If we’ve learned anything over the past few years, it’s that we should never take our water supply for granted, especially if you live on an island. Take Hawaii, for example. Water for drinking or irrigation isn’t easy to come by there, making it very expensive. The average water bill for a one-acre estate with a house, pool, guesthouse and landscaping can be as much as $5,000 per month. And the allotments are strict. Go over the amount you’re allowed to use, and your water is simply shut off.

The numerous resorts on the island use reverse-osmosis systems to obtain water, with a backup to the county supply. Tapping the county mains is avoided as much as possible, according to Kevin Moore, an irrigation consultant and landscape designer who works and lives in Hawaii. “Water is so expensive, it’s like buying liquid gold.”

In such an environment, there’s a tremendous need to boost the efficiency of existing irrigation systems. When Moore is called in on a retrofitting job, either commercial or residential, one of the first things he does is install soil-moisture sensors.

“At one of the resort living estates on the west side of the big island, the property manager said that they were using 32,000 gallons per month,” he said, “which they needed to cut back drastically.” Moore placed nine soil-moisture sensors on the one-acre property. “After we installed them, we adjusted the settings. When we were done, we’d cut the resort’s usage from 32,000 gallons per month down to 8,500, without losing any of their lush tropical plants.”

Soil-moisture sensors

So what is a soil-moisture sensor? It’s a device that has at least some part of it buried in the ground, anywhere from three to eight inches deep. It is connected to the controller either by wire, or it could be wireless. It works by sending out an electrical impulse to detect how much moisture exists between the soil particles. Some soil-moisture sensors are compatible with various brands of irrigation controllers, while others are brand-specific.

After installation, the next step is to set the controller with low and high thresholds. When the sensor detects a moisture level that’s below the low threshold, it will tell the controller to go ahead and irrigate when scheduled. Conversely, if it senses that soil moisture is at the high threshold, it tells the controller to ignore the next scheduled irrigation time.

Most smart controllers use ET (evapotranspiration) data from weather stations. While better than conventional controllers, in Moore’s opinion, it’s still guesswork. He prefers using soil-moisture readings, because he knows he’s getting a precise picture of what’s really going on in the soil.

Across the Pacific, in the San Francisco Bay area, home of Roger Waters Environments, Inc., water is also very pricey, with tiered-rate structures. “People who have large estate properties are always up in the third or fourth tier, where the rates are just outrageous,” said company president Roger Waters, a soil and water management specialist. “So cutting the amount of water use makes a big difference.”

When there are problems with sensors, he says it’s usually because someone hasn’t taken the time to install the devices properly, putting them in too deep, or in the wrong areas. He’s found that with the area’s clay soils, a depth of three to four inches works best, giving him much more sensitive readings.

Soil-moisture sensors helped Waters save the day for the owner of a winery residence in Napa, who’d been hit with a nasty surprise. Once her winery was built, the County suddenly limited all water to the entire property, probably because of the drought. Previously, there’d been no limit on how much water she could use. One July, the owner saw that she was close to exceeding her allotment, which would’ve resulted in a cutoff, threatening both her home and her business.

Waters installed several sensors on the two acres of landscaping around the home and winery (not the vines themselves). He dramatically reduced the amount of water used—from 8,000 gallons per month down to 3,200, and knocked 40 percent off the water bill to boot.

But there are more benefits to using soil-moisture sensors beyond water conservation and lower bills. Plants that get just the right amount of moisture—no more, no less—are healthier for it.

Waters put the sensors in while the owner of the property was away. “When she returned, she called me, saying the plants on the property had never looked as good—and they’d only been in the ground for a week. Just taking into account the amount of water that’d been cut back, she thought I must have killed her entire landscape—until she actually saw it.”

Drew Barna, irrigation manager, Mid-Atlantic region, for BrightView (the new name for the merged ValleyCrest and Brickman companies), loves soil-moisture sensors, not just because they’re excellent tools for managing irrigation, but because he’s seen how plant material responds when they’re used correctly.

“Think of the soil, of the planet, as a living, breathing organism. If you tell the controller, ‘I want 22 percent water in the soil,’ you’re creating a condition where all the air space doesn’t get filled up with water, which is what over-watering does,” says Barna.

“The plant life does better, not only because it’s getting a consistent amount of water, but also because you’re aiding that oxygen exchange.”

The sensors also help with horticultural practices. “Almost all of them will also read the soil temperature. We use that information in determining when we should apply pre-emergent herbicides. They’re very useful sensors to have in the ground.”

They’ll also tell you how much salt is in the soil. This is a function of how the devices work, because they rely on soil conductivity to transmit their electrical signals. The more salt, the more conductive the soil is.

But salt’s not good for plants, and unfortunately, much of Hawaii’s soil is full of it. Because nutrients tend to leach out very fast there, a lot of chemical fertilizer is used, leaving a great deal of the compound behind. Moore uses the sensors’ salt-sniffing capabilities to tell him if a landscape’s been over-seasoned.

Moore says the sensors are a source of remote telemetry for “innumerable” amounts of data. “I used to have to drive around and physically check all of these places. Now, I can just sit at home with a cup of coffee and a computer. I can do a diagnostic, or see if a solenoid somewhere isn’t working right.”

Barna says there’s good reason to learn about soil-moisture sensing products, if you’re not already familiar with them. BrightView services only commercial customers, and this group is demanding more eco-green solutions on a daily basis.

Soil moisture-sensing technology has come a long way since the devices first started hitting the market. The products have become durable, reliable, accurate and very easy to install. They’re the type of tools thirstier times require, and they’ve come up to speed just when we need them.

Rain Sensors

Rain sensors are installed on roofs and work in a variety of ways. All of them collect rain until a certain threshold is reached, then they send an electronic signal to the controller to shut off. Some rain sensors use a hydroscopic disc that swells with water and shrinks as it dries out, turning the controller on and off accordingly.

Other devices have a ‘tipping bucket’. When it fills up with rainwater, it tips over, cueing the controller to stop irrigating.

Both kinds are usually wireless.

“These products have been on the market for a few years, so most people using them are intimately familiar with them,” said Daniel Hunter, product manager for sensors and remotes at Hunter Industries, San Marcos, California.

“Not only are they standard practice, in some areas they’re mandated.

The most horrifying thing for some water boards is to have their people drive around and see properties with their sprinklers running in the rain.”

Rain and soil-moisture sensors save you the trouble that can result from this kind of unsettling discovery. Not only that, but they can help make landscapes healthier and more beautiful by providing just the amount of water they need—no more, no less.