Raised bed question - building on rock?

16 years ago

I just moved from an area that I used raised beds on because of poor soil, to an area that has NO soil. I am not kidding - the previous owners of the property trucked in about 20 tons of topsoil to get grass growing in the front yard. It took us 2 days to get a small swingset set up in the back because all our set stakes kept hitting rock! The landscape is composed of many large, flat rocks....some over 5' wide. There is some dirt visible, but you can't go down too far without hitting rock again. We have plenty of natural grasses and weeds in the area, so SOMETHING grows.

There is only one logical area on the property for a raised bed - an area that is relatively flat, gets enough sun and is out of the range of our septic sprinklers. I can easily get about 6-8 decent sized beds in there, which is great. I think I can get a good garden growing if I can make each bed 12-18" deep, since of course, this area is all rock. The only problem is, ALL of the beds I've seen constructed this deep require some sort of staking to the ground, and that simply isn't going to happen. I would end up with either skewed beds or odd shapes in order to avoid the rocks, and having done that in the past, I don't want to go there again.

What I need is some sort of bed that maintains its support from the outside of the bed to keep it stable and not shift - like a frame of sorts, but one that isn't anchored. I want to avoid railroad ties and treated lumber. I've done searches on this site and online, but have yet to find something that isn't anchored.

Any tips? Hubby would be pretty ticked off if I took over his front yard grass for my beds. :)

Stacy

Featured Answer

celestial

16 years ago

I've got 2' tall raised beds constructed from pressure treated wood, lined with plastic, no stakes into the ground. I did put gravel in the bottoms of each--provides plenty of support along with the soil.

Here's a picture (note: they are pretty old so they've warped a little over time)

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Comments (8)

sandra_christie
16 years ago
How about constructing raised bed boxes from 2by 8 or 10's and then bracing them inside with extra wood so that they will keep their shape. By bracing, I mean having another piece of wood running through the center to hold the shape. Should work.
neognomic
16 years ago
You have to build a retaining wall out of stone to get higher than 10-12".

Small beds of one foot height or less can use the standard garden centers' Retaining Wall Blocks. Pick your style and color. ...I suggest a small bed, very small, like 2'x4' x 1' high as an initial project. It is not hard but is labor intensive. You will need to get an inch or two into the subsurface...

4"x6" lumber will work for that size too; even 4x4's. However, you will want to stack heavy stones on them so 4x6 is best(with the 6" side on the ground). You can use cedar or redwood if pressure treated cannot be used. ( BTW, That is expensive lumber. )

For taller structures you need some larger blocks such as those from Anchor Wall Systems. They are called gravity walls because gravity is the equalizing force to sub' for those stakes. You can build 18", 24" and for some of the blocks even 36" gravity walls.

Anchor Projects
AWS Products

Now if you have big, heavy(50+lbs) stones in the area, you could use those rather than purchase cut ones. Gathering them should be fun -take your shotgun. ...
You can make a frame of 2x12xY' lumber and stack stones on and around it too. The frame by itself will not work without support in the long term. (short term w/b okay)

I rec' that you look for landscaper in the area and tell them what you want. So many beds is a big project and take my word for it, those retaining wall blocks used for 2'+ high walls are very heavy(70 lbs ea.) when moving and stacking a hundred or more of them. And you will need a lot more than 100 ...

Now, here's a novel idea for you if you want to be earthy and practical. Use bales of hay. Entire homes have been built that way. I have no experience building structures with them so you have to Google...

Just so you know, pressure treated lumber is not harmful to people, plants or the environment when used properly. Five years or so ago, it might have been ( and was dangerous to some really stupid people ) but that kind of PT lumber is not made anymore or sold anywhere in the USA.
It would be, relatively, simple to construct a 8'x8'x1' bed with landscape timber; I've done it here and am about to do it again for a base wall for a garden house. It is like making toothpick houses but instead of glue at the corners one uses 5" spikes ... there are plans on the net. E.g.: Raised Bed Project
Again, however, without stakes in the ground you will have to add weight to the top. The taller it is, the more that is needed. Sides will need support too if the structure is very long...

