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.