Friday, October 16, 2009

Raised Bed Garden or Sunken Bed Garden

When starting a garden, the goal is to maximize your advantages while minimizing your disadvantages. A raised bed garden is a good way to do this. By building a raised bed, you give your garden great drainage. You don't have to worry about poor native soil, either. You need to fill up the raised bed anyway, so you might as well fill it with the good stuff. A raised bed garden also helps bad backs by making it so that you don't have to bend over so far to get to your garden. In addition, there are other minor benefits, such as getting the garden up high enough that certain pests, like rabbits, can't get to it. If slugs are a problem in your area, a simple strip of copper ringing the raised bed will keep them out. Slugs and snails won't cross copper.

When I started my first garden at my first house here in Arizona, my goal was to create a raised bed. Materials were expensive and I didn't have a lot of money, so it was a long term goal. But after a few years, I discovered that a raised bed garden would not necessarily be a good thing here. In the arid southwest, good drainage is a bad thing. The more water you let drain away, the more you have to supply. All of our water here comes from the ground. The more you use, the more you pump out. The more you pump out, the less there is to go around. So conserving the water you have is a very good idea. In addition, we get close to enough rain to water everything. It just doesn't always come down when you need it. So the real way to make that work is to store as much water as you can when it does come and use it for the dry periods. A rain barrel makes for a good storage device, but it can become cost-prohibitive to buy enough to fully meet your needs. What you really need is a way to keep the water that hits your garden, a way to store the water in the ground.

A sunken bed garden does just that. It keeps the water from flowing away long enough that it can soak in. You then have the water stored in the soil itself, which will help the plants last longer between waterings, which means less water used overall. If your terrain allows, you can even shape the earth so that your sunken bed catches runoff from elsewhere. I will cover methods for doing that in a future post.

Another advantage for sunken bed gardening here lies in the soil itself. There isn't as much sand in Arizona as you might think. In fact, there is a lot of clay, and lots of that is expansive. Expansive clay works much like water-grabber crystals. When they are exposed to water, the microscopic particles expand and hold on to the water. In its natural state, this is a bad thing, and not just for your building foundations. During a rain event, the clay particles on the surface swell and seal off the pores in the soil. This means that the water can't really penetrate deep into the soil and just runs off, wasted. A sunken bed will help the water sit long enough for it to soak in. It still may just sink in a few inches, just enough to saturate the surface and cause troubles for the plants that don't like wet feet. To really take advantage of the water-holding properties of the soil, you need to amend the soil. When you first create the garden dig down a foot or more and amend with organic material, preferably composted wood chips. The wood will have some lasting power in the soil. Ideally you'll have as much as 50% of the volume of the soil as organic material. This will open up the pores of the clay and let the water soak in deep. The clay particles will still swell and hold the water, but now more of them can do the work, delivering it slower and holding it over a greater area. Also, if you treat the soil with a mycorrhizal fungus, the fungus will travel through the organic material, better surviving than in a soil that is poor in organic material. It will then send its filaments throughout the soil and grab the moisture that the plants can't reach and deliver it to the plants.

So, how do you design one of these? I'll cover that in a future post as well, so stay tuned. For now I'll just say that factors like local rainfall, soil and what you are planting all come into play. I will also say that this sort of design is ideally suited for landscaping and will give you a lower-maintenance landscape. It is a little trickier for vegetable gardening.

9 comments:

  1. Hi Edmund,
    Looking forward to your blog on sunken beds. We have soil here in northern California that is high in clay. Wondering if this would work better than the raised beds we have now.
    Thanks!

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  2. Regina, I covered that in my November post on Collecting Rainwater Through Grading. But beware of collecting too much water in clay soil. Clay holds on to water really well and can suffocate your plants' roots.

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  3. Hi Emund, I was wondering if you have covered how to build a sunken bed again in one of your topics. I'm live in Prescott as well and if you have sunken beds in your garden I wouldn't mind taking a look! Thanks.

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  4. Anonymous, I did a post in October called Making a Water Budget, which is the first step and another in November on how to design and build one. I also did another in December covering a more complicated (and probably unnecessary) way to calculate rainfall. I don't have any sunken beds at my current residence, but I did co-design some for my neighborhood, which are currently in use, and probably functioning right now, since it rained last night. Write me at ed(at)holodeck.com if you want to try to get together.

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  5. Edmund,

    Am very anxious to hear your ideas on designing a sunken vegetable garden. I live in Mesa, have very hard clay soil, and flood irrigation. My grandfather grew all of his own food in a Mesa garden years ago. Unfortunately I did not learn his methods. I have an acre lot and believe I could become very self sufficient with the right approach to a garden that could be watered efficiently. Have lots of Bermuda Grass that would need to be dealt with if you have any ideas.

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  6. I recently started a vegetable garden in Phoenix and am very curious about the sunken beds! I started two plots - one raised and the other slightly sunken. Basically, I dug out a bunch of native earth and filled the cavity (maybe 5"-8" deep) with rich soil and mixed it with a little bit of the native soil, which appears to be decent quality. I planted tomato transplants in that plot. I really hope they grow and produce lots of tomatoes!
    Looking forward to your comments on sunken beds! Thank you!

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  7. I posted more details about creating sunken beds at http://madbioneer.blogspot.com/2009/11/collecting-rainwater-through-grading.html, but I haven't actually tried vegetable gardening in a sunken bed myself, and likely won't get the opportunity myself in the forseeable future. So for those of you trying it, let me know how it goes. Oh, and my suggestion would be to lower the whole bed but then grow the veggies on little mounds to keep food away from possible contamination in the water.

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  8. Hi Edmund, I am Jim Hogan, an amateur heirloom tomato grower. I am experimenting with: ground level beds, raised beds, sunken beds, and containers. I have a web site for my low-desert friends: AZ-HeirloomGarden.org
    I would like to use some of the great information from your blog "Raised Bed Garden or Sunken Bed Garden" on my “Sunken Garden” page. Please let me know how you feel about this. jhoganaz@msn.com

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