Good Luck!
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Thank You Sylvia and Ceth... Glib, I have done Decomposed Granite from the outset, so I don't know any difference, I learned before I built my beds the possibility that it enhances Clayish soils while also contributing to the soil food web, this is my initial take on it and what motivated me to try it, so far ,it has worked splendidly ;-) Decomposed granite is a rock powder and is an excellent amendment for clay soils. All rock powders are great sources of minerals and micronutrients. All growing soils need them. As the microbes and macrobes like earthworms, digest the insoluble minerals, they break down into the various soluble micronutrients that all forms of plants need. For example, limestone rocks are rich in calcium. Granite rocks are rich in potassium, etc. Seaweeds of course are the king of micronutrient fertilizers and soil amendments. There can be up to 70+ trace elements in seaweed. In locations where seaweed is not readily available; rock powders is one answer to the problem. Research continues to reveal that insoluble tiny particles like rock powder minerals, can be easily digested or absorbed via microbial activity, over time, into the anatomy of growing plants, thriving in the presence of rich organic compost. Obviously a heavy clay soil is very difficult to initially dig, whereas a sandy loam is much easier. Heavy clay soils can be improved considerably by adding a lot of sawdust and mason's sand. DO NOT use beach or river sand! Mason's sand, being crushed granite, has sharp edges and actually loosens the soil. One of the least understood aspects of gardening is that of capillary action of the garden soil. It is a proven fact that compressed earth has a better capillary action than undisturbed soil. That is why gardening books tell you never to walk between rows in a regular garden, as that encourages weed growth. And of course people are told never to step in a French Intensive bed and thus compress the soil. But the capillary action is absolutely necessary to bring water up to the roots of growing plants to promote good and deep root growth. In regular garden rows, it is easy to roll a heavy 2 or 3 inch wide weighted wheel (mounted on an axle and handle) down the row directly over the freshly planted seeds, which will then promote deep root growth along that line. The spaces between the rows are than rototilled to eliminate footprints prior to the initial watering. The basic concept of French Intensive gardening is that raised beds are created which have humus added, and the whole bed is light and fluffy to a depth of two feet to promote fantastic root growth. The beds are typically 4 feet wide and 12 or more feet long, with 3 foot paths between the beds.
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I'm not that concerned about sliding, as the area is flat. I'm more concerned about shifting blocks due to the weight of 12-18" (or more) of soil on the interior, combined with variables such as possible flooding rains drenching the beds and the occasional times I have to step in the beds for one reason or another (I try not to). I've seen problems arise where stacked CMUs (which was my first thought) would shift and require rebar as staking. I want to make sure that I have a system in place ahead of time, since staking after the beds are made is going to be near impossible (unless I'm really, really lucky and happened to pick spots that are pure soil, and in that case, I'm going to give up gardening and start playing the lottery!!!!). Sarah Lynn - love your beds. They look almost exactly like the garden I used to have - I used untreated 2x6s with pea rock paths. They last a good 7 years before they needed replacement due to rot. gamebird - hadn't thought of using the interlocking stones for retaining walls. I'll have to price that out. I know it will be grossly expensive, but if they will last 15-20 years then it may be worth it. Stacy
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Gardengal, I have similar questions about a rock wall we just built. We don't have a lot of problems with dirt running out yet, partly becuase the rains haven't come yet (that is, they hadn't until today) and I think because my husband quite solidly compacted the soil behind each course of rock before putting the next one on. My back-up strategy is to put plants with fairly fibrous root systems near the biggest gaps and hope that they will catch and hold any remaining debris over time. The book we used for guidance did say that the gaps would eventually stopper themselves with bits of stuff.
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neognomic
16 years ago
violet_z6 posted a bunch of great pics here:
Raised Bed Insulation (Follow-Up #8)

With some creativity you might get around having to build stone walls to get depth greater than 12".

Here's a pic' of "dry-stacked" flagstone beds to get an idea of what you can do:
ruthieg__tx
16 years ago
I live in the same kind of area...The Texas Hill Country has a few pockets where you can find dirt but mostly rock...the answer is raised beds.....We used landscape timbers...yes I know some people object to them but I chose to use them...and we used spikes from Lowes or Home Depot and anchored the three together...they are layed out so that they overlap each other including on the ends and I promise you...these beds are not going anywhere....the spikes are like huge nails and we pre-drilled to drive them into the timbers.
girlgroupgirl
16 years ago
We have an old walkway beside the house we wanted made into a garden space. We dug up some other walkway areas and pounded the cement with sledge hammers. I used the small junks as the drainage mechanism over the solid cement walkway and then used the old walkway for "stacked stone" - I did this right over the stairs too. It looks great and nobody ever guesses it was a walkway, or that the cement is not real stone!

GGG
flora2b
16 years ago
Here is a thought, you could look around and get old bath tubs, and skirt them in with wood, or rock I suppose. I've seen these done at a seniors center and they looked great, couldn't actually tell what they were. Would give you more depth than 18 inches as well.
missinformation
16 years ago
You could use a zillion different things. White wash some old tractor tires and set them directly onto the rock. You'll want to whitewash them so they don't get heat up the soil too much in hot weather. I love the stacked stones. We used to have a raised bed garden directly on an old cement slab patio in our backyard, and I just used old boards to build a 4x4 box and ran a few extra pieces acros the bottom for additional support. I didn't grow carrots or potatoes in that one, but it was great for growing shallow-rooted veggies.

girlgroupgirl - do you have a picture of the walkway garden you describe? I can't quite imagine it, but it sounds interesting.

Raised bed question - building on rock?

celestial

Comments (8)

sandra_christie

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