During periods without rainfall, trees should be thoroughly watered on a regular schedule. Slow, deep watering provides a cool drink for thirsty trees and creates a water reserve, much like filling up a sponge. Tree roots draw from the reserve until the next watering session. This encourages development of deeper roots and helps sustain trees during hot dry summers. A thick layer of woody mulch around your tree is also valuable to help retain soil moisture.
Current science suggests deep, soaking irrigation applied every 3-4 weeks is best. Water your trees during summer months without significant rainfall, typically June - September, adding May and October as needed. Trees For Seattle recommends that newly planted young trees need 15-20 gallons of water twice per week. Mature trees need substantially more water, potentially 1-3 hours of soaking irrigation each month, depending on tree size.
Urban landscapes often have compacted soils which makes it difficult for water to penetrate top layer of soil. A lawn sprinkler is NOT adequate and can promote shallow tree roots. Trees do not receive sufficient water from lawn irrigation systems and schedules. We suggest using a deep root watering probe that connects to your garden hose to deliver water directly to the tree’s root zone.
Start 3-6 feet from the base of the trunk and push the tool 6-12 inches into the ground to get below the surface compaction layer. Turn the water on low and let it run for 15-20 minutes. Then, move the probe to a second location (1-2 big steps) within the drip line of the tree - the area between the edge of the foliage and the tree trunk. For trees up to 12 inches diameter, water in 2-4 locations around the tree. Increase the number of locations for larger trees.
You can also water your trees with just a hose. Set to low flow (just above a trickle) and thoroughly saturate the soil. It is important to run the water long and slow to avoid run-off and evaporation. Move the hose every 15-20 minutes.
Do you have questions about when and how much to water your trees?
Contact the certified arborists at Tree133. We are happy to help!