By Gary Deters
The summers of 2021-2023 presented significant challenges for those managing lawns or turfgrass with drought conditions, extreme temperatures, and a lack of rainfall. Many homeowners likely relied heavily on underground irrigation systems to maintain healthy grass or to follow municipal odd/even watering restrictions. Even smart irrigation systems with rain sensors, soil sensors, or weather-based controllers allowed irrigation based on weather and soil conditions due to the lack of rain.
Rain sensors (Figure 1) are designed to prevent automatic irrigation systems from operating during or after rainfall. When a specific moisture threshold is reached, the system shuts off if it is running, or it prevents a scheduled cycle from starting. Both wired and wireless rain sensors require proper setup to ensure that irrigation does not occur during rainfall. One of the disadvantages of rain sensors is they do not monitor soil moisture and can allow the irrigation system to operate even when there is sufficient soil moisture. For example, an area might receive a soaking three-inch rainfall, but if the weather following the rain is sunny or windy it could dry out the rain sensor corks which could allow irrigation despite adequate soil moisture.
Soil sensors (Figure 2) are typically buried in the soil at specific depths to measure moisture levels directly at the grass roots, allowing for continuous monitoring. It is difficult to accurately assess soil moisture by simply observing the surface. While a homeowner may feel the need to water the lawn based on the appearance, there could already be sufficient moisture for the grass roots. Soil sensors can eliminate guesswork, ensuring more efficient irrigation decisions.
Weather-based irrigation controllers use real-time weather data to adjust watering schedules and optimize water use, as opposed to traditional controllers that operate on fixed schedules. When properly installed and managed, smart controllers and sensors can significantly reduce water waste and prevent unnecessary irrigation.
At the Minnesota Landscape Arboretum (Figure 3), we just finished a first-year trial with rain and soil sensors that was designed to determine how much water each of these technologies can save over a 15-week period. The research area has a unique setup with each of 24 individual plots being controlled by a separate irrigation controller; furthermore, flow meters allow us to record how much water each plot is receiving over a given amount of time. The irrigation controllers (Figure 4) were programmed to run every Friday morning between 3:00am and 4:30am, resulting in a total of 15 watering days from June 7 through September 13. While September has been mostly dry, the 2024 growing season experienced several timely rainfalls, with some instances of excessive precipitation, making it an ideal year for this trial.
Of the controllers, 4 were set up with soil moisture sensors and 8 had rain sensors. In addition, another set of 4 plots was irrigated based on readings taken from a handheld moisture meter that determines soil moisture. Data analysis from the trial revealed that rain sensors prevented irrigation about 33% of the time, while the in-ground soil sensors and manual moisture meters reduced irrigation over 85% of the time. This disparity shows fairly clearly that soil moisture sensors and a handheld moisture meter are far better at reducing water use. Although rain sensors performed poorly, they still demonstrated their value in reducing water use, which is important because these are very commonly installed on home lawn irrigation systems. We plan to repeat this experiment during summer 2025, and it will be interesting to compare the results with this year's findings.
This project is supported by the Metropolitan Council as part of our collaboration to reduce water use on Twin Cities lawns.