Remember the advertising campaign for a brand of cigarettes aimed at women? “You’ve come a long way baby,” was their slogan. To borrow that catch phrase, the same can be said about the moisture sensors now available for irrigation systems. They’ve more than come a long way, they’ve arrived and become an indispensable tool in the management and conservation of two precious resources—water and time.

While moisture sensors have been available in the market for years, they’ve only recently begun to gain acceptance in the world of the landscape contractor. Part of the problem has been that early moisture sensors failed in the field a short time after they were installed. After so many broken promises over the years by manufacturers of the next best thing, the “once bitten, twice shy” mindset is understandable.

But with advances in technology, the accuracy and reliability of the sensors designed for the landscape market make them a perfect fit for the contractor wanting to better manage his time and optimize his conservation of water.

Photo courtesy: Irrometer

“The newer no-maintenance type sensors are the perfect fit for the urban irrigation market,” says Tom Penning of the Irrometer Company in Riverside, California. “Sensors like our Watermark can provide ‘set it and forget it’ type systems, once they are properly installed and tuned in for the site conditions.”

Now before going on, let’s define what we mean by moisture sensors. After all, there are rain sensors, and there are other devices used to measure evapo-transpiration rates. While those are excellent, useful tools for monitoring weather conditions, they are devices placed above ground. The moisture sensors we will be discussing are devices placed in the soil.

The way a moisture sensor works is that it’s buried in the root zone. Many of the products now available are digital moisture sensors, which are capable of giving you an accurate measurement of moisture in the soil at any given time.
“A moisture sensor is the ultimate rain sensor,” says Dale Hansen of Acclima, in Meridian, Ohio. “It’s in the ground, taking advantage of what rainfall is actually usable.”

What you do with that reading depends on the system the sensor is hooked up to. It can be as simple as an on-off switch that keeps the irrigation system from turning on when it rains, to a supplier of data to measure moisture content and soil temperature that can be adjusted from anywhere in the world.

Worldwide access of the properties you are caring for may not be an immediate need, so several companies offer a basic device that can be installed on just about any irrigation system.

The concept is simple. The units are an add-on box to an existing irrigation controller that ties into the 24 volt side of the controller. Basically what you’re doing is breaking the common whenever the sensor determines the soil doesn’t require any more moisture. It’s what’s called a suspended cycle system.

Here’s how it works. Whenever you have rain or irrigate the landscape, the moisture content of the soil goes up. The sensor measures the upper and lower threshold of the soil’s capacity to retain moisture. The upper threshold is your field capacity -- how much that soil can hold without any of it leaching through or running off.

The lower threshold is the 50 percent depletion point, or half-way between field capacity and wilt point. At wilt point, the plant dies. You don’t want it to get that low. But when you reach the 50 percent depletion level, that’s when you’ll start irrigating. And you’ll want to irrigate until you get back to field capacity.

So with a suspended cycle system, the sensor waits until the soil moisture drops down to the lower threshold before allowing an irrigation cycle to operate. The sensor then lets irrigation take place until it detects that the moisture level is at the upper threshold. When it reaches that level, it shuts off the system.

“You still set the run times; you still set the day schedule, and everything you would normally do,” says Hansen, “but what the sensor does is set the lower threshold, and whenever it drops below that level, it allows the next irrigation cycle on that controller to operate. If you’ve got enough moisture in the soil, it suspends your next irrigation cycle.”

By preventing the irrigation cycle until the soil really needs it, you are not only doing what’s best for the landscape, but you are on the way to conserving water, lots of water. Many studies have shown water savings of 50 percent and more on irrigation systems that utilize moisture sensing technology.

Installing a moisture sensor is easier than it sounds. There’s very little disturbing of the soil. You dig a hole to accommodate the sensor. How deep a moisture sensor is buried depends on the plant material. For turf, you’re looking at four to six inches depending upon soil type; you want the sensor to be in the root mass. You not only want to monitor evaporation, but also transpiration, which is moisture leaving through the plant material. If you’re placing a sensor in an area with shrubs or trees, you’re going to go deeper.

Next, lay the sensor in the ground and place the sod back on top of it. Then with a flat blade shovel, v-trench over to the valve box, lay the wire four to eight inches deep, compact the dirt back into the trench, wire the valve and you’re done. Hook it up to your controller: five wires to the controller, three wires to the valve.

Sensors come with up to 25 feet of wire, but you don’t have to run the sensor wire to the controller. You just trench into the nearest valve. Using the power wire for the zone that goes to the controller is what powers the sensor. Because you’re already using pre-existing irrigation controllers, you don’t have to change anything. It’s very cost effective.
“Simple add-on systems are very affordable,” says Penning. ‘They often pay for themselves in water savings alone within a few months.”

