If you garden in an area with poor soil fertility and drainage, you're probably familiar with the advantages of planting in raised beds. Less familiar, however, may be the concept of sunken beds - beds dug down to sit slightly below the soil surface.
The Advantages of Sunken Beds
For gardeners in arid climates, sunken beds offer several advantages to raised beds and ground level beds. They are designed to capture maximum rainfall and retain moisture, so they dry out less quickly, keep roots cooler, and lessen the need for irrigation. Below grade walls also cast a small amount of shade, which gives garden plants a slight reprieve from scorching summer temperatures.
Assessing Your Site
Unfortunately, not all arid garden sites are good candidates for sunken beds - for example areas prone to frequent flash floods. As many gardeners in the southwest can attest, desert soil can also be hard to excavate - sometimes literally as hard as a rock. The usual culprit is caliche, a hardened deposit consisting of calcium carbonate. Also referred to as deadpan, caliche beds can form impermeable cement-like layers in the soil that prevent water from draining, roots from developing, and cause soil to skyrocket to the alkaline end of the pH scale. Layers of caliche can vary from a few inches to a few feet thick, and may be nearly impossible to dig through with a shovel unless first soaked with water. Even then it becomes very heavy and clay-like and removing it is a lot of work. Gardeners who find themselves unable to penetrate layers of caliche by hand need to consider if their garden is large enough to make enlisting the help of experts worth the cost.
Digging Sunken Beds
If you decide your garden site is a good candidate for digging sunken beds, you will prepare them the same way you would any other new planting site. First you'll need to dig down at least 24 to 30 inches deep, screen out any existing caliche, and fill the beds back up with a mixture of soil and compost. This ensures the soil will have sufficient drainage, and that plant roots have sufficient amounts of organic matter and adequate space to grow. Adding organic matter will also help neutralize imbalances in the soil's pH. Beds should be refilled so that the top of the bed is a few inches lower than the original grade. Leave native soil around the sides of each bed as a path. Its compact nature will also help direct rainwater toward the beds.
Mulch Like Mad
Mulching reduces the stress of plants growing in dry soils by slowing down the rate of evaporation. Add a thick layer (3 to 5 inches) of organic mulch around plants as they emerge to keep soil cool and help retain moisture. Using straw or compost will also add extra organic nutrients to the soil. Native plants (those naturally adapted to dry conditions) should not be smothered in mulch.
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