Summer Watering Guide for Mature Trees

The mature trees in your yard and along streets need water during hot, dry summer months. Even with established root systems, urban trees should be watered consistently and deeply to reduce the risk of pests and disease induced by drought stress (https://extension.oregonstate.edu/gardening/flowers-shrubs-trees/how-drought-stress-affects-landscape-trees).

The frequency of water depends on soil type, exposure to sun and wind, and tree species and size. However, these general guidelines (https://www.oregon.gov/odf/Documents/forestbenefits/watering-fact-sheet.pdf) apply to most large trees in the Portland metro area.

Method of watering

Consistently saturate the area beneath the drip line.
Water deeply and slowly to encourage a healthy root system using a drip irrigation system or soaker hose.
Avoid overhead watering. Sprinklers don’t saturate the soil, and watering the foliage can increase growth of leaf spot, mildew and other fungal diseases.

Frequency and amount of water

May through September, water mature trees at least once per month if it’s dry. During extreme heat or drought, increase frequency to once per week. Saturate the soil 18-24” deep.
During mandatory water restrictions, prioritize watering trees over lawns. Lawns will recover with the rains, trees may not. Mature trees take years to replace.
Avoid fertilizer near trees as it spurs growth which increases water needs.

Other tips and guidelines

Add 2-4” of mulch along the drip line to conserve soil moisture and reduce temperatures.
Look for signs of drought stress, drooping or yellowing leaves. If you see these signs, increase the duration and frequency of watering using depth of saturation as your guide.
Do not add supplemental water to Oregon white oak (Quercus garryana), Ponderosa pine (Pinus ponderosa) and other drought tolerant species. (https://extension.oregonstate.edu/collection/trees-shrubs-drought-tolerance)
Trees near driveways or street trees will have greater watering needs as the adjacent paved surfaces reflect heat. These soils are likely to be compacted from foot traffic. If the water pools, turn it off. Wait for it to soak in, then turn it back on.
Pay particular attention to Western redcedars. You may have observed the die-back of Western Redcedar (https://extension.oregonstate.edu/video/tree-health-webinar-series-western-redcedar) in the Willamette Valley which is believed to be caused by drought stress resulting in fine root death and vascular collapse and ultimately in foliar and decay disease, causing trees to thin and die from the top down.

Consider adding more water-wise trees to your yard, trees that will tolerate the long, dry summers: