In climates that receive adequate rainfall during the growing season, or where trees can access a high water table, watering is primarily recommended only for new and establishing trees.
For many areas of the Klamath Basin, access to water is a major consideration. Experienced nursery professionals caution that it takes longer for trees to reach full size in our region, a factor of both our cold winters and limited water during the growing season. Drought stress can also take a less obvious toll on landscape trees, making it more difficult to connect the dots between lack of water and the problem. Let’s take a look at some ways drought stress affects our landscape trees.
Failure to thrive: Most folks realize that a newly planted landscape tree needs to be watered in order to establish roots, but often the amount and frequency of water needed are underestimated. Consider the transplanting transition: the tree has been nurtured in a nursery for years, watered every summer day, sometimes twice, in its container. Suddenly, it’s transplanted, destroying most of the “feeder roots’, tiny root hairs best at absorbing water and nutrients. The tree is then plopped into a hole, followed by a weekly application of water at the base of the trunk. Sometimes, the well-meaning new tree parent will adjust the lawn sprinkler to reach the tree’s patch of yard, not realizing that the matted roots of the lawn will likely absorb all of the water. Many of these challenges can be avoided by fall planting, a counterintuitive but useful tool for avoiding stress in newly planted trees.
Wrong tree, wrong place: Plenty of trees grow just fine in dry, wild areas, without additional summer water. These are specific species adapted to doing so- the notorious juniper, for instance. Landscapes, however, are full of trees that prefer different environments- birches, maples, fruit trees. There is a tree for every environment and ecological niche: however, landscape tree choices are often based on aesthetic preferences or the desire for fresh fruit, not ideal conditions for the tree. Some might argue that nurturing the wrong tree in the wrong place well enough for it to become the right place is the essence of gardening: The thrill of growing something “they” said could not grow here. Success, however, will require that larger trees receive additional water to approximate a more appropriate climate. For many large, mature trees, ten gallons of water per week is a good baseline — and difficult to achieve with a lawn sprinkler.
Flagging: When trees leaves are wilted, browning, or limp, this is referred to as flagging, a sign that something is wrong within. Single branches may flag, as in when wood boring beetles first infest. One side or section of the tree may be considerably more wilted, or yellowed: this is an indicator of Verticillium wilt, common in some maples. Or the entire tree may flag, possibly indicating drought stress. In some cases, the tree appears fine early in the day, but takes on a wilted appearance, with leaves hanging limply from their petioles, during the warmer hours.
Calls to the wood boring beetles: The Pacific Northwest is home to a multitude of wood boring beetles, most of which infest trees already stressed for some other reason — drought, lightning strike, or disease. The Bronze birch borer, discovered in Klamath Falls in 2017, is an example of this type of pest drawn to drought stressed trees. The larvae of this insect feed right in the cambium, where water and nutrient exchange for the tree is taking place. Adequate water drowns the little larvae, preventing them from taking hold. Once established, the Bronze birch borer will kill the tree unless there is chemical treatment. Pruning during the summer also causes some trees to send out stress signals that attract wood boring beetles.
Strong to the last gasp: A large, mature tree has a significant reserve of water and nutrients. One season of drought stress may not result in visible clues to a problem. After multiple dry summers, however, a tree may seem to “collapse” overnight, when the problem has actually been ongoing. When diagnosed, these situations are often hard for the homeowner to digest.
A lack of summer precipitation does have some advantages. There are far fewer foliage and fungal diseases of trees and shrubs in our climate compared to those that have more summer rain. One reason for this is that the presence of the pathogen, moisture, and the right temperature must exist on the leaf in order for disease to take hold. Lack of moisture on the leaf prevents many plant diseases in our area. So keep watering those trees — at the root zone, not at the trunk or on the leaves.
Nicole Sanchez is horticulture faculty at OSU’s Klamath Basin Research and Extension Center. For more information on this or other gardening topics, contact Nicole at Nicole.email@example.com or 541-883-7131.