Q: I want to grow an organic garden but want to actually have it above the ground. If I place the raised garden frame securely on posts, what can I use for the bottom of the garden bed? Also, how do I prevent too much water from gathering at the bottom? I want to make three 8' x 5' raised off-the-ground beds.
A: First you should try to use cedar or a hard wood for the frame and you can either use wood or plastic for the bottom but the key will be the amount of support that you are able to give the bottom. In the bottom of the box I would recommend lining it with pea-gravel and a layer of filter fabric this will help give excess water a place to go. Give this area about 3 inches.
At about the 3-inch line you can drill holes in the side, only a few are needed. You should place window screen over the holes on the inside to prevent the gravel or soil from leaking out. There will be some soil that gets through though. You can then have a series of ropes that lay in the gravel and extend up into the soil as long as these are cotton or natural material that will wick the water then you will be able to use the water that is in the bottom of the planter to help keep the bed moist.
It is a good idea to have a pipe that you extend into the gravel that you can use to both fill water into it and check the water level.
Just to step back a bit the bottom can either be solid sheet of wood or plastic or it can be boards with filter or geo-fabric/plastic over it depending on how much water you are willing to have get through the bottom.
Here is another site with images and products that should help you.
-- Answer from Scott Munroe, a landscape architect on JustAnswer.
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