Saturday, July 18, 2009

Landscape Irrigation without the Numbers – Step one.

Step 1) Discovering your watering rate:

Basically in step one, we want to figure out how many minutes it takes your system to wet the root zone of your plants.

First we need to let your yard or garden dry out a bit.

Then you will water it once for the normal amount of time.

Then you will measure the depth of the wetted area under your plants.

Let the soil in your yard or garden dry out for the regular interval between irrigation cycles, plus maybe one or two extra days. Watch your plants carefully. You may need to hand water certain plants, especially your pots and containerized plants that begin to stress.

When you are satisfied the ground is dry, or when many of your plants are beginning to wilt, turn your irrigation system back on manually for one cycle. Everything should get the normal drink of water. If your plants normally get 30 minutes, run it for 30 minutes. The best time to do this is early in the morning or just after the sun sets. Wait about an hour after the normal watering cycle is finished, then grab your soil probe. You are going to gently push it into the soil about 4” - 6” away from the drip emitter or bubbler of each plant. Remember, I said gently.

**** But first, a word of caution. ****

Every yard has utility lines, water, electric, phone, cable TV, natural gas and sewer. Add to that, landscape irrigation pipe and hoses, pool piping, landscape lighting, electrical for outdoor kitchen, patios, fountain pumps, drainage line, etc. Anytime you dig or push a soil probe into the ground, you should understand where these things are and take proper precautions not to hit any of them. There is a chance that you could make contact with an electrical line and receive a lethal electrical shock. Never use a hammer or anything to drive or pound your soil probe into the ground. Also make sure your probe is rounded or blunt on the tip. This way, if you do bump a drip line or pipe, your probe will probably slide off rather then puncture or cut the line.

Always call before you dig or excavate! Arizona Blue Stake will mark your utility lines in your yard for free.

The soil will be wet so the probe should push in easily. My wife can do this with one hand. If you have very little hand and arm strength, you might need to use two hands, but be careful. If the probe doesn’t go into the soil easily, don’t force it. You can move to the other side of the plant and try it, or move to another plant and try it again. I have been using soil probes for a very long time and I have never damaged anything. However, I use them very gently as if I expect I might hit a drip hose or pipe.

When the probe hits dry soil, it will be become harder to push it down. Don’t force it. You are measuring the depth of the wet soil only. Hold your finger, or mark your probe at ground level and then pull it out. This is the depth of the wet zone or soil reservoir your irrigation system has provided for your plant.

The 1, 2, 3 rule works well for mature plant root zones. Here in the desert, ground covers have a root zone about 1 foot deep. Shrubs feeder roots will usually run about 2 feet deep. Tree feeder roots usually run about 3 feet deep. Lawns should be watered to about 8”.

Adjust your timer till you have the right wetted depth for most of your plants. You don’t have to be perfect, just get close and write down your results. I usually make a simple drawing with the wetted depth for each plant along with the number of minutes used on the timer. This is very close to the amount of minutes you will use to irrigate each given zone or valve.

Don’t go out and buy a bunch or drip emitters and try to balance each plant just yet. In the next step we will talk about proper wetting of the entire root zone and balancing the water rate at each plant. You may need to change or add some emitters in this next step. That will be when you can go out and buy stuff.

Work through your watering depths and check back here for Step 2. Wetting the Root Zone.


  1. I hope you don't mind my asking but how deep on average are utility lines? I found a wire last summer that was literally an inch down. Of course I left well alone and reburied it but it still makes me nervous. Thank you!

  2. Hi Mo,
    You probably found a Television Cable wire. These are very often just below the surface. I think Cox did many homes this way in the 80s and 90s to keep up with the demand. On the bright side, they will most often repair the lines for free. The path they follow is usually right along the right or left property line and then truning in toward your house at the electrical panel.

    In subdivisions, much of the time the other utility lines follow a similar path. The Phone lines are usually 12"-18" deep with electrical 24" and deeper. I say usually because you cannot depend on this to be true in your yard. I have uncovered very shallow, gas, telephone, cable and electrical lines that seem to zig-zag across yards for no obvious reason.

    The safest thing to do is call AZ Blue Stalk and have them mark your utility lines before you dig. The service is free and you can take pictures of the markings and save them for future reference. Sometimes I do this before I start a design because I don't want to conflict with a utility right-of-way or set my client up for problems later.

    I always have a property marked because even though I can pretty much tell you where your utility lines are, there is always that one situation where for some reason the original contractor didn't follow the standard form.

    Hope that helps you, Mo.