Long-Winded Explanation on How to Irrigate Landscapes

Q. I read your piece in the LV Review/Journal regarding the subject.  You recommended not increasing the length of watering time when it get hotter, but increasing the number of days one waters. How much time or number of days is adequate? 

I have:  Heavenly Bamboo, Gold Spot Euonymus and Golden Euonymus, Box leaf Euonymus, Pinkie Hawthorne, Med Fan Palms, Variegated Mock Orange, Pineapple Guava and more.  I live in a newer neighborhood where our plants are 1-3 years old and the landscaper put our drips on for 10 minutes twice per day when they were installed.  I can understand this when the roots are shallow, but 3 years later I’d think that is too much. Presently, I have two drips systems and have both on for 15 minutes 3 times per week for all plants.

A. These are very difficult questions to answer for a specific person because they are site-specific. The difficulty in giving you precise watering information is because of the soil at your site and how your irrigation system was designed. Whenever you have a site that has a number of different types of plants growing in the same location, it's difficult to answer with any kind of precision. I will try to answer your email as specifically as I can and in general terms.

Distribution of water. When we water plants we fill the soil with water that surrounds their roots. The water should be distributed beneath the plant so that the area receiving water is about half of the area under the canopy of the plant. In very general terms, this means spacing drip emitters usually about 18 inches apart from most soils. If irrigation basins surround the plant, the basins must be level even if the soil is not.

In soils with a lot of clay we can space emitters much farther apart. In soils that are mostly sand, the emitters must be placed closer together. So we strike a happy medium at about 18 inches apart.

Make sure you have enough emitters for plants and make sure they are spaced far enough apart to spread the water applied to the soil more evenly and over a greater distance.

Emitters can be located from six to 18 inches from the base of the plant. So for instance a 1 gallon plant would only require one drip emitter about 6 inches away. A 5 gallon plant would require 2 to 3 emitters about 12 inches away. A 15 gallon plant might require 3 to 4 emitters about 12 to 18 inches away. A 24 inch box plant might require 6 to 8 emitters scattered 18 inches apart under the canopy. 

Basically we want the water delivered to the plant so that it spreads as evenly as possible over the roots.

How many minutes. As I said above, when we water plants we fill the soil with water that surrounds their roots. The number of minutes to run the cycle depends on how deep the roots are and how fast the soil fills with water from the emitters. 

We generally figure that lawn roots are watered 10 to 12 inches deep, most annual vegetables and annual flowers are watered to the same depth as a lawn at 10 to 12 inches deep. There are exceptions in the vegetables such as onions, garlic at 6 inches deep. Small shrubs we figure have roots 12 to 18 inches deep and need water delivered to this depth. Big trees and shrubs we figure have roots 18 to 24 inches deep and need water delivered this deep. 

Depth of root system of different plants and depth of watering based upon their size

We set the minutes on our irrigation clock after we find out how many minutes it takes to get water to these depths. The only real way to determine the number of minutes needed is to run the irrigation for a certain number of minutes and "see" how deep the water goes. We can "see" how deep the water goes after a certain number of minutes using a little trick. This is the trick: "Sharp objects pushed into wet soil are pushed in much easier than the same sharp object pushed into dry soil." So we run the cycle for, let's say, 10 minutes. We wait another 10 minutes so the water finishes draining and push along sharp object into the soil next to the drip emitters in several locations. By "feeling" the resistance it takes to push these objects into the soil gives us the approximate depth that 10 minutes of water delivered. If the irrigation is not deep enough, we increase the number of minutes until we get the depth we want. 

This sounds complicated but once we do this we seldom need to change the number of minutes on the irrigation clock. This is because irrigating plants is like filling the gas tank on a car. If we drive the car until half of the gas tank is empty, we will fill the gas tank to the brim. I am willing to bet that if we fill that gas tank at the same pump each time, it will take the same number of minutes to fill half the gas tank every single time. This is the reason we can leave the minutes unchanged when we irrigate provided we use half of the water that we put into the soil each and every time before the next irrigation.

How often? This refers to the number of days we irrigate each week or every two weeks or every month. The unit of measurement (week, two weeks, monthly) depends on the irrigation clock. For simplicity let's use weekly.

I know this looks complicated. This is the general water requirement for plants on a daily basis for different months of the year in Las Vegas Nevada.

We know that plants use more water in the summer than they do the winter. Remember we are filling a gas tank so we need to let the water in that soil get "used up" before we fill it again. In the winter, that might take a week or two. In the summer, that might take one day to several days depending on the "gas tank" or the soil reservoir that we filled. Soil reservoirs that are deep and wide (large trees and shrubs) hold more water than smaller reservoirs (lawns, annual flowers, vegetables). Large trees and shrubs require more emitters and they require more minutes to fill their "gas tank". We water the shallow rooted plants more often than the deeply rooted plants but we water shallow rooted plants with fewer minutes and fewer emitters. 

Make sure basins that collect water for plants are level when these plants are growing on slopes.

For most woody plants that are not from the desert, we let them use up about 50% of the water in their "gas tank" before we irrigate again. For desert trees and shrubs, trees and shrubs that originate from deserts, we water them less often because they can tolerate soils with less water in them for longer periods of time. 

Basically, we run the "gas tank" down to 30 or 40% before we irrigate desert plants again. The way that desert plants conserve water is by surviving longer between irrigations when there is less water. When water becomes available to them, they respond quickly with new growth. Plants that are not from the desert don't respond this same way.

What is reality? The reality is there is no perfect irrigation system. Residential or commercial landscapes with lots of different types of plants are going to "waste" water. An ideal irrigation system would irrigate desert plants separately from non-desert plants. A perfect irrigation system would irrigate trees and shrubs separately from lawns, flowerbeds and vegetable gardens. An ideal irrigation system would irrigate the South and West exposures differently from the North and East exposures. 

Probably the closest to an ideal landscape and irrigation design is to follow the mini oasis concept developed out of Arizona which has evolved into what is now more commonly called "hydrozones".

Minioasis Landscape design concept from Prof. Warren Jones at the University of Arizona featured in Sunset Magazine a number of years ago

To get to your specific question, landscapers irrigate plants twice a day, daily, to 1) flush salts from the soil, 2) keep the soil wet so that any salts in the soil are "diluted" and cause less damage, 3) prevent callbacks by customers and 4) promote establishment of the plants. Certainly after a week or two they should no longer be irrigated this way. It is at this point the landscape maintenance company or manager or homeowner should take over the irrigation system and apply an appropriate schedule.

This is the reason I encourage people to determine the number of minutes for each irrigation valve using a long screwdriver or a piece of 3/8 inch rebar ground to a point at one end and bent over for a handle. Once they determine the number of minutes needed, they should seldom have to change it. Instead, I would like to see them change the number of days they irrigate each week as the seasons change from winter, spring, summer and fall.

I know this is long-winded but I hope it answers some of your questions. I will post it on my blog as well because you are not alone concerning these questions.