Senior Gardening

One of the Joys of Maturity

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A Dry Sump for My Raised Bed
October 8, 2008

Even though I've installed a semi-raised bed for the Senior Garden, drainage is still a concern. Our raised bed is on gradually sloping ground, so only two sides of it really benefit from the raised bed effect. In wet weather, the center of the bed and the "high side" still can have standing water.

Raised bed

Our soil in the Senior Garden is clay. At about one foot deep, you hit a strata made up of some really nasty orange-gray clay that doesn't drain well. Penetrating that clay layer as deeply as possible with a dry sump drain helps dry out the garden in wet periods. The process is really pretty simple (and somewhat tiring). The short version is that you dig as deep a hole as possible with a post hole digger and fill it with coarse, builders' sand.

With the weather still nice outside and garden space opening up as fall approaches, I gathered my tools (rake, shovel, and post hole digger) and some sand and got to it. I've forgotten where I read about this type of drainage aid (maybe in Mother Earth News), but used it successfully to help dry out a perpetually wet spot just outside the back door of a house where I once lived.

Area for the sump Deep hole begins
I raked away the mulch from the area where I wanted the dry sump. I first dug away a circle, carefully setting aside the topsoil to return to the hole when done.
I dug about one foot deep and maybe sixteen inches or so wide with the shovel before beginning with the post hole digger.
About as deep as I could go The hole
I ended up going about three and a half to four feet deep. At that point the post hole digger's handles began to bind on the hole preventing going any deeper unless I was willing to widen out the hole. Yep! That's a nice hole in the ground!
Post hole backfilled with coarse builders' sand a bit more sand to about four inches below soil surface
I filled the hole with sand and packed it in as tightly as I could. A tamp would have come in handy for this job. I also filled an inch or so of the wider hole, as the sand will settle some.
Hole fillled with topsoil Clay on compost pile
The topsoil that I'd set aside goes back in the hole on top of the sand and is packed to a slight dome to allow for settling. The heavy clay went onto the compost heap where it will "learn" to be topsoil!

If you have a persistent wet spot and like me, garden on heavy clay soil, the dry sump trick should help some. But don't expect it to be a wetvac or sewer. The improvement is only moderate. But for my rather large (16' x 24') raised bed, it may prove to be an ideal solution.

Update (4/5/2019):

The terraced or "semi-raised bed" I mentioned earlier had its two other sides enclosed with landscape timbers to create a true raised bed a year after I wrote this piece. I'm not sure a 16' x 24' raised bed is ideal, but we've made it work for us. All the ones I've built since have been around 4' x16', easily workable from outside the bed.

The dry sump in the center of the bed does help dry the soil in wet springs, allowing earlier tilling and planting. It also tends to really dry things out mid-summer when our rainfall tapers off.

Since I've mentioned raised beds a lot in this piece, here are links to our methods of building raised garden beds.

From the at Senior Gardening


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updated 4/5/2019