plants and the edge) I can still have root plants like garlic, onions, or smaller herbs. It is possible to add a trellis on either end to support beans or other vine produce.

This an old picture that I need to update. But it shows stacking bricks in the foreground, uncapped cinder blocks on the left and painted, capped blocks on the right.

If you decide to build a raised garden using cinder blocks, be sure to read some of the watering considerations given below before starting. It's easier to run PVC earlier than later.

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Construction Process

  1. Plumbing ... if you are going to tie into a sprinkler system or have multiple gardens sharing a timer for watering, you should read the notes below and install any watering options first. PVC is cheap so this mostly adds labor on your part.
  2. Scalp the lawn ... even if you are putting weed cloth down, this is a good idea. Scalp an area that is about 6" outside your exterior dimensions so that the forms (next step) will be flush with here we want the based to be. Once we build the base/foundation, it may be a good idea to remove even more dirt from the inside and replace with better soil but don't do that up front. We need at least 4 inches of the interior for the inner form to be level with the outter form.
  3. Build forms and pour a base ... pouring a base (foundation) insures that the cinder blocks stay aligned and don't shift over time. If it slightly lower than the surrounding area it will also help prevent grass from growing into the garden. It is also makes it very easy to get all the cinder blocks abutted to each other. Failure to get tight alignment will allow soil to wash out with watering. Personally, I would never skip this step. That said, it is possible to just put the cinder blocks directly on the ground and save a little time and money.
     
    The forms can easily be built with 2x4s that laid flat on the ground so that the concrete base is only 1.5 inches thick. Note that in the concrete calculations below we assume 2 inches because we won't have the ground scraped completely flat. We certainly don't want a raised section of dirt making our base thinner, so if anything we dig the center parts inside the form a little deeper or even add a narrow center trench. We use wooden spacers to keep the forms from moving in and long nails (spikes) to keep the form from moving out while we are pouring.

  4. Place cinder blocks and anchor ... after the concrete base has dried, we can gently place the cinder blocks on the base. (Gently because concrete is still soft for days before it fully dries). It will help to have a taut string on the outter edge to make sure you get them straight. Once they are in place an tightly against each other, pour an inch of concrete in the cinder block holes at the corners of the garden. This will keep everything from shifting.
  5. Block caps and anchor ... we put the caps on to make the garden look nice and to give us a good place to sit while working. Again, when everything is in place and tight against each other, remove the outter caps and put a bead of concrete cement, then replace the caps. This will mostly lock the caps in place. Although they can still move (and be moved back), in general they will be very stable.
  6. Weed cloth and dirt ... Once all the concrete is dry, we can put a layer of weed cloth down (to keep any grass roots that remain or crawl under the base from getting into our garden).
  7. Optionally paint the cinder blocks, add bird netting and/or install automatic watering.

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Calculating Materials Based on Size

Almost the first thing on everyone's mind is how much will it cost to build the garden. This section intends to describe a little bit about the materials required and how calculations will be performed so that the next section which calculates costs will make sense.

One of the calculations required is how many bags of concrete are required for the base. An 80 pound bag of concrete is typically 0.6 cubic feet (stated on the bag). We first convert that to 1036 cubic inches (0.6 * 12 * 12 * 12). If we assume a base depth of 1.5 inches to 2 inches (thickness of a 2x4 laid flat on the ground plus uneven soil), the we need ~233 cubic inches (2 * 7.6 * 15.6) per cinder block. Therefore one bag generates a base for 4.3690958164642 cinder blocks.

We also reiterate that cinder blocks are a standard size and therefore using them restricts us to integral multiples of the length or width. For our calculations, we will use 7.6" because you can not get these together without having a small gap. The following table gives you a feel for how many blocks are required for a certain length.

Integral Distances for Cinder Blocks
Blocks 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Inches 31.4" 47.1" 62.8" 78.5" 94.2" 109.9" 125.6" 141.3" 157" 172.7" 188.4" 204.1" 219.8" 235.5"
Feet 2'7" 3'11" 5'2" 6'6" 7'10" 9'1" 10'5" 11'9" 13'1" 14'4" 15'8" 17'0" 18'3" 19'7"

Note is that we can also arrange our cinder blocks so that we can add a 7.6" ( width of a block) increment to our garden size. The diagram below demonstrate the principal better than words. Later I will refer back to these layouts when running the garden calculator.

