Building a raised garden bed using concrete blocks on a slope


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I have tried to search for a tutorial on you tube. The only ones I found use wood. My environment in Mobile, Alabama is extreme humid year round and we are usually in the top 3 if not #1 in the continental us for total rainfall. Not that I would use pressure tested wood for a garden but it doesn’t last but a few years.

I want to keep my cost low and so I thought going with concrete blocks would last and cost very little monetarily although I am sure it will be more labor intensive. I am hoping that there is a video that I can study to help me with this project
 
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Actually you need to understand one problem (Birmingham here!:)) and it is the same problem Hugelkultur faces with establishing a grow zone across a slope. Essentially, you can create a landslide. Here is a windy but informative article that may turn your thoughts to building a proper retaining wall.

 
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How steep is the slope? This is the key to this whole question.

If you put a 4 foot level on that slope how many inches off the downhill surface is the end when it is held with the bubble centered?
 
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How steep is the slope? This is the key to this whole question.

If you put a 4 foot level on that slope how many inches off the downhill surface is the end when it is held with the bubble centered?
Good Point Mr Yan

I measured and it was around 14-16% slope where I would try to build the bed. Not sure if I did it correctly. I have a 18-24 inch level & I took at 48 inch 2x4 and placed the level on top. I raised it on the lower end until the bubble was in the center. It varied along the way between
7-8 inches. I divided that by 48 x100 and it showed between 14-16% slope
 
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I took at 48 inch 2x4 and placed the level on top. I raised it on the lower end until the bubble was in the center.
Yup you did it right. Don't you wish you had at least a third hand to do that task?

Anyway. In terms of a roof you have a 1.5 - 12 pitch. That is too shallow of a pitch to put shingles on.

@DirtMechanic has a great point about retaining walls and land slide hazards but, in my opinion, I don't think this is a steep enough slope to worry about it if you keep 3 things in mind:
1) don't make the beds too tall - 6 inches tall on one side is only 14 inches tall on the down hill side - I don't see much risk
2) don't have the "on contour" dimension more than 4 feet long - this way you keep the retained height low
3) don't have many rows of beds going down the slope - you're not making Maucu Pichu and mowing between lots of raised beds in rows is a ton of work

I built my raised beds to be 6 inches tall on nearly flat and smooth land. I used corrugated metal roofing panels. These look cool but will not work for you.

I used to work extensively with a community garden in Illinois where most of the raised beds were PT lumber but some were stacked cinder block. The block beds were always a problem and eyesore even on flat ground.

There are strong corrugated metal garden beds you can buy but you'll pay for them. You can also look into converting a galvanized stock tank into a raised bed and sinking part of it into the ground to level it off. Lehman's has some nice looking options or Tractor Supply co has stock tanks that could work
 
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Besides the block beds being an eyesore, what other problems did you have?
These were dry stacked with rebar driven down in the cavities. Some of the walls had dirt packed in the blocks thinking they could grow plants in the blocks too. The blocks were always shifting, angling out, and generally being loose. It always happens when you build a block wall on anything other than a level and smooth footing.

The block walls, word was, would wick water from the bed making the soil dry out much faster than the wooden beds or the inground parts. But, I have no experience with that.
 
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The entire backyard has this slope. If you are asking if I have erosion in that area. No
 
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How tall will that downslope wall be when you are done?
I have no idea. I have not committed to doing this project. I don't have a lot of confidence, since I have never tried to build
anything on a slope. Ii can see the possibility of building this, and later having issues when it rains very hard. Regardless if
I used blocks or wood. I need to talk to a friend who is in the landscaping business and get some advice from him.
 
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I have no idea. I have not committed to doing this project. I don't have a lot of confidence, since I have never tried to build
anything on a slope. Ii can see the possibility of building this, and later having issues when it rains very hard. Regardless if
I used blocks or wood. I need to talk to a friend who is in the landscaping business and get some advice from him.
Yes if it is attractive it actually adds so much to the enjoyment of being at home!
 
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The more I have observed the area that gets the most sunlight has the most slope. It also has the least amount of grass, mainly weed although there is a small amount of grass. So all in all I will probably layout the area with small posts and string and spray the area with round up

I will wait a couple of weeks and use wood. Knowing that it will not last a long time. especially the parts that are below the soil.

I have not decided on the size of the lumber. No sense in spending a lot since it will have to be replaced.
 
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The more I have observed the area that gets the most sunlight has the most slope. It also has the least amount of grass, mainly weed although there is a small amount of grass. So all in all I will probably layout the area with small posts and string and spray the area with round up

I will wait a couple of weeks and use wood. Knowing that it will not last a long time. especially the parts that are below the soil.

I have not decided on the size of the lumber. No sense in spending a lot since it will have to be replaced.
put a bunch of borax laundry powder down the holes.
 
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I assume you would be digging some kind of stop on the the bottom side. Are you just going to make wood boxes that lay upon the ground?
I am sorting that out. I have watched a number of you tube videos and some just lay them on top. Then drive a post down and secure it on each corner. Others use A pick ax and scrape out the area the frame will lay on and After that build up the lower side with pavers or dirt to get it level.

I would like yalls opinions. If it was on Level ground it would be a piece of cake

I checked out the book at our library by Simon akeroyd raised bed gardening but nothing about building it on a slope
 
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I have put in retaining walls with the castle block style interlocking concrete blocks. Are these actually what you meant or were you thinking mortared together 16" x 8" x 8" dimensional cinder blocks? For some reason my mind went to cinder blocks when you said concrete blocks.

Castle blocks sit on a packed base of crushed stone to level them out as you set them. They're also designed to lock together and work in curves. With these a curve adds strength but a long straight run will be weak and want to lean over to the non-dirt side over time.

I have also put in retaining walls with pressure treated landscaping timbers. The PT differs from standard lumber by only offering a few more years. Construction is the same.

(My opinion) If you're putting in a wooden wall longer than 8 feet or higher than 2 feet you should have fence posts set into the ground on the non-dirt side of the wall then the wall material stacked between the post and the dirt. This way the full strength of the post is holding the wall not just the fastener.

As you don't want modern pressure treated wood there are old school DIY preservatives worth looking into if you go with wood. One I have heard of but not researched is boiled linseed oil mixed with charcoal dust - linseed oil is made from flax seed and charcoal is just burnt wood.

Think about the life time of each material. The biggest cost of this project will be the labor putting it in, not the materials. Trust me it sucks to spend hours rebuilding something in 3 years because I tried to save $100 the first time.

Either material works but both have the limitation that a long straight line will be weak and want to cave over. There were a lot of retaining walls leaning over in the neighborhood I used to live in where everyone had a flatish lawn but the street was on a hill. Then within 10 years everyone spent a lot of money to have someone else replace the walls - most of those houses are 100 years old now.

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For raised beds I prefer setting the garden bed wall a few inches into the soil. I think it gives a cleaner looking line and I don't have soil wash out of voids where the ground dipped a little. Plus it plays into my engineer OCD mind to have the bed set level and square to the other beds but that is a different topic.
 
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