Drainage in planter boxes

Question

I have to build some 6' x 2' planter boxes, I'm not sure what's going to be planted in them; probably "garden-y" stuff like oregano, basil, etc.

I know how to build things but I know nothing about plants except that every one I touch dies (I even killed my cactus).

I'm basing the boxes on instructions I found here:

My questions are:

1. How big should drainage holes be and how far apart should they be spaced?
2. The screen at the bottom will be vinyl, nylon, or some other inert plastic. How fine should the screen be? Does it matter?
3. How deep should the boxes be? I don't really care about minimizing volume, I just want to make sure the plants have a nice place to grow.

The box will be cedar (non-treated). Underneath the boxes will be a few inches of open air, then concrete.

I'm assuming the person I'm building them for will be filling them with a bit of gravel and some soil.

Answer 1

That looks good and of a decent size. If you're growing herbs depending on what type e.g. Mediterranean requiring good drainage I would expect to have about 12" depth of planter or thereabouts. That can give space for about 2-3" of good sharp drainage. Less drainage can be used if growing herbs that require a more moisture retaining soil. The container can also be used for small alpine plants as its deep enough to not dry out too quickly. The drainage holes about 9" apart across the bottom, but avoid where you think the supports will be placed, and approx 3/4" in diameter. The screen at the bottom? What is this going to be made from? Marine ply perhaps? If this is to be a mesh of some description make it a fine one so that the drainage doesn't drop through and get wasted.

Answer 2

Any wood will rot when in contact with moisture. It's just a question of how long. As a DIY project if you are happy with it lasting a few years then what you have is fine.

There are those among us who aspire to build long term. I am usually reminded of this when I want to change something in my garden and find it to be immovable. Then I know I was having one of those moments...

In this case if you want the planter to last ten or fifteen years you need a liner between any wood and soil. Some methods I have seen used include using:

• 20 mil plastic sheet on the inside of the box with drainage holes over holes drilled in the wood
• same as above only with EPDM food grade pond liner
• using a disposable inner box of moulded plastic
• keeping the plants in pots inside the planter

Answer 3

The problem with that design is that all raised beds have a problem with drying out readily. This design doesn't allow for much tolerance to keep water in the box, but only prevents over watering. So it shares the same fault as every garden pot with a drainage hole or holes.

It would be better to line the bottom with impervious plastic to a depth of 2 inches to collect water, and put drainage holes in the sides just below the 2 inch line. You could also put some piping in there.

Having water at the bottom encourages the roots to go deep to get water, and if you feed this sub surface water reservoir with a pipe, you avoid irrigating the surface preventing much fungal growth.

Look for designs on sub irrigation planters. You'll need a minimum of 30 cm depth for most vegetables. If you increase the depth to 60 cm, you'll reach the depth of a double dug garden. This then allows much closer spacing of your plants as they can grow their roots vertically rather then horizontally. The close spacing also creates a living mulch which reduces the soil evaporative losses. But then you'll also need a strong box to hold all this weight, or, well spaced supports.