I should also caution you that when you are placing your top slabs over the cinder blocks you need to align them with the blocks underneath. That is because 2 times the width is less than the length and you would end up with a 1/2" gap between slabs if you do that.

Layout A Integral lengths and widths of full blocks have two possibilities for virtually the same size garden. Use the one that provides the front or side view (show only full blocks) that you want. Since the block width is slightly less than 1/2 the block length, the layout does affect the concrete base dimensions.
Layouts BL & BW Extend one side by a half block length using only full size blocks. Layout BL extends length. Layout BW extends width
Layout C Extend both dimensions by half a block length while still using only full size blocks

Another interesting note is that if our base is poured the correct size (a multiple of 9") we can simply start laying blocks from any corner and it will be apparent whether we will reach the next corner with a straight block or need to turn the block.

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Interior Dimensions Table

This information is in table form so that the reader can get an idea of the amount of material and costs for different size gardens.

Some users will want to calculate their garden based on the interior dimensions, based on the number of plants and suggested plant spacings. For example, if you want peppers two feet apart, you might want an interior width of 4 feets so that each plant is 1 foot from the side and there is 2 feet between them. In the next section we will discuss exterior dimensions.

Calculating a few example sizes, we have the following table for different interior sizes. I used current Home Depot prices for materials to calculate just the materials cost. NOTE: These estimates used a 8x8x16 cinder block (instead of 7.6x15.6 Use the calculator in the next section for exterior sizes to get the actual needed materials for your garden. That is, these tables were just to give a rough estimate of what it cost to build different size gardens from cinder blocks.

Interior
Length x Width
Interior
Length x Width
Cinder
Length x Width
Cinder Blocks
$1.68
Top Slabs
$0.99
Concrete
$3.90
Cost
24" x 72" 2' x 6' 1.5 x 4.5 12 12 3 $ 43.74
24" x 96" 2' x 8' 1.5 x 6.0 15 15 5 $ 59.55
24" x 144" 2' x 12' 1.5 x 9.0 21 21 6 $ 79.47
32" x 144" 2.66' x 12' 2.0 x 9.0 22 22 6 $ 82.14
48" x 144" 4' x 12' 3.0 x 9.0 24 24 6 $ 87.48
48" x 192" 4' x 16' 3.0 x 12.0 30 30 8 $ 111.30

COST NOTES ... the material costs above DO NOT include:

Calculator for Exterior Dimensions

If a person cares about garden size and desired plant spacing particularly with larger plants. Those people can easily use the tables above to calculate how many blocks to buy. However, a person might also be restricted to a certain size of garden driven by a combination of where there is adequate sunlight and available yard space. For those people, we have an online calculator to determine your best option.

Use the following calculator to get an estimate of costs for a garden of an arbitrary exterior size (specify in inches). Enter your desired or a maximum size in the Desired Length and Desired Width boxes and then press the 'Calc' button. The closest size EQUAL TO or UNDER the desired dimensionss will be calculated. NOTE: Exterior material calculations WILL NOT match the interior examples above.

Desired
Length
inches
Desired
Width
inches
Actual
Exterior
Length
Actual
Exterior
Width
Full Blocks
$1.68
Top Slabs
$0.99
Concrete
$3.90
Use Layout
Plan
Material
Cost
140.4
11'8"
46.8
3'10"
22226 A $82.14

Again, the cost above ONLY includes the material cost of the garden walls.

Finally, if you absolutely needed exactly 48" (as an example) for you planter, you can follow all of the instructions on this page EXCEPT make your base the precise dimensions that you want. Leave a gap in the middle of a length or column between the cinder blocks that has the additional amount that you require when installing them. Clamp 2 short sections of board against the cinder blocks to make a form that covers the gap and then pour a concrete spacer. Repeat the process once the top slabs are in place.

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Painting

Some people may think that the grey concrete look of cinder blocks is undesirable. I happen to be one of those people. So we painted the cinder blocks with a Rustoleum Exterior White Oil Paint. I chose white since it would reflect more heat away from the garden. In colder climates, you might chose a brown to help heat up the garden. Yes the white gets dirty but a water and a stiff brush once a year return it to an acceptable level without too much work.

