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Community and Q&A

French drain

Lisa_Tom | Posted in General Questions on

I have terraced planter beds with cinder block walls and footing.  The wall heights are two feet.  The inside width of  each soil bed is 3 feet.    The inside of the walls are waterproofed.  On wall is 30 feet long, the other is 20.   The beds will be irrigated with drip irrigation with occasional deep watering with hose, and plants will be a combination of  vegetables, small fruit trees (maybe two or three at most), and flowering shrubs (two or three), and then perennials. I live in southern California. Soil is clayey here, but have modified it a slot with organics.

So, do I also need a french drain with outlet?  If so, where should it be placed? For ex. above footing, below, how deep, how far, etc.   Also outletting the french drain may be a problem because I don’t have a lot of slope in the rest of the yard.  Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    The answer depends entirely on the type of soil under these planters. If the underlying soil has a high clay content, and the water table is high, the planter may be damp. If the underlying soil has a high sand content -- I know yours doesn't -- and the water table is low, the planter won't be damp.

  2. Expert Member
    BILL WICHERS | | #2

    It's all about keeping the soil from turning into soup and drowning your plants. As Martin said, your soil type is important. Drainage is too, just like how you want drain holes in pots. It's possible the water will drain to the ground through the terraced beds, but come soils like clay will take longer to drain than sandy soils.

    If you aren't sure, you can put in the french drain and have the option. It won't hurt anything. My guess is that if you are careful with your watering and have decent soil you probably won't have problems either way.

    With my potted plants (BIG pots outdoors), I use drip irrigation and have good luck with it. The pots are filled with potting soil, and have drain holes. I'm careful to keep the timing cycle for the drip system set so that it matches the needs of the plants and doesn't over or under water. Some systems, like my "smart" Orbit controller, get weather data and can adjust their sprinkling settings based on the weather. This can help limit overwatering in cooler, damp times and make sure your plants get enough water when it's hot too.


  3. Lisa_Tom | | #3

    Do you mean a natural water table or from the irrigation water?. No groundwater here in this area of southern Cal. And I am inland from coast about 10-15 miles. I need a french drain? If so, need to know placement. Also, with french drain and gravel, will I be drying out the soil too much? Water is expensive here in southern Cal. Just wanted to be sure. Also, wanted to add that the cinder block walls do not have any weep holes.

    1. Expert Member
      BILL WICHERS | | #4

      Water table shouldn't be an issue for a garden, unless the water table was REALLY high. I wouldn't worry about that.

      The only issue I could see you having is if water accumulated in your terrace steps, keeping the soil in the immediate area of the garden too damp. Assuming your terrace steps are open on the bottom to the natural ground underneath, it's unlikely you'll have excessive water accumulation unless you have unusual soil, like hard-packed clay, that won't drain well.

      A french drain won't dry the soil excessively, it will just prevent bulk water accumulation. If you're worried about water waste, you can collect any water draining out the french drain and use it to water again using a small pond pump to provide pressure. This is a green building site after all, and collection of rainwater for irrigation is not that unusual of a practice. There is no reason to dump the water down a drain in this case.

      Typical cinder blocks won't have weep holes, but they are fairly porous and aren't really waterproof. Water will slowly make its way through a typical cinder block wall unless you use some type of sealer on the surface.


  4. Lisa_Tom | | #5

    The split faced cinder block walls will have a waterproofing membrane behind them which I am paying dearly for. They are really nice walls and I want to see them last. However, I also want to put plants behind the walls. The standard retaining wall advice for a french drain behind the wall of a foot of gravel along the wall and above the drain and compaction of soil behind it is not the greatest set-up for plants (especially woody shrubs) and will take up a lot of room in these relatively small beds.. If I do need a french drain do I really need all that gravel and must I compact the soil? As I said the soil is a clay type but will be amended with organic matter and drip irrigated for the most part.

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