Watering Trees

dead tree_COS_PLH_3The tree had clearly expired. What leaves remained had turned a sickly yellow-brown, and hung limply on the branches—in mid-August! Yet, when our neighbors planted it last year it had been perfectly healthy. Something was obviously wrong, and I had a hunch I knew what. (I’ve often said that master gardeners kill just as many plants, we just know why they died.)

Surreptitiously moving the cobblestone mulch aside (and wondering if the homeowner was watching through the closed curtains), I looked at the drip irrigation set-up. There was only one emitter, and it was directing water right to the base of the trunk. No wonder the tree was dead! (That and the fact that it was planted too deeply; there’s no sign of the root flare.)

close-up tree irrigation wrong_COS_PLH_1

dead tree_COS_PLH_2We have a strange tendency—even among professional landscapers, who should know better—to water a tree’s trunk instead of its roots. Maybe that’s because we can see the trunk, while the roots are hidden under dirt and mulch. Obviously, a newly planted tree will have a restricted root ball, but still, it’s those roots that need irrigating. Watering the trunk is worse than useless. Constantly wet bark can no longer protect the tree, making it susceptible to a myriad of lethal bacterial and fungal diseases. There is a reason the average life expectancy of a tree in suburbia is a mere eight years.

Assuming it was planted correctly, as a tree grows, its roots spread outward, staying within the top two feet or so of soil. After all, that’s where the nutrients, air, and water are. In the case of a mature tree, most of the “feeder roots”—those that absorb those nutrients, air, and water—will be found away from the trunk, in the area shown in this illustration:

tree drip line

As you can see, the roots begin about halfway between the trunk and the tree’s drip line, and reach at least an equal distance past the drip line, although they can extend outward up to five times the diameter of the canopy. That may be all the way to the neighbor’s yard! (While there are roots closer to the trunk, their main purpose is structural support—keeping the tree in place.)

You can see why these drip arrangements are so silly:

Of course, both of these trees get their water when the lawn sprinklers go on. The drip hose does nothing but wet the trunk. Here is a close-up of another tree with a similar problem—all the water is inside the root zone—and this one isn’t planted in a lawn:

watering tree 9

Shoveling gravel mulch over irrigation hose around planted tree @CarnegieLib 16apr07 LAH 971When a tree is planted, it’s easily watered with a ring of drip hose, positioned over the root ball (not tightly around the trunk), as shown here. But as the tree grows, it’s important to add concentric rings, watering the entire root zone. In a traditional landscape, eventually the roots will extend far enough that watering the lawn or surrounding shrubs will take care of the tree as well. But in a xeric landscape, depending on the water needs of the tree, it may need to be on irrigation “life support” for its entire life.

That will certainly be the case for the English oak we were required to plant as a street tree. Situated between the street and the sidewalk, its only companions are low-water western sandcherries (Prunus besseyi) and some assorted native wildflowers such as penstemons and desert four o’clock (Mirabilis multiflora). None of these will require nearly as much water as the oak, making for a challenging irrigation problem. (We’ve temporarily solved it by using drip tubing with more, higher-flow emitters around the tree, along with some extra hand watering during especially hot and dry weather.) That will continue to be true even as the tree grows, and its roots reach under the sidewalk and into our front yard—which also planted with water-saving species.

tree water bagOne more thing. In the process of writing this post I came across a new product being widely sold online: the Treegator. Apparently, you fill the bag with water and it irrigates your newly planted tree. Knowing what you know now, it should be obvious why this is a really bad idea—just one more way to kill an innocent tree.

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This entry was posted in Botany for Gardeners, Gardening, Xeriscape and tagged , , , , , . Bookmark the permalink.

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