Rules for Watering Your Trees (by Season) From A Leading Tree Expert

This summer has been a scorcher. Drought conditions and sizzling temperatures have spurred the Oregon Department of Forestry to remind residents to water trees. In a July press release, the department emphasized the importance of consistently saturating the area beneath a tree’s drip line to mitigate the effects of hot, dry weather. In the same spirit, we’re sharing our Portland certified arborists’ tips on how to water your trees throughout the year.

Multiple factors influence watering needs. Established trees will generally require less water than newly planted trees. New trees’ root systems are less established, so they require careful watering during the first one or two years. Plant placement is also important. Buildings, walls, and fences can reflect heat, putting plants at risk of damage. These areas will need more water, as will windy zones. Soil composition matters, too. Sandy soils drain quickly and therefore need more frequent watering. Clay-heavy soils tend to hold onto moisture longer, so they can be irrigated less frequently. Portland tree care will also vary by species. Crape myrtle needs just monthly summer watering, while stone fruit trees have moderate watering needs (twice per month) and birches need the most water of all (three times per month). Water-hungry trees naturally grow along streams and rivers; species from arid region plants will survive far drier conditions.

hose water in front of treesPortland Tree Care: General Watering Tips
Water at the Drip Line. Large, established trees have a root spread as wide as the tree is tall. Water the most critical part of the root system by soaking the area beneath the drip line.
Slower is better. Trees can best soak up water when it is distributed slowly to a depth of 12 inches. Slow watering allows for best absorption. Irrigation lines are ideal for this style of watering. A good rule of thumb for each watering session is to slowly distribute 5 to 10 gallons of water for every inch in the tree’s diameter.
Avoid soaking right around the tree’s trunk. Doing so can increase the chance of disease, fungal infestation in particular. Instead, focus on watering the area underneath and beyond the tree’s canopy. (Young trees are an exception to this rule; their roots will only occupy the planting hole area right around the trunk.)
Be generous with damaged roots. Root systems that have been damaged will require more water for repair.
Mulch for protection. Lay down a blanket mulch to a depth of three or four inches beneath the drip line. This will conserve moisture and offer some protection against cold. Leave the space right around the trunk free of mulch.
Avoid extended soil saturation. Too much water can smother developing roots. If the underlying soil is soggy, hold off on watering. You can use a soil probe to determine soil moisture levels. A ¼” to 3/8” diameter rod will slide easily through moist soil, and become difficult to push into dry soil. You are looking for moisture at a depth of six inches below the soil surface.
Water near the soil, not on foliage. Avoid spraying leaves with water. This can deposit salt and damage foliage. Overhead irrigation creates a disease-happy environment, where leaf spot diseases, mildew, fungus, and bacteria can take hold. Soaker hoses, standard hoses, and irrigation systems are the best water delivery systems.

With these overarching watering approaches in mind, let’s examine seasonal watering nees.

Summer Watering in Portland: Trees’ irrigation needs are at a zenith in the mid to late summer, when winter moisture has been exhausted from the soil. Deep watering will satisfy a tree’s summer needs for 10 days to four weeks, depending on the species and the conditions. During the most extreme conditions, watering once per week or more may be required, depending on how the soil holds moisture.

  • Honor soil absorption rates. If you notice that the water is pooling, turn off the water for a while and it soak into the soil. Keep watering in this way—until the water begins to pool, in an off and on cycle—until the soil has been wetted to a depth of 18 to 24 inches. This may require several days to accomplish, unless you have drip irrigation systems installed.
  • When to water in the summer: Water early in the morning and late at night, when evaporation rates are at a nadir.
  • Look for signs of heat stress. If leaves are wilted, crisp or yellowing around the edges, they are struggling under the heat, and deep, consistent watering is needed.

Fall Watering in Portland
Here in the Rose City, the early fall is often dry and warm. Soils may still dry out under the crisp blue skies, so it’s best to maintain the same vigilance regarding soil moisture as in the summer.

Assuming moist soil conditions and cooler weather, fall is a good time for Portland tree planting. Be sure to support the development of root systems by watering weekly or more frequently if soil dryness calls for it.

Winter Watering in Portland
Portland’s winters tend to be soggy affairs. From time to time, Stumptown does see snow flurries. In those cases, wait to water. Winter watering should occur when air and soil temperatures are above 40 degrees Fahrenheit, with no snow cover.

  • Water during dry spells. Monitor weather conditions. During extended dry periods, water once or twice per month.
  • Avoid cracking soil. Winter freezing and thawing causes cracks in the soil. These cracks expose roots to desiccation and cold. Prevent this damage with regular watering and mulching.
  • Water midday, so that water has an opportunity to soak in prior to nighttime freezing.

Spring Watering
Watering is typically less critical during Portland’s spring, as winter and spring rains tend to leave our soils damp.

  • Check soil conditions weekly; water if you discover dryness at a depth of six inches.
  • Water once per week during the growing season, if soil is dry.
  • Deeply areas exposed to de-icing chemicals. Wash away any remnants of de-icing materials by deeply watering once or twice per year. Use up to three times what you would typically water, to wash away minerals and salts.

As this dry, hot summer winds down, it’s important to recognize that drought conditions leave trees more susceptible to disease and insect damage. Therefore, deep summer watering is key. We include watering advice in our Portland tree services. For a complete picture of your trees’ health needs, contact us. Our leading certified arborists are founts of tree knowledge. They can tell you how much water your species need, and how frequently to water each during the different seasons of the year.

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