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Thursday, September 10, 2015

Please Define Deep Watering

Q. Please define "deep watering." In particular, how would you describe "deep watering" a pomegranate bush with a 3" to 4" trunk?

A. Good question. I throw these terms around and I assume people know them or understand them when they don't.
            Deep watering is talking about applying enough water in one irrigation so that the entire root system becomes wet. When deep watering it is also accompanied by less frequent irrigations. This means giving the soil and roots a chance to dry somewhat before the next irrigation.
            There are two ways to water. One way is to give a small amount very often, perhaps daily or twice a day.
            The other way is to apply a large amount and then wait a long time before you give it more. The second way is preferable to the first way for plants that are deep-rooted. Deep-rooted plants are larger, woody plants such as all trees, all large and medium-sized shrubs, all woody vines, and all woody groundcovers.
            Water these types of plants deeply by applying enough water so that the applied water drains to a depth of 18 to 36 inches. The water is then shut off until that soil begins drying. Technically, the plant is not watered again until about half of the water in the soil has been used.
            When half the water is gone, water is applied again until it reaches the same depth as before. This is a continual cycle with the number of days between irrigations decreasing as it gets harder and increasing as it gets colder.
            Deep watering normally applies to woody plants but not to lawns, flowerbeds, vegetable beds and small fruits like strawberries. These are shallow rooted plants and their applied irrigation water is shallow, not deep.

 Deep watering a pomegranate that has a 1 inch diameter trunk is really no different than deep watering a pomegranate with a 3 to 4 inch trunk. Water is applied to the same depth with both of these trees. 

The difference between them is where the water is applied and the amount. In the case of a pomegranate with a 1 inch diameter trunk, we might use only two drip emitters. Two drip emitters might deliver enough water to the soil under the canopy while it is small. 

However, when the pomegranate gets to 3 to 4 inches in diameter, more emitters are needed because the larger tree requires more water. These emitters are spaced far enough apart so that the water is applied over a greater area under the canopy of the tree. The number of minutes on the irrigation clock may not of changed but the number of emitters has so the larger tree now gets more water.


  1. I've had this same question. Is there a general rule of thumb about how long to time these deep waterings? Most of my trees have bubblers off PVC lines, but I'm going to be planting some more trees and using drip, so any kind of rule-of-thumb about how long it takes to get the water that deep would be great as a jumping-off point!

    Thanks, and love the blog!

    1. Because drip emitters come in various volumes measured either in leaders or gallons per minute, it is very difficult to come up with the general rule of thumb. It is easier run the drip emitters for a certain length of time and then determine how deeply the water has moved in the soil. For instance, set your irrigation timer to 15 minutes. Run the timer for 15 minutes and wait at least five minutes for the water to thoroughly drain into the soil. Take any kind of long pointy thing that is strong enough to push into the soil and push it into the soil a couple of inches away from the drip emitter. Long pointy things will push into wet soil much easier than dry soil. Push it until you feel a lot of resistance. Grab the long pointy thing near the soil surface and pull it out. This will tell you the depth of your irrigation. Do this with several drip emitters to get an idea of the average depth of water has moved. If it is not deep enough, add another 15 minutes and do it again. Once you have done this, you will seldom need to do it again because the number of minutes that you run the timer doesn't really change. What you will be changing seasonally as temperatures increase or decrease is the frequency the water is applied. In hot weather you apply the water more often than in cooler weather but the number of minutes will remain constant unless you change emitters or add or subtract emitters.Running water through the drip emitters into the soil is kind of like filling a gas tank on a car. You fill the root zone with water and then you hold off in filling it again until it used about half of the "tank".