Digging a Water-Harvesting Basin Around a Young Desert Tree With a Round Point Shovel: Out Past the Drip Line, 6 Inches Deep, and Berms Four Times as Wide as Tall

Round point shovel beside a shallow water harvesting basin around a young tree

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Most young trees on a dry lot get their water in a little ring right at the trunk. That keeps them alive, but it also keeps their roots small, and the summer storms run right past them down the slope.

Dig a shallow basin that reaches out to and past the tree's drip line, at least 6 inches deep, so rain soaks in where the feeder roots are. Pile the soil into a low berm about four times as wide as it is tall, compact the berm, leave the basin floor loose, and give overflow a rock-lined exit.

Why the basin goes out, not in

University of Arizona Cooperative Extension's guide to passive water harvesting says feeder roots, which take up most of a plant's water and nutrients, are often found near and beyond the drip line. It warns that trees watered only near the trunk, even several years after planting, often never build an extensive root system and become vulnerable to blowing over.

The same guide says basins should be at least 6 inches deep to collect water well, and up to 12 to 18 inches where large volumes have to soak into a small area. It also says lining inlets and spillways with rock prevents erosion.

The classic mistake here is a deep, narrow moat hugging the trunk. It holds water against the bark and root flare while the roots that actually need it sit dry a few feet out.

Laying it out

  1. Find the drip line: the outer edge of the branches. On a young tree this might be only 2 or 3 feet from the trunk.
  2. Mark a ring a foot or two beyond the drip line with flour or a garden hose. The basin will widen as the tree grows.
  3. Watch where water runs during a storm, or run a hose upslope, and note where it enters. That is where the inlet goes.
  4. Pick the low side for a spillway, where overflow can leave without cutting a gully.

Digging it

I dig with a round point shovel, because the pointed blade bites into hard, dry crust better than a square spade. If the ground is rock-hard, I soak it the evening before.

  1. Leave a slightly raised, undisturbed pad around the trunk itself so water does not pond against the bark.
  2. Dig the ring to about 6 inches deep, sloping the inner and outer edges gently rather than cutting straight walls.
  3. Throw the soil to the downhill side and shape it into a berm.
  4. Make the berm about four times as wide as it is tall: a 6-inch-high berm should be about 24 inches across.
  5. Tamp the berm firm with the back of the shovel, then stop. Do not walk on or tamp the basin floor.
  6. Cut a shallow notch in the berm at the spillway and line it with fist-sized rock.

Arizona Extension's residential water harvesting presentation gives the same build rules: dig when the soil is moist, build berms four times as wide as they are tall, compact the berm, do not compact the bottom of the basin, and keep the slope into the basin gentle.

Finishing the basin

A bare basin crusts over and sheds water. I cover the floor with 3 to 4 inches of wood chips or small gravel, kept back a few inches from the trunk.

After the first good storm, I walk out and look. If water pooled for more than a day, the basin is too deep for the soil; if it overflowed early, it needs to be wider.

Hard ground and caliche

On caliche ground a basin can turn into a bathtub that never drains. Test by filling it with a hose and watching how fast it drops before you plant anything new in it.

Home Depot and Tractor Supply both carry round point shovels and bagged wood chips, which covers one or two trees. A local tree service often gives chips away by the truckload.

If the drain test fails, see this walkthrough on breaking a drainage chimney through caliche.

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