Magazine article Sunset
Watering Deep Where Trees Really Need It
How deep is deep when it comes to deepwatering trees? After three relatively dry winters, it's an important question. Heavy rains in March went a long way toward replenishing soil moisture in parts of northern California, but in many other areas of the state, the soil is still quite dry. This could be a critical summer for many specimen trees you might not normally consider watering-valuable, tough-toreplace ones that have the greatest impact on your landscape. One or two efficient, deep waterings to wet most of the root zone could make a big difference to a tree's overall health and survival.
A tree's height and spread don't reflect the depth of its roots
Most tree roots are in the top 3 to 4 feet of soil. More importantly, if the soil is mulched or shaded, 90 percent of the absorptive roots are usually in the top 12 inches. Laterally, many tree roots grow within the drip line but often extend well past it. However, watering to just outside the drip line should supply the tree with adequate moisture.
Irrigating much deeper than 3 to 4 feet is unnecessary and probably a waste of water. It takes about 4 inches of water to reach a depth of 4 feet in sandy soils, about 7 inches in loam soils, and as much as 16 inches in clay. (Clay, however, holds water 3 to 4 times longer than sandy soil.) Use a slim rod to check water penetration; it should move easily through moist soil and stop when it reaches dry.
Here are three ways to get water down this deep without wasteful runoff.
Use sprinklers or drip irrigation. A lawn sprinkler or soaker hose, turned on very slowly or cycled on and off, lets water penetrate the soil without runoff. Use a cup to keep track of the sprinkler's output, and move the sprinkler around to wet the entire root zone.
Drip emitters or mini-sprinklers on black polyethylene drip tubes can also water deeply, and runoff is less likely. This is a more expensive (though permanent) alternative, but not practical for trees growing in lawns.
Build a soil basin. This time-honored technique is still an effective way to wet the root zone. …