Plywood won't last very long. Why bother with a bottom anyway?

I'm planting it in place of a large, unruly and ugly shrub. Last year I cut the shrub nearly to the ground, but this spring it has come back. Rather than trying to dig it out, I thought I could build a bed around it. The floor would be to keep it from growing up into the bed.

Perhaps I have the wrong approach?

Yep, wrong approach. Chances are that the shrub will find a way despite you. You'll have to remove it - at least the big parts - or it will haunt you for years.

Beside, 12" isn't all that deep for many plantings. What do you intend to grow there?

The square-foot gardening method only requires a depth of 6 inches for most plants. You might head over to that forum and re-post. Your project sounds like it's more of a raised bed than a container. Best of luck.

I have 6-inch deep beds in my square foot gardens, and the only bottom I use (unless the beds are raised off the ground) is weed fabric.

Grant

Hey Kluttery,

Just cut that shrub down to below the soil level, use a shovel to chop as much root out as possible. Depending on what kind of shrub it is, chances are it won't come back after that.

Then, simply build the raised bed without the floor around the dead shrub and cover it all with soil. No need to use plywood, it's far, far better to have your soil in contact with the earth underneath. Add good fertilizer with mycorrhizae, humus and other organic elements (like Dr. Earth or Jobe's) and the natural biology of the soil will eat the rest of the shrub. In fact, leaving a lot of the cut up shrub in there will help grow new plants. The roots of the new plants will eat away at the decaying plant matter over time. It's natural composting.

Ditto. The shrub has to go. It's a little more work but you will be done with it after that. No Plywood.
Good luck...

The roots do what? Eat the decaying shrub?

Um no.

If you want the roots and stump to decompose, you feed the bacteria that will do the decomposing with Nitrogen. That's why stump remover is usually Potassium nitrate. Mycorrhizae won't do anything to decompose the root ball of the shrub. You need saprophytic fungi for that. That's already in the soil. No need to add that. If you have a decent amount of organic matter in the soil (5-8%), there is no need to add humus. The decomposition of the root ball will result in humus.

Listen to Nil13, he actually knows what he's talking about :)

If the shrub keeps coming back, you basically have three options. You can dig it out, keep cutting it back until it runs out of energy in the roots and stops coming back, spray it with glyphosate until it stops coming up.