I just planted a California pepper tree. A 36-box. How much water does it need per day in the deep root pipe?

Question

It’s in the ground now.

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Answer 1

You should discuss that with the nursery or contractor who did the planting. A tree like that is a big investment and information on watering is best given by the local service who planted it, and who knows more about the backfill soil that was used and the details of the installation of the watering tubes. Frequency for watering depends on soil type and climate and how fast the soil dries.

The watering should be based on an "as needed" basis and not a specific number of times a day or a week. The need should be assessed with a soil moisture meter.

I only see two tubes in the photo. Are they perforated watering tubes or are they solid wall inspection tubes? If only two watering tubes, then don't depend solely on those for water supply, also water the soil surface. Although there does not appear to be an adequate water well formed to hold four to six inches of water.

For a general recommendation, without benefit of a moisture meter and site specific information:
Fill the watering tubes and then monitor with a dry "dip stick" to determine when the water in the tubes is gone, (no standing water in the tube). Then water again.
Use some of the soil piled on the left side in the photo, move it around to form a burm in a circle just outside the mulched area for watering purposes. Fill that well at the same time you fill the watering tubes.

Do this for another month until weather cools down in November, then go to less frequent watering during winter. Although in your locality drought conditions and drying winds could occur, so keep up the watering as needed. Next spring-summer-fall will be critical to provide appropriate water for establishment of the root system. Leave the watering well in place and gradually extend it to water a larger area of soil surface for encouragement of an expansive absorbing root system. Mulch the soil surface and use compatible plantings that don't require frequent water in the long term; do not install lawn grass around a Peruvian (Calif) Pepper. Long term excessive watering will cause root rot.

https://www.gardeningknowhow.com/ornamental/trees/pepper-tree/california-pepper-tree-care.htm