Wilson Bros Gardens LLC

To soak most types of average soil to a depth of 3 inches it takes about 1/2 inch of water, either from rain or a sprinkler system. Doing some simple measuring will help you to know how much time it takes your sprinkler system to spread 1/2 inch of water over a given area. 

To measure how much water your sprinkler system puts out in a set amount of time, you can use 3 rain gauges to conduct a test. If you don't have or don't want to buy rain gauges, tuna cans or straight-sided coffee cans will work. Set one gauge near the sprinkler, one near the outer spray perimeter, and one halfway between these two points.

Now run the sprinkler zone for 15 minutes. Then combine the water from the 3 gauges into one gauge and use a ruler to measure the depth of water collected. Divide this measurement by 3 to get the average output of your sprinkler system. For example, if the measurement is 1/4 inch, then you now know that this sprinkler zone would need to be run for 30 minutes to soak the soil to a depth of 3 inches, or 1 hour to soak to a depth of 6 inches, and so on.

Note:  If there's a significant difference in the closest and farthest gauge measurements, you may need to adjust your sprinkler heads, or add new sprinkler heads to your system so that two streams overlap when watering.

Not all soils are alike. While 1/2 inch of water may soak to a depth of 3 inches in a loamy soil, it may only soak to half that depth in a dense clay soil, and maybe even less if there is a slope which allows water to run off. So you need to take one more measurement to determine the depth 1/2 inch of water applied will soak to in the soil. Run the sprinkler system for the amount of time necessary to spread 1/2 inch of water, as explained above under Measuring. Then dig down in the soil to measure the depth of soak. If not 3 inches, adjust the watering time accordingly up or down.

Let's assume that watering with 1/2 inch of water soaks the soil to at least a depth of 3 inches, and that this occurs with a 30 minute watering cycle. When watering newly planted shrubs, trees and other plants it's important that the entire root ball of the plant, which could be 6 inches or more in height, remains consistently damp to moist during the first growing season. This would mean you would have to run your sprinkler system for twice the amount of time (1 hour) in order to apply 1 inch of water that would soak to a depth of 6 inches. 

Here's the conundrum. What if there are established or other plants in the same area that don't have the same water needs? This is where it gets tricky when attempting to properly water landscaped areas where various types of plants with various water needs are growing. In this situation, the best I can advise is to run the sprinkler zone for only the amount of time needed to meet the requirements of the plant(s) in the area that require the least amount of water. Then provide supplemental water by hand to the other plants in the area that require more water, such as newly planted plants.

In average garden soil you should not have to water your newly planted plants every day. In the absence of sufficient rainfall, water only as needed to keep the entire root ball and surrounding soil damp to moist. Keep in mind that deep soaking to a depth of 6 or more inches less frequently, and allowing the soil to dry some before watering again, is much better than splashing just a little water on plants every day. The same is true for established plants. 

During the first few weeks after planting, to avoid oversaturated soil or overly dry soil, use a soil moisture meter or your finger to test soil moisture before watering with your sprinkler system. Run the sprinklers only as needed to maintain a damp to moist soil to the necessary depth. After a week or so you should know how often to run your sprinkler system to maintain proper soil moisture. At this time, you can set the system to run automatically at specified times. That said, if there is sufficient rainfall turn the system off until it is needed again. 

If you're sprinkler system can not be run long enough to deeply soak your newly planted plants to a sufficient depth, provide ample supplemental water using a garden hose during the first growing season. remember, it is best to deep soak the soil less frequently than to splash a little water around plants every day.

When watering with an automated irrigation system, water during the early morning hours and not in the late evening or at night, which can lead to the onset of fungal diseases. 

When plants have gone dormant for winter, which means they are not actively growing and drinking as much water, plants will require much less water.