Determining how long to run sprinkler zones requires a precise calculation based on landscape needs and the system’s water delivery rate. Guesswork often leads to inefficient watering, causing shallow root growth, runoff, or plant disease. The ideal run time is variable, determined by site-specific factors like soil composition, plant type, and the unique output rate of the sprinkler heads. Understanding these variables allows you to transition from arbitrary timer settings to an efficient, precise watering schedule that promotes a healthier landscape.
Determining Water Needs Based on Plant and Soil Types
The first step in setting an accurate sprinkler schedule is determining the target water amount the root zone requires. Most established turfgrass needs about one inch of water per week during the peak growing season to maintain health. Watering to this depth encourages roots to grow deeper, making the lawn more resilient to drought and heat stress. For turf, the goal is to saturate the soil six to twelve inches deep, while deeper-rooted shrubs and trees may require saturation to eighteen inches or more.
Soil type dictates how quickly water can be applied and how often the total weekly amount must be delivered. Sandy soils, characterized by large particles, have a high infiltration rate, absorbing water quickly (often one inch per hour or more). However, sandy soil has a low water holding capacity, necessitating more frequent, shorter watering sessions to prevent water from draining past the root zone.
In contrast, clay soils are made of fine particles that pack tightly, resulting in a low infiltration rate (as slow as 0.1 to 0.3 inches per hour). Applying water too quickly causes immediate pooling and wasteful surface runoff, even though clay soil has a high water holding capacity once absorbed. Loamy soils offer the most balanced approach, with moderate infiltration rates and good water holding capacity, allowing for a more flexible watering schedule.
Calculating Sprinkler Application Rate
Once the target water depth is established, the next step is determining the specific application rate of your sprinkler heads, which often differs from the manufacturer’s rating. Water pressure variations, nozzle wear, and system design influence the actual precipitation rate (PR), measured in inches per hour. To accurately measure this rate, a simple physical test using catch cups is necessary for each zone.
To perform the catch cup test, place several straight-sided containers (such as tuna cans or specialized catch cups) in a grid pattern throughout the zone, ensuring they are not obstructed by a sprinkler head. Run the zone for a set time, typically 10 to 20 minutes, and record the exact run time and the depth of water collected in each cup.
Next, calculate the average depth collected across all cups and convert this into an hourly rate. For example, if a 20-minute test yielded an average of 0.15 inches, multiplying by three gives an hourly rate of 0.45 inches per hour. This calculated hourly rate provides the delivery speed needed for precise scheduling. The final total run time is determined by dividing the target water depth (e.g., 1 inch) by the application rate (e.g., 0.45 inches per hour).
Implementing the Cycle and Soak Strategy
The total run time calculated from the catch cup test must be translated into a practical watering schedule using the cycle and soak method. This strategy involves breaking the total required watering time into two or more shorter cycles, allowing the water to soak into the soil between applications. This technique is especially important for compacted soils, sloped areas, and clay-heavy zones where the soil cannot absorb water as fast as the sprinkler delivers it.
For instance, if a zone needs 30 minutes of watering, split this into two or three shorter cycles, such as three 10-minute cycles. The “soak” period allows the water from the first application to penetrate the ground before the next application begins. This pause eliminates surface runoff that occurs when the application rate exceeds the soil’s infiltration rate.
The soak time should be a minimum of 30 minutes for most soil types; heavy clay soils may benefit from an hour or more between cycles to ensure deep saturation. Programming your controller with multiple start times separated by this soak interval ensures the required water amount is applied without being wasted as runoff. This deep, non-continuous watering promotes a healthy root system capable of sustaining the plant through environmental stress.