If your goal is more of a decorative planter, it would also be possible anchor all the blocks with a little concrete to the base, so they don't shift relative to each other, and then put a layer of concrete and sand on the outside, seal with KILZ and paint really artist scenes. My only point here is that cinder blocks provide a long lasting planter or garden and it possible to decorate it so that also look really nice.

Deeper Gardens

WARNING: The suggestions below are for ideas only and only for one aditional layer. Stacking cinder blocks more than that could create an unstable structure that could topple over and cause serious damage. If you are going higher than a second level, you should use better construction techniques than described here.

I have found that the 9" depth (plus some portion of the base) is adequate for growing vegetables. However, you may be building a planter along one side of your patio and would like a taller structure. You may just want a taller garden for aesthetic reasons. Maybe you want to also use it as seating. Obviously we can just double the number of cinder blocks. However, when we are stacking then DO NOT put one full block directly over another. Start off by having your first block on the second layer rest on two blocks underneath. That will set the whole pattern and it will make the structure more stable.

A second design feature might be that you would like only one wall higher. For example, if this garden is next to a patio, you might want one side higher to keep the flowers from falling over to the inside or to act as a shelf for people to set their drinks. Again, we can obviously deduce the number of extra cinder blocks, although we might need a half block to make the one wall complete. If so, set the half block in the middle of the wall.

In either of the two cases above, if you are going to stack more than 2 levels, you may want to consider anchoring the layers to each other by adding some mortar between layers and/or adding rebar and concrete inside of the cinder block holes. If only making one side higher, you should consider stair stepping the adjacent sides and again adding rebar and concrete.

Other options include buying topslabs that are 2" or 4" thick and/or stacking these (no more than two high) if you require a more precise height.

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Suncover / Bird Netting / Frost Protection

There are any number of ways to construct a cover. The most stable, least obtrusive and relatively cheap method that I came up with is the following. Take 6 or 8 foot pieces of rebar and drive them into the ground in the inside corners of your garden no more than 6 feet apart. Slide pieces of 3/4" PVC over the rebar. Add PVC tees and corners to complete the 'box' frame. Drape bird netting over the frame and simply lift a side when working in the garden.

Adding a suncloth to the top can reduce watering requirements during the hottest part of the summer, however, it will also tend to make your plants grow very leggy and spend more time on green growth than vegetable growth so you should only add it if trying to keep the plants alive through a short period. Shade cloth also tends to catch the wind so you will need to wire or tie-wrap the cloth to the frame.

If you have a small enough garden, you might also be able to use the frame to support a sheet to keep an early garden protected from an early freeze. The sheet can be attached another of ways, but an easy one is just to tied some heavy twine to the posts on the bord poles and then also criss-crossing where possible and draping the sheet on that. Maybe add some binder clamps to hold it in place. Add a small wattage light bulb under the sheet to create enough heat to prevent freezing OR if a light freeze fill some empty milk jugs with hot water and place inside.

WARNING: Using too hot a bulb that could contact with the sheet could create a fire hazard.

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Automated Watering and Multiple Gardens

For a smaller garden and someone who really enjoys watching the flowers or vegetables grow, I recommend using a watering can and as much captured water as possible. City water often contains chlorides and flourides that kill the microorganisms in the soil. Using rain water will make the plants grow faster and healthier. I often hand water my young plants since a sprinkler system covering the entire area is also wasteful. However, when the plants are well established and the time required to water during the hot summer becomes annoying, I switch over to using a separate zone on my sprinker system but use it to drive a drip irrigation system. In the past I also used a simple timer on the faucet for the garden to do the same.

So here is the important tip for users that want to have more than one garden. I ran PVC under the ground between the two gardens and brought up a spigot inside each garden area. That allows me to share on controller or zone, but turn on/off the drip irrigation inside each garden. I can hear you asking "Why?". There reason is because I might have winter crops, have started plants in one garden but not another, I finish harvesting and let some plants die in one garden while keeping the other garden with a different crop growing. Effectively these are different watering zones, but since they rarely change it is simple to manually enable them.

If you want the multiple garden watering option, put the PVC in the ground between them before pouring your bases and installing weed fabric and dirt.

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Pictures

 

 

 

 


 

This page modified Feb 20, 2021