In the DIY greenhouse article I talked about using sunken garden beds. This article goes into a little more depth (ha, sunken depth!) on that idea.
And why would you do this?
This is a technique taught in my PDC with Geoff Lawton. It’s specifically for drylands water conservation, whereas you might use raised garden beds in a wetter climate.
In the desert, success lies in using anti-evaporation strategies. This is another Geoff-ism.
The idea behind sunken garden beds is to use the shade and coolness of the earth to overcome the soil heating from the sun and drying winds. It will also catch any water used to irrigate it, or rainwater, and make is seep int the bed rather than letting it flow away.
Sunken garden beds process
It’s a little like double-digging, a technique used to loosen soil and improve drainage and fertility. But it’s not nearly as hard.
Essentially what you want to do is remove soil from where you want a garden bed, and use it to make a elevated walkway between beds. It’s really just the reverse of a raised bed, without the wood sides
Make sure to reserve the good topsoil in the bed, and put it in a wheelbarrow or set it on a tarp so it can go back on top when you’re done. Most of the soil life is in the top 6″ of the soil, so no sense in buying it and starting over building soil.
Your subsoil below the topsoil will be lower in quality and fertility. You want this subsoil for that elevated walkway. Make sure to compress it, but NOT the bed. You don’t want the walkway eroding into your garden beds.
However, NEVER step on the bed directly if you can help it. It crushes the air and water pores out of the soil, and the soil life needs both to do well. Soil is what feeds and takes care of your plants. Not “Miracle-Grow” or any other synthetic fertilizer.
If you need to step on the bed, put down at least a two-foot square piece of board or plywood to step on. This spreads the impact of your foot out to a larger area, and damages the soil life less than just a foot stepping on it.
After you’ve dug as deep as you want and made your walkway, put that good soil back in the bed, on the top. You may need to dig a little deeper bed because of this. Smooth the soil and break up clumps, preparing the seed bed.
Go forth and plant!
Now plant up the bed as normal, with either seeds or transplants.
One issue with these kind of beds is that they are physically harder to plant. Get some free range kids to help you. They’re closer to the ground.
It you’re direct seeding it’s a good idea to cover the bed with a floating row cover or shade cloth directly on the ground. This really helps improve germination.
Thanks for this technique go to the awesome Curtis Stone of Kelowna, BC Canada, owner of Green City Acres. See his video on this technique.
Just water like a normal bed, though you should find that it takes less water to keep your plants happy. You probably want to water overhead until roots at at least one inch long, then you can use drip irrigation.
Another tip I learned from Curtis is that some crops do better than others on drip, and it’s not what I’d expect. Check out his YouTube channel for more info.
This technique also works great in the greenhouse, but again it’s harder to plant and harvest. It may not give you as big of a benefit since the greenhouse itself takes away some of the problems with desert gardening.
But as you can see (right pic), my experience with sunken beds in greenhouses is pretty good!
Issue #1: Access. Already mentioned, it is more difficult to access the sunken garden bed for planting, weeding and harvest.
Fix: Mechanical advantage. You can use planting tools and machines to save wear and tear on the back and knees. These can se as simple and inexpensive as a dibble/dibber stick, to a very nice but expensive Jang seeder, or a wheeled seat and foam kneeling pad.
Issue #2: Erosion. You also may see issues with bed sides erosion, dirt sloughing off into the bed.
Fix: Soil barrier. You can reduce erosion of the walkway sides by putting in wood or rock sides, just like a reverse raised bed. It may be helpful to use landscape fabric to prevent dirt from falling in under the side supports.
If you combine this technique with others (some mentioned in my desert gardening article) like shading, wind barriers, native seeds and deep mulch, you can multiply the benefits and reap (literally) the rewards.
If you have any questions about sunken garden beds or desert gardening, or anything permaculture, please comment below and let me know.
I’d love to talk to you!
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The Crew at TPL