More About Sunken Beds

July 3, 2012

I talked a bit about sunken beds in last month's article about raised beds, but I got a question from one of you the other day that told me that I need to talk about them further, because in many cases sunken beds are even better than raised beds.

Sunken beds are like raised beds in reverse. The idea is to make a huge pot using something similar to a raised bed system, but with the soil at or below ground level.

The advantages of this are:

  • You can make the soil as it should be by filling the bed with good soil or compost. This works great in areas with contaminated, salty, or dead soil, just as with raised beds.

  • You can add protection to your garden from digging creatures such as rabbits, moles, and gophers using hard sides and a wire mesh bottom, just as you can with raised beds.

Here's where sunken beds really shine:

  • Since the bed is below the ground, the soil around the bed helps cool it off, perfect for desert areas.

  • The bed edges help keep moisture in the bed, so you don't have to water as often as you have to water pots or raised beds.

  • If you get the base of the bed below the frost line, the mass of the earth around the bed will keep the plants warm. Because of this, you can keep plants in it year round -- you don't have to lug pots in and out every year. Of course, depending on the plant and the climate, you may need to add other frost protection strategies.

    How do you make a sunken bed?

    The components of a sunken bed are: the hole, the side barriers, the (optional) floor barrier, and the soil.

    First you must dig the hole, which is the hardest part. If you're not up to that, it shouldn't cost too much to get a neighbor kid and his or her friends to dig the space out for you. It helps to mark off the area you want to plant first. Put it somewhere fairly permanent.

    Use your sunken beds as part of your design! They don't have to be square or rectangular -- since you're digging them, make them any shape or size you want.

    If you want to put wire mesh on the bottom to keep out digging animals, do so before you set up the side walls. Because we want to keep animals out, I would use something made of metal with a mesh of 1" (2.5 cm) or smaller -- chicken wire, metal hardware cloth, metal screening, etc.

    Don't use landscape cloth or any sort of fabric that an animal can chew through or that will rot, or you'll be wasting your time.

    You can use anything you want to for side barriers, but don't use anything that's going to rot such as wood or bamboo. Remember that this is going to be more or less permanent.

    My husband and daughter (who was about eight at the time) made sunken beds for me at our first home using concrete and river rock (which was lying all over the ground at the nearby creekbed).

    Other items you can use are: cinder blocks, brick, metal siding (smooth off sharp edges), stone, and so on. Use inexpensive items that are readily available in your area for underneath (which won't be seen once the dirt is in), and decorate the top with the expensive stuff if you like to make mosaics or decorative walkways.

    This can be a fun family project and give your children some useful skills.

    Now for the soil. If you have made compost, use that. If your soil is clean (as in not contaminated or salt) and you don't have your own compost, mix the soil you dug out of the hole with as much compost or other organic materials (manure, hay or straw) as you can get.

    Make sure you use materials that have not been sprayed with chemicals! Sometimes hay and straw have been sprayed with herbicides (straw especially) that will kill your plants too. If you can, find manure from animals that have been fed chemical-free food.

    If your soil is not clean, you'll have to have compost or soil brought in. Once your initial expense is done, however, you'll have a wonderful deep rich soil to plant in.

    If you love planting long tap-root plants such as carrots and parsnips, plants with special soil needs, heavy feeders such as corn, or invasive plants like mint, sunken beds are the best way to go. They are also excellent in desert, high-latitude gardens, and gardens which suffer periodic drought.

    Although heavy clay is a serious challenge to dig (you may have to hire machinery to do so), a sunken bed can permanently fix your soil problem and open up plants to you that you could never have planted before.

    Where I would not use a sunken bed is in wet areas, especially those that flood constantly, unless you are planning to use plants that like having their roots wet all the time. Use raised beds instead.

    What do you think?

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