Should planter be "self-contained" or open at the bottom?

[hstraf]

Hello,

I want to build a cement block planter down one side of my driveway. So it would be about 2 feet high by 1.5 feet depth (inside measurement) by 40 feet long. On the house side, it would spread out to a big square planter next to the house that is around 6 feet on each side. This part of the planter would be about 4 feet high.

So kind of like this:

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______________________________|                      |  (This part would be 4ft high by 6ft by 6ft.)
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This part about 2ft high by 1.5ft depth)

Should I leave the bottom of the planter "open" to the dirt (currently grass) below the planter? Or should I put plastic around the entire inside of the planter so it is completely self-contained and isolated from the dirt under the planter?

Also, how should I "fill" the planter? Should I just put regular dirt into it? Or rocks at the bottom, then some sand, and then planting dirt on top? Will the cement blocks "leech" out the water from the dirt?

I live in a warm climate (a Mediteranean island) and will have plants that don't need much water. I'll probably have an olive tree or small palm tree in the middle of the square part, and then small shrubs and bushes around it and down the driveway side. There will be a simple drip irrigation system on a timer to provide the water.

Thanks for any advice!

[rainbowgardener]

DEFINITELY open at the bottom. Especially if you are going to plant big stuff like trees and shrubs, they really need to be able to put their roots down in to the native soil. If you have a bad weed problem you could put a couple layers of cardboard at the bottom, wet it down well and then pile your filler stuff on top. The cardboard will suppress all the weeds and then it will rot away. If your soil is clay-ey, punch some holes in the soil and then in the cardboard for drainage.

Do not put rocks or gravel in the bottom, just fill it with soil. If there is a boundary between soil and gravel, water will not cross it very well, so the soil will stay too wet. People used to put rocks/gravel in the bottom of containers for drainage, but that was a myth/ misunderstanding.

So what to fill it with. You want some combination of good topsoil, compost, peat moss or coconut coir, coarse sand, and some mineral ingredient like perlite, vermiculite, crushed granite, etc. For the Mediterranean plants, you want yours heavier on sand than I would or you can use bagged cactus mix potting soil as one of your ingredients. But you want a slightly sandy mix, not a layer of sand.


Don't fill them with just garden dirt. It is too heavy and over time will compact down into a brick, excluding air and water. And don't make your mix more than about 1/3 topsoil.

[hstraf]

Thanks! That is very helpful.

Would you put plastic around the inside sides, so the dirt does not directly touch the cement blocks? (I'm wondering if that will help retain moisture in the dirt so it doesn't leech out as quickly during the hot weather?)

[rainbowgardener]

No, no plastic. Holds too much water, impairs circulation of air and water. You will have irrigation system. If your soil mix is drying out too fast, you can add more peat moss/ coconut coir, to make it more moisture holding. You can mulch over the surface to hold moisture. So you aren't worried about holding moisture. What your Mediterranean plants want most is good drainage.

[hstraf]

Excellent. Thanks again!

For the top layer I was thinking to put decorative rocks since it would fit in well with the surrounding area and hopefully keep the weeds out. (So a layer of geotextile weed cover stuff, and then the rocks on top of that.)

Do you think that will work ok instead of mulch layer to keep the moisture "in"?

[rainbowgardener]

Others can chime in and there are mixed opinions about it, but personally I hate the landscape fabric/ weed barrier stuff. It works well for the first year or two and then I think the pores start relaxing or something and weeds start growing right through it. But then they are tangled up in it and you can't just pull them. I have always ended up pulling the landscape fabric back out eventually, which is a big chore.

Rocks would help as a mulch layer, but they don't add anything to the soil. If you mulch with organic stuff (bark chips or pine straw is most common for ornamental areas, for my veggies I use pulled weeds, grass clippings, and fall leaves), then your mulch breaks down over time to feed the soil.

[tomf]

I second using mulch, rocks will just be hard to take care of.

[ElizabethB]

Ditto on mulch instead of rocks. Like RBG I dislike landscape fabric. On the issue of creating a barrier between the soil and the concrete blocks that may be desirable because the blocks will leach lime and raise the pH of the soil. When I had my landscaping business several of my customers had concrete planters and could never get anything to grow. I did line the planters with black garbage bags - hoes punched in the bottom for drainage. The containers were planted with small annuals - not large plants. If you are concerned about the lime line ONLY the sides. Leave the bottom open to the soil. Don't use heavy plastic like visqueen - just leaf bags. Pay close attention to your watering practices. Make sure you soil does not get too wet.

Good luck