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.

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)

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).

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
picture.

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.

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.

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
beds.

Can't have wet feet, that's for sure.
I guess I could go the hole-drilling route. That makes sense.

Be sure to create drainage at the front and rear still.

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.

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.

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.

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.

James