“It’s just the best product for water conservation, environmental protection, and a healthy landscape,” says Hansen. “It’s going to give you the healthy landscape because you’re only keeping water within that root zone; you’re not wasting your fertilizer by leaching it through. You’re just keeping the root zone moist constantly. That’s what plant materials need. They don’t want to be stressed; they don’t want to be over-watered, driving the oxygen out. They just want to have moisture when they need it”

Depending on how much control you want, or need, will ultimately affect the system you decide to install. In many instances, a one-sensor system that ties into a pre-existing controller is more than sufficient. It will turn on your irrigation system when you reach the lower threshold, but you’ll still have to dial-in the controls. If you don’t have the run times set properly, it can still waste water by applying it and exceeding field capacity. With a water-on-demand system that won’t happen.

Photo courtesy: H20 Strategies

A water-on-demand system will set both the upper threshold and the lower threshold. They water whenever the soil gets to the low threshold, and will continue to water until it gets to the upper threshold, or until it reaches a water restriction which you can set, whether you can water only so many hours in a day, or so many days a week.

“In our system,” says Kye Hellmers of H2O Strategies in Manhattan Beach, California, “the combination of the moisture sensing probe and encapsulated electronics make a decision whether there is sufficient irrigation in that zone. It will then instruct that irrigation valve to either continue irrigating because not enough moisture is present, or to stop because sufficient moisture is present. When the system is installed, it’s hands-free.”

Water-on-demand can be as simple as one per valve or one per system common. Or it can be more complex for larger, more diverse, landscapes. With many of these systems, several irrigation valves with similar watering requirements can be grouped together, creating a hydrozone. Individual hydrozones have a sensor location controlling that group of valves.

You can also have multiple zones and run multiple sensors per zone. You have a lot of flexibility especially if you have varying irrigation heads, or if you’ve got some tight clay on one side of the property and sand on the other. You might have microclimates that vary from shade to sun; those are areas where you may need additional sensors to monitor them.

How big your property is and the variables on that property will factor into the decision on what type of system you ultimately decide on. If it’s a large piece of land that has the same exposure to the sunlight, with the same plant material and the same soil type, you’ll probably only need one sensor, no matter how big the land is.

But say you’re on a site regularly, and you’ve always manually adjusted the irrigation schedule, depending upon the observed needs of the landscape. Why should you incur the expense and work of adding moisture sensors to the system? You’ve gotten along fine without them, why change now? What’s in it for me?

“A system like this is a huge time management saver,” says Ed Mathieu of Baseline in Boise, Idaho. “It has the capability of completely automating itself to water appropriately throughout the whole season. Typically, the contractor has to run around and guess what day to change the controller, when to add more water, when to add more days, and when to cut back watering. That’s a time management issue that a contractor will never overcome unless he automates his system.”

“Savings of water alone is not the only factor to consider when evaluating moisture sensing systems,” says Penning. The labor savings gained from eliminating seasonal controller re-programming adds up. Also, indirect costs such as fertilizer and pesticide savings from creating a healthier plant environment through proper irrigation management need to be considered.

Once you’ve decided to install a moisture sensor, the next step is to determine what type of system will best fit your needs. The price range is a few hundred bucks for a simple residential system to 20 grand or more for a unit that can handle a large subdivision with 600 zones. The larger systems are typically installed during new construction, but can be added after the fact.

With all these smart technologies it’s important that you do your homework and know the advantages and disadvantages of each system. The key is to understand exactly what you’re trying to purchase and what you hope to do with it.

A good way to do your homework is to check websites, then talk to local distributors and people who are utilizing the equipment now. Find out exactly what they liked and what they didn’t like.

Water purveyors are also starting to enter the picture. Some water districts offer rebates for systems that have soil moisture sensors. And some areas are mandating that all new construction will be required to have soil moisture sensors attached. More and more water agencies are encouraging the use of smart technologies.

But it’s also important to note that these technologies are not a magic bullet. They are an aid to help you manage and do a better job of applying water appropriately. But they need to be used properly.

As water conservation becomes more and more of a hot-button topic, anything the landscape contactor can do to advance the cause also advances the longevity of the green industry.

“We’ve got to show the powers that be, that we’re capable of monitoring and managing ourselves,” says Mathieu, “and doing the right thing, especially when it comes to water use in the landscape. We’ve got to get back to watering based on need and not based on what’s convenient for us.”

“It’s not about how quickly we can get these technologies thrown into the system and how much money can the next guy make off them. This is something that has to really move us forward, because if the industry doesn’t learn from this and do a better job, we may not get a second chance.”