Soil basins are circular dams constructed to direct water to the root zones of trees and large shrubs. They are also useful for watering widely spaced vegetable plantings, such as tomatoes.
Build a shallow dam, about 4 inches high, just outside the plant’s drip line by drawing soil into a pile with a hoe. To protect the trunk of trees sensitive to water, make a second ring about 6 inches away from the plant trunk. As the plant grows, keep moving the basin outside the drip line.
To thoroughly water trees in loam soil, fill the basin once, let it drain, and repeat. Trees in clay soil will need the basin filled at least three times; those in sandy soil will require the basin filled only once (if the soil is extremely porous, let the hose run for several minutes to fill).
Basins are usually built as a temporary watering measure until trees are established and can survive with the same watering as the rest of the yard. When the trees are established (their roots have grown well into the native soil), break down the basin to allow good drainage.
Basins can also be a permanent part of your watering system, even if you have a permanent underground system. Position bubbler heads close to the base of any area you wish to water this way, then build a basin to contain the water. Each time the system is turned on, the bubbler will fill the basin with water. This watering method is particularly useful for roses and other plants prone to leaf diseases. It give good deep watering without getting the leaves wet.