Watering Urban Trees, May 21

As rainy as western Oregon often seems, the truth is that typically in May and June no rain falls on more than half the days of the month in cities inland from the coast. With trees leafed out and putting on new growth, urban trees need to be watered during these two months to survive and thrive. Yet many people in Oregon cities only start watering their trees in the sunny and hot months of July and August. By then it may be too late. Kristin Ramstad is manager of the Oregon Department of Forestry’s Urban and Community Forestry Assistance Program. Ramstad said the majority of trees planted in urban areas are native to places that receive ample summer rain. “Their main season of growth comes right when rainfall in Oregon is decreasing. That can lead to trees’ growth being stunted,” said Ramstad. Ramstad points to the Learning Landscape arboretum at Meek School in northeast Portland’s Concordia neighborhood as an example of how irrigating trees in spring and summer has helped non-native trees grow faster. “Watered every spring and summer since being planted in 2010-12, trees in that arboretum are already much taller than similar trees which got watered only for the first couple of years,” she said. Besides growing more quickly, younger non-native trees will also be less stressed if they receive supplemental water. “If trees are already somewhat water deprived in spring, they can really suffer once temperatures soar and rainfall ceases altogether in July and August. Lack of water can cause dieback and even kill non-native trees in Oregon,” she warned. “Trees under stress are also more likely to be attacked by insects. That seems especially true for conifers.” How much water do trees need? Ramstad said trees less than three years old should get 10 to 15 gallons once a week from the time they leaf out until early fall. Older non-native trees might need 20 to 25 gallons. “Let the water slowly soak in. The goal is to have the water get deep into the root zone.” In the Willamette Valley, rainfall averages only 2 to 3 inches in May and June. An inch of rainfall equals 1.6 gallons per square foot. A young tree’s roots may only reach into an area of 4 square feet. That means they can access only about 6 gallons from an inch of rain falling in their root zone. “For the whole month a young tree might receive less than 20 gallons from rainwater. But for healthy development they need two to three times that – about 10 to 15 gallons every week. And if grass is allowed to grow up to the tree’s trunk, almost no rainwater will be available to the young tree,” said Ramstad. She suggested keeping grass at least 2 to 3 feet away from a tree’s trunk to reduce competition for water. “Mulching around the tree also helps,” she said. All newly planted trees require watering to survive their first couple years. For those looking to minimize watering beyond that, Ramstad said oaks native to Oregon and California are a good alternative. “They usually don’t need to be watered after their first two years because they are adapted to hot, dry summers. In fact, watering a native Oregon white oak can cause problems that could cause the tree’s death.”