My wife and I are interested in using up an old but sturdy concrete pad
in the back of our yard for a raised bed. The pad is about 10' x 14',
and we'd like to build the beds about 3' high, in a U-shape so that it's
possible to easily reach all areas of the bed. I haven't found anything
in the archives of this newsgroup that really discussed raised beds that
were not built on top of soil, and I'm a pretty new gardener, so I have
a few questions:
1. Is 3' of soil adequate for this kind of bed? We want more than 2'
since the plants will have no chance to get into the soil beneath the
bottom of the bed.
2. Should I make special plans for drainage? I was already thinking of
some gravel to fill in a dip in the center of the pad and to make a nice
path in the center, is it also worthwhile to have a couple inches of
gravel in the bottom of the bed boxes too? Hopefully I don't have to
knock some kind of holes through this pad itself.
3. I've considered both timber and block construction. The problem
with block is that I'm uncomfortable building it three feet high without
some kind of reinforcement, and I'm a little uncertain about the best
way to anchor, say, rebar into the existing pad. I'm not sure if I can
put in support posts outside the layout of the pad, because of adjacent
structures and the way the foundation of the pad flares out below grade.
Has anybody done this?
4. I had also considered 4x4 timber. I don't want to use the treated
wood, and cedar's a little expensive around here (Chicago area). How
long would regular untreated pine 4x4 last? What would be a good
exterior coating to help extend that life? (I already plan for a liner
on the interior against the wood, although maybe not at the bottom
depending on the drainage situation.)
5. I've seen a lot of people recommend 2x lumber, but I wonder if it
can support a 3' high bed without bowing out a lot (especially at the
bottom). I had been thinking about using 2x4s laid flat, so that the 4"
thickness through the wall is still there. This would save some money
vs. 4x4 lumber but this would be double the drilling and cutting
compared to 4x4s. Also, it may look very bizarre.
Thanks for any advice that comes to mind, and sorry if some of these are
sort of off the deep end :)
What are you planning to raise in that bed? I made a raised bed on a on old
asphalt driveway in back that was unused, had some thick uneven blocks to
hold in the dirt and put in about 1 foot deep or less of soil. I have grown
lettuce, collards, tomatoes, parsnips, sugar snaps peas, etc in there. They
have done fine. The soil packs down with time, so I have to add two-four
bags plus manure each spring. If you are planning to grow trees or shrubs,
obviously your bed will need to be deeper, but for vegetables or annual
flowers, you certainly don't need 3 feet of soil. (However you will need to
water in times of drought, more than for your regular beds).
It's going to be a vegetable and herb garden, no trees or shrubs. To be
honest, we mostly thought about 3' because it would be less bending over
when planting or weeding. But I'm not sure if I want to do a lot more
effort in constructing and filling the boxes, just for that :).
Thanks for the feedback; as a newbie this is the kind of thing I really
didn't know about.
Depending on what you plan on planting, a 3' depth is more than adequate.
If you're looking to put in trees and shrubs you might have some issues
though. For veges, you could probably get away with much less -- many beds
laid on top of soil are just 2x6 or 2x8 pine boards nailed together
Question: How close to your house is this pad? What is the layout of the
area behind the house (sloped towards/away, level, cut-out in a hill, etc.)?
Basically what you're building here is an oversized pot, so you should think
along the same lines as container gardening. Drainage is an obvious
issue -- you don't want the plants to have wet feet or they'll likely rot
out and/or suffer a variety of diseases. You mention a dip in the center of
the pad -- water WILL collect at this point.
You'll want to measure how level the pad is all over to see how water will
drain. Given this information, you can design a drainage system. Gravel
and screen with some sort of weep system along the walls is likely your best
bet -- be sure to have somewhere for the excess water to go though, and keep
it away from the foundation.
What adjacent structures?
You can mount brackets into the concrete; lay cement blocks, drill holes for
rebar, then fill the holes in the cement blocks (where the rebar sticks out)
with cement; use landscape block or stone.... My immediate concern though
would be these adjacent structures.
A liner will cause issues with drainage. Pine will last a few years
untreated before it becomes part of the soil in the bed. ;) Treated lumber
will last longer obviously. Anything you use will have the bottommost
pieces in direct contact with the soil, liner or not, so water has a place
to drain. Anything you coat the wood with may leach back into the soil.
You could just as easily get away with using landscaping timbers.
You might be able to make this job a little easier by reducing the height of
the beds. What kinds of things are you looking to plant?
Thanks for the detailed response, see comments below.
