Raised beds on concrete

nboldock

Hi there

I am seriously considering setting up a vegetable garden - something I've been thinking about for a while. It's not just a money-saving venture (though that's a bonus), I want to do it as a hobby, to get my little 'un involved, and I also love the idea of cooking food we've grown ourselves.

My problem is I have a small patch of grass and the rest is concrete. So I was thinking of just building a large-ish bed on the concrete and filling it with compost or whatever. I'm told it would need to be about 18 inches deep if on concrete.

Does anyone have any tips they'd like to share? All pointers appreciated! Particularly about what soil/compost mix to use? Will it be expensive to fill? I think the finished thing will be approx 6-7 foot long and maybe 2ft wide. That's not set in stone yet though.

I will be growing organic if that makes any difference. I want to do root veg (carrots, parsnips, swede etc) plus things like peas, peppers maybe, onions, garlic. Also potatoes but I was thinking of doing those in pots.

Wooh, I'm rambling...

Fire away gardeners!!

ScratchandSniff-->[X]

nboldock wrote: »

Hi there

Does anyone have any tips they'd like to share? All pointers appreciated! Particularly about what soil/compost mix to use? Will it be expensive to fill? I think the finished thing will be approx 6-7 foot long and maybe 2ft wide. That's not set in stone yet though.

What are you going to build it out of, and are you going to line it?

• If it is lined, it will be a mud bath as drainage will be limited.
• If it's not properly lined, the soil will leech out of the "walls" and onto the concrete over which it will wash down in a nice soggy and slippy delta to your drains and block them.

I wouldn't do it. Try big pots instead - much less problem with run-off and a whole lot more flexible.

ceridwen

I want to do the same - and am thinking along the lines of Square Foot Gardening and using the plastic readymade boxes (by Link-a-Board I think?).

Do hope this is a viable option - without blocking my drains - so would also appreciate further advice on this. (Crikey - I'm halfway sorted ready to go with this in fact - got both the Square Foot Gardening books and a set of those make your own organic liquid fertiliser thingies to put under the sink for my food waste). Hope I'm not going to be stymied by some previous owner of my house having paved out the back yard.!:eek:

Davesnave

A perfectly feasible idea, but you'd need to build it out of concrete blocks, line the bottom with drainage material, screed the sides and provide drainage holes at the bottom. I built a small one once about 2' 3" deep and my drains didn't block! Presumably, your concrete drains somewhere now, so there shouldn't be much extra run off, though there will be some after heavy rain, watering etc.

However, if it were me, and I wanted to go down this route, I'd still take an angle grinder to the interior concrete, break the centre section out and remove it to give some interior drainage.........which of course brings us to ask, 'Why not remove some of the concrete first, then build a raised bed of normal height?' If the answer is because you are in a rented place then all the above will be irrelevant.

I suppose what I'm saying is that it's possible, but you need a good reason.
In my case, I had two existing walls and it was only about 2m2 of otherwise dead space.

ceridwen

thanks for that....can I have a bit of clarification please? Not sure what you mean by:

- drainage material?
- "screed the sides" (what is screeding?)

Also wondering what an angle grinder is please?

Davesnave

Ceridwen,

Drainage material = anything that stops the soil/compost compacting at the bottom and blocking drainage holes. It could be broken bricks, blocks, concrete etc, plus a bit of gravel to close the larger gaps.

Screeding is just adding a thin layer of cement to the interior/exterior of the blocks to improve water retention (blocks are quite porous) and improve appearance. It's like plastering. I also add waterproofer to the cement for this, but can't say if it helps. It should. I bring screeding over the top of the blocks in a small hump, which means the edges are rounded and kinder to the legs/knees. If, like mine, your screeding technique is not very good, an old hacksaw blade drawn over it when almost dry can scrape away the worst lumps.

An angle grinder is a tool with a circular cutting disc, which will cut lines in concrete to weaken it, helping to define the edges where it's being broken. This might help if you were going to use, say, a sledgehammer to break out some of the concrete base, whilst also leaving other concrete intact. A small angle grinder (4") would probably be no use and a 9" one is recommended. They all make a lot of dust! If you just wanted to remove the middle bit of a base, this tool would probably not be necessary. I have invested in one, so I look for reasons to use it. A power (Kango) hammer (like a mini road drill) is an alternative to a sledgehammer and gives a better-defined area of breakage.

Hope this helps.

Gardenia101_2

At my daughter's school, the caretaker has built a simple raised bed directly onto the playground, and the salad veg. in there seem quite happy.

- The bed is made with wooden decking planks (think Ground Force!).
- It is lined with Mypex, the ground cover that lets water through, but not soil/compost (garden centres sell it under different names).

Hope this helps,
Ray

ceridwen

Ah - thanks. Will look out for it.

nboldock

Wow, thanks for the advice. Plenty to take in there.

I don't want to remove any of the concrete as this would entail quite a bit of expense (tool hire, skip etc), not to mention backbreaking effort!! And drainage isn't a major issue - the concrete runs off onto grass at one side and a thin flowerbed next to a fence at the other (nowhere near the house drain) so should be fine. The concrete used to have a garage on it, if that helps explain - I knocked it down a couple of years ago and now have a small shed instead.

I was going to build it out of wooden planks screwed to a piece of 2x2" at each corner. Should be sturdy enough especially once it's filled.

As for lining it, I'm not sure - that is the sort of thing I was querying. I don't think soil will leak through the sides of the structure as I intend to build it nice and tight. I was considering just raising the bottom a bit (inside) to improve drainage at the foot of the bed.

So, I think the key questions now are - (1) do I line it, and if so, how? and (2) what is the best compost etc to fill the thing up with?

Hoping to buy materials tomorrow if possible.

Fern_Merkin

I was lucky enough to get 2 x metre cube packing cases and after coating with wood preserver filled them with top soil to about 3/4 then mixed compost in the top bit to fill up. They have preformed well for the last 12 years giving me loads of space to grow initially flowers and over the last decade rhubarb and mange tout

Gaps in the planking have been ideal for Nasturtiums which are perfect in salads and look great.

One is starting to crumble a bit this year but I've definitely had my wear out of them. Thinking of just building a wall around them now and allowing nature to take it's course with the wood

Maybe try local engineering firms for these sort of things? I would recommend them.

nboldock

I went out and bought materials today. Had a great time with my little 'un (he's 4 and thought it was great fun buying wood and stuff with Daddy).

Bought 8x six foot tanalysed planks of wood from a local timber yard (£16 all in) which are perfect for what I want. Then bought two enormous bags of compost from B&Q plus one bag of.. erm.. poo. (Well it says "Farmyard Manure" in fact).

I also got some bricks from a friend (for free) which I'm going to smash up and put in the bottom to help drainage.

Going to build the thing tomorrow. The boy and me are both v excited!

I'll keep you posted if anyone's interested!