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Monday, July 3, 2017

Irrigating Big Pine Trees Requires Lots of Water

Q. What is the optimal watering and fertilizer schedule for desert pines? I started taking care of a large horse property two months ago. Many of the trees were in distress from lack of water and fertilizer. I've been watering about 5 gallons per tree per day and the trees have responded with new growth and color.

A. Five gallons of water per tree per day is not much. Watering this way forces tree roots to grow along the surface of the soil. It does nothing to encourage deeper growth. Deeper root growth is important to  prevent them from blowing over during high winds. It is also important for their tolerance to dry conditions.
            I understand they were in bad shape but with the amount of water you are giving them they will never become lush and dense. The density of most trees increases with increasing amounts of water. Tree density decreases with lesser amounts of water.
            Larger trees require more water. Smaller trees require less water. You can decrease their water use through selective pruning to control their size if their water use is a problem.
            It is difficult to estimate how much water they need. But the amount of water they will need in one year is roughly equivalent to the volume of water 4 feet deep under their canopies.
Let's do some math. Estimate the cubic feet, 4 feet deep, under the canopy.First calculate the area under the canopy. Use 3.14 x half of the diameter squared. If the diameter of the canopy is 20 feet, multiply 3.14 x 100 which is 314 square feet. Now multiply 314 x 4 to get the cubic feet of the area 4 feet deep. The cubic feet of water needed by EACH pine is 1256 cubic feet.

Multiply the cubic feet by 7.48 to estimate gallons of water. This will be about 9320 gallons of water consumed by each tree this size each year. About 2/3 of this water, or about 6100 gallons, will be used from May through October. The remaining 1/3, about 3100 gallons, will be used from November until May. 

Water less often but with more water. Depending on the soil you have, water once or twice each week during the summer months. Water less often than this during the fall, winter and spring months.Give each tree enough water so the water will drain at least 2 feet deep around the tree. It is very important to give these trees the same amount of water each time you irrigate them. A surface mulch of wood chips in these basins, at least 4 inches deep, would help these trees considerably.
You can't see it very well but there is a basin around this pine tree. I would make this basin deeper so that it will hold at least 2 to 3 inches of water. Large basins may require 2 bubblers or 2 rings of drip emitters around the tree.
            Trees of this size might be best irrigated using a basin around each tree trunk. The trees are watered by filling this basin. The basin should be constructed at least half the size of the trees canopy.It does not have to be round but should be located where you want most of the roots to grow. If these trees are close together, you could group 2 or 3 trees with a shared basin. Just make sure each of the basins are level.

The basin is also a convenient place to apply fertilizer or horse manure, in your case, once or twice a year.


  1. Thanks for all the information and the formula. Excellent reminders. Even when I think I am practicing good water techniques I forget just how much it takes to make this part of the world green. Now, I need to go check and see if I really am watering for the size of trees today vs when I last set the bubblers.....maybe 2 years ago? Thanks Bob, I so appreciate you.

  2. Could you please explain why the pine trees surrounding the office complex called Quail Park IV at 2820 West Charleston planted right next to the buildings and surrounded by asphalt are so huge? Where are they getting the water? Thank you