Dear Bill,

Thank you for contacting us about watering your dogwood trees. In the Willamette Valley, you should try to provide one inch of water per week over the entire root area (ie. beyond the drip line of the tree canopy.) Since about half of the soil by volume is just space, the moisture needs to reach a depth of two inches in the soil. 

This includes rainfall and irrigation in normal temperatures. If the temperatures become extreme, more water may be required. 

For larger single-trunk trees, the recommendation is to provide 5 gallons of water per week per inch of trunk diameter (measured about 4 feet above the ground.) Again, the water should be distributed across and slightly beyond the drip zone of the canopy. 

Irrigation water should be applied slowly and deeply. The best way of accomplishing this is to use a soaker hose. Lawn sprinklers contribute little to the moisture needed by trees because so much is lost to evaporation or absorbed by the grass roots. The full amount of water does not need to be applied in one session. You can divide into two waterings per week.

I hope this information is helpful. Please write again whenever you have questions.