Definitely no trees/shrubs, so maybe we will consider less than 3'. The
biggest plant in there would probably be tomatoes or squash. And I
might just grow tomatoes in the regular 'ol ground anyway.
It is a level pad that used to have a smokehouse or a lean-to on it. It
is bordered on one side by the garage (which used to have a door opening
onto the pad area) and on another side by the neighbor's fence. So on
two sides it will be difficult to have bracing that is outside the
footprint of the pad (although I guess the garage wall is enough bracing
on that side).
That's a really good point as far as container gardening; I hadn't
thought of it that way before. The pad has been exposed to the weather
for many years apparently, and it is cracked and sunken in the center
although the raised lip around the outside (6" wide, 2" above the pad
surface) is still all there. There is apparently some drainage in the
center fairly large crack, because even after heavy rain I never seen
standing water in it. I could probably break out the center if I needed
to, but taking out the whole pad would be a bit of work, which is why
we'd rather just build something useful on it instead.
In the worst case I guess I'd knock some notches into the surrounding
lip, so that the water would drain before it backed up too far into the
A fence, and the garage back wall.
I guess I could go the hole-drilling route. That makes sense.
Right. For pine, are we talking 2 years, or 5 years, or 7 years, or
what? If I went with treated but lined wood, I suppose I'd be in fairly
good shape as far as any leaching from the chemicals - they would only
be able to leach at the very bottom of the bed where the liner stops.
For some reason (possibly the time of year) I have not found landscape
timber around here. I've tried Home Depot, Menards, local lumber
stores, etc. Maybe it is strange to build something like this in the
fall, but in my case:
- I had other projects that took most of the spring,
- I already have the dirt and compost that's going into the beds, and
- I need to move the dirt pronto so I have a place to store this
year's leaves for next year's mulch :)
Reducing the height sounds like a good plan - that way, I could still
build it out of cedar without the cost being too prohibitive. And I
guess I could always add on the extra foot at some point in the future,
as long as I secure it properly. I'm not sure if I have enough dirt to
fill up a 3' high bed that will work out to about 30' long, anyway.
Thanks for the ideas,
Don't forget to provide drainage. Otherwise you could drown everything.
I would place gravel on the bottom, then sand and then soil. Also, the
sides would have places where the water could run out. Usually that
will not be a problem.
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3 feet is massive overkill. I'd go with closer to 18-20 inches -- 3 inches
of gravel followed by screen and landscaping fabric then a rich blend of
soil, compost, organic matter, etc.. I'd also toss in a few different types
of worms as well including red worms and earthworms (not likely they'll be
working their way through the concrete into the bed)
This is a problem. You do NOT want water draining on either side. Are the
front and/or back graded in such a way that water will drain away? If so,
I'd use cement block on either side, mortared onto the pad and each other.
To keep them firm to the pad you can use some short lengths of ribar
inserted into holes you drill in the pad -- the other end goes through the
hole in the cement block and you put cement in the hole to complete the
Your best solution here would be to remove the pad -- it's near structures
and that can pose issues. At the least you need to build a drainage system
into the front and back to accomodate any excess then of course run it off
somewhere productive. The simplest system is to use perforated plastic
tubing, covered with a piece of landscape fabric to prevent clogs, that is
completely ensconced in gravel/pebbles. This gives the water a place to go
and then you can put holes in the front and back walls to get the runoff.
Can't have wet feet, that's for sure. :)
Be sure to create drainage at the front and rear still.
Probably about 3 years assuming decent drainage. That's really no biggie
though -- it's dirt cheap. If you did the sides in cement block and the
front/back in a pair of 2x8's you'd probably be looking at less than 30
bucks for the whole thing.
Think of it as an experiment -- go with less expensive pine this time around
and if all goes well, you like it, etc. then move to cedar.
We'll be doing a similar project here although on a slightly lesser scale.
I have a side yard area that's about 10 feet wide and 20 feet long. Part of
that is a sidewalk that is lined by a concrete wall that meets up to soil --
it's also all uphill. I'm going to build a small form along the sidewalk,
extend it at the bottom of the decline across to the fence, tap ribar into
the existing concrete and pour the form. At the bottom it will be about 3
feet tall, at the top it will be about 18 inches. The only complication is
a tree that is there -- I'll extend the form around that as well. When all
is said and done, I'll have a plantable bed that will be level although it
edges an incline, and it will have about 4 feet of plantable area. I've
debated turning it into a water feature rather than a planting area (it's
just outside a bedroom window) but who knows -- have to see what I fancy
when I start work on it next spring. :)
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