build garden on top of concrete?

I have a question that you might be able to help me with. We have a large hole in the back yard where the previous owners planned on building another house. The hole is 40x60' and about 3' deep. I has concrete walls. If I were to push in the concrete and cover it with soil, will the concrete effect the quality of my soil?

Heidi Miller wrote:...has concrete walls. If I were to push in the concrete and cover it with soil...

Hi Heidi, 'concrete wall': how high are we talking?
Unless there's issues with it blocking sun and so on, I'd leave it in place.
Maybe cut/smash easy (read wide) access.
I've dealt with gardens on top of broken and solid concrete, and it totally sucked: most plant roots want to go really deep.
I'd also prefer adding organic matter to what I assume is subsoil, rather than spreading huge amounts of 'soil' etc on top of concrete.

Aside from all that, concrete is quite alkaline and I imagine freshly broken could leach quite a bit.
Unless you're dealing with extremely acid soil I'd be wary, as most veges prefer a ph a bit on the acid side.

Hi there Leila!

Thanks for the response.

Here are a couple of pics of the hole (covered up with plywood). As you see they sawed off part of the porch roof to make room for the building they wanted to put up. we plan to fix that.

Around the edges the concrete is about 6-8" above ground. The sidewalk on the South side is slanted towards the house. There are also supports with screws sticking out that are a trip hazards.

I would rather put in a little bigger back deck and just let grass grow over the rest.





I was thinking about putting in a sunken garden with raised beds, it would have a bit of protection from the wind storms we get. But I just found out that it's not a good choice because on the other side of the foundation there is a grove of old black walnut trees.

Seem like it might be an ideal opportunity for a pond, or perhaps a wetland garden of some sort. Just add a liner, or do a "carpet sandwich" ...

Is it just a foundation wall, ie a perimeter wall? Or is there concrete under the soil in the whole area? I'm guessing it's the former,a s the latter would only be for if there was a basement.

Leave it as it is, and build a hugel mound on that perimeter, or a fence, or something. Knocking it in and dealing with a wide strip of rubble fill (and having to dig out to knock it in and the rest of the possible problems) seems worse than dealing with a 6-8" raised lip. The more I think about it, mounding a hugel around the perimeter to hold in water/protect from wind seems a better and better idea.

One thing you may be concerned about is the previous owners probably had the topsoil dug out and just put fill dirt in, so you're going to need a pile of organic matter to make good growing soil.

Thanks for your responses!

We have yet to remove the plywood and get a really good look inside, but as far as I can tell it is jus a perimeter wall and there is gravel at the bottom with an occasional cat living down there.

It might be hard to tell in the pictures, but its really, really big and pretty close to the house. A pond of that size would just be too much and too close for comfort. It's maybe 5 feet away from the door, would hate to trip and drown coming out of the house...lol.

Down the line we do want a green house and an aquaponics system, so filling most of it will be the option for us.

I like the idea of a mound on the perimeter so we won't need to go to the effort of knocking down the wall, but near the porch I guess we could rent something that we could use cut it level to the ground, then use the pieces as stepping stones or something like that near the shop out back. Same with the wide path with the pillars....has to go.

Would the leaves from the black walnut be a problem for the pond? I'm sure the roots wont because there will be a liner.

Hey there,

First post here, so bear with me! Not sure how practical it would be, but could you use the walls as a foundation for your greenhouse, or if you built up the floor or knocked a whole in the walls to provide access, could you use it (or part of it) for a chicken house, it would stop any critters from digging in. It would seem a waste to not benefit from the walls. Cheers

Fiona Martin wrote:Hey there,

First post here, so bear with me! Not sure how practical it would be, but could you use the walls as a foundation for your greenhouse, or if you built up the floor or knocked a whole in the walls to provide access, could you use it (or part of it) for a chicken house, it would stop any critters from digging in. It would seem a waste to not benefit from the walls. Cheers

Aloha Fiona!

Thanks for jumping in.
I originally planned on putting the green house to the back part of the hole. Something like 40ft x15ft or so, but when I look at google earth it seems like it might get too much shade from the pine and the walnut trees.. I don't want it up on ground level because it would block the view of all my windows on that side of the house (we have beautiful sunrises out back)



Also would have the issue with the toxic stuff from the black walnuts.

We have more than enough land and concrete pads (two buildings burned down before we purchased the property) to put the green house and garden.

That is a beautiful sunrise, I can see why you wouldn't want to block that view! I'm new to this game so have to ask, what's the problem with black walnut? For the bit near the house could you build a platform of some sort over it, that way you have a seating area to sit and watch those lovely sunrises. I'm out ideas for the rest of it I'm afraid.

Hi again Fiona! Thank you! As for the trees, it seems that black walnuts are kinda toxic for other plants, mostly garden plants. The roots contaminate the soil. I think, in that line behind the house, we have 5 or more black walnut trees. There are at least 30 or 40 years old....they have a well established root system and even if we did cut them down it would take years for soil to be good again.....That's what I was told....if anyone knows better or different, please let me know...would be a lot less headache for me if it wasn't so! lol

Could you do a trial raised bed in the area, if it was soil transported from elsewhere or compost, would the walnut trees effect the soil? That way you could do your sunken garden with raised beds. The concrete walls might act as a barrier to the toxins, maybe, just a guess though! You probably need to check the drainage in the bottom though to make sure it's free draining.