Irrigation Systems Irrigation Scheduling

In the best of all possible worlds your irrigation system would supply your landscape with exactly the right amount of water to keep it healthy. The system would shutoff when it rains and would increase watering times during dry spells. The perfect system would supply different amounts of water to different types of plants and would eliminate over-spray onto sidewalks and pavements. The perfect system would only run at night to minimize evaporation losses and it would alert you if there was a broken head or a stuck valve or some other problem.

Proper irrigation scheduling is a skill that surprisingly few have mastered.

Many people don’t realize that they must change their irrigation program regularly as the seasons change. Ideally you should program your sprinkler clock weekly or even daily to maximize efficiency. But even monthly changes to the irrigation schedule will result in substantial water savings and improved plant health. You should turn the system off when it rains and make frequent adjustments to the timing of each zone so that just the right amount of water is applied. Few people have the time or inclination to take this micro-managing approach, so these tips are designed to help you maximize efficiency with your sprinkler system through sensible scheduling. The biggest problems encountered are watering too much and too frequently. Many of the common turf grass and landscape shrub diseases are made worse by, or even may be the result of, watering too frequently.

Irrigation Scheduling Tips

The Basics

  • Know how to run your controller and change watering times.
  • Adjust the watering times (number of minutes.) and the frequency of watering (daily, twice a week, etc.) based on weather conditions.
  • Change your settings to adjust for seasonal differences and reset the timer when needed.

Water At Night

Make sure your system only operates when the sun is down to reduce evaporation losses. If you like to watch your sprinklers run, set the start time for 8:00 p.m.  there is still plenty of light outside, but the sun is usually down. Many experts say the best time to water is between midnight at 4 a.m. because evaporation in kept to a minimum.

Understand the Water Needs of Your Plants

Plant roots need a combination of both air and water to survive. Some plants, like ivy, can grow in a jar of water. Others will die if the roots are wet for longer than 24 hours. Thus, irrigation scheduling must begin with an examination of the plants to be watered.

It is important to understand the needs of drought-tolerant plants.

These plants are often native to arid climates where it rains heavily for short periods, followed by long periods with no rain at all. The drought tolerant features of arid region plants allow them to survive and even thrive under these feast or famine water conditions. Drought tolerant plants may be found growing in all types of soils, from sand to clay. Sandy soils do not hold moisture well, and drain quickly. They are the easiest soils to grow drought tolerant plants in when irrigation is available. Clay soils hold water tightly for long periods of time, and cause the most problems with over-watering. Watering needs to be much less frequent in clay soils to allow the drying time that these plants need.

Never Water if the Soil is Wet

Irrigation scheduling is simply a matter of close observation and dedication. Ideally, the irrigation control clock should be adjusted on at least a weekly basis to conform with current weather conditions, but even with monthly adjustments plants can be maintained healthy and happy.

The first basic irrigation scheduling rule for drought-tolerant plants is never water if the soil is still wet. The old rule for landscape care was “if it doesn’t look right, water it”. This is often the worst possible thing to do. Plants wilt for any number of reasons other than needing water. Wilting for some perennials happens on hot afternoons no matter how much water they have.

Wilting in drought tolerant plants is often the first sign of too much water. The roots die from too much water, then the plant wilts from lack of water uptake by the roots. Any number of other diseases or even insect damage can also cause wilting. Some drought tolerant plants fold their leaves on hot afternoons to conserve water, which can be mistaken for wilting. So never assume a plant needs to be watered because it looks wilted. Check to see if the soil is wet first. When You do Water, Don’t be Stingy

The other rule for irrigation scheduling is when you do water, don’t be stingy. Saturate the soil throughout the entire planter. The soil should be completely saturated (the technical term is that the soil has reached field capacity) throughout the root zone  often a six inch depth. The primary feeder roots for most plants will be growing throughout the top 6 inches of the entire planter, not just under the plant’s foliage. These feeder roots are so small that they are not even noticeable in the soil. The plant’s lower roots are primarily to physically support the plant, although these lower roots can sometimes take up water if they need to.

Cycle Your Sprinklers

If you’re irrigating using sprinklers, the water will probably start to run off into the gutter, or into a low spot, before the soil is wet through the root zone. This is because the sprinklers put out more water in a given amount of time than the soil can absorb. In technical terms the precipitation rate of the sprinklers is greater than the infiltration rate of the soil. (Both, by the way, are measured as inches/hour in the U.S.A.) Fortunately, solving this problem is easy. As soon as the water starts to run-off, just turn off the sprinklers! Wait an hour or so for the water to soak in, then run the sprinklers again until run-off once again occurs. Continue this run-stop-wait-run cycle until the soil is saturated through the root zone. This process is referred to as cycling the sprinklers. Almost all sprinkler systems need to be cycled for proper irrigation.

Technical note:

in large areas of turf you may not notice the run-off because the water doesn’t run into a gutter or over a sidewalk, but runs off to the lowest area in the lawn. It’s still critically important to prevent the run-off. If you don’t, muddy, wet areas will result where turf diseases will thrive, mosquitoes will breed, and your mower will leave ruts.

Avoid Cycling

Drip Systems With drip systems the goal of saturating the soil through the root zone in the entire planter is the same, but a different approach is necessary to achieve the goal. Drip emitters slowly trickle water into the soil at the location of each emitter. Because the water comes out of the emitter so slowly it easily soaks into the soil, making saturating the soil easy. The problem with a drip system is saturating the soil throughout the entire planter area, not just the soil directly under the emitter. To saturate the entire planter area the water has to move outward in the soil from the emitter locations. In all but the sandiest of soils the water can be forced to move at least 36 inches in each direction away from the emitter through a combination of positive displacement and capillary action. To achieve the positive displacement part of this action it is necessary to avoid cycling the drip system. Run the drip system as long as possible at a time. Create small berms if necessary to control run-off. In some clay soils you may need to cycle the drip system like you would a sprinkler system to avoid run-off, but try to keep it down to just one repeat cycle if possible. Remember, if you can’t achieve saturation of the entire planter area, you at least want the wetted area around each emitter to be as big as you can make it in a single 24 hour period. You may even need to add more emitters to achieve the goal. If you do add more emitters, space them at least 36 inches apart. Remember, the goal isn’t to add more water to the areas that are already wet, the goal is to wet more area for the roots to grow in.

Multiple Start Times

If your irrigation controller offers the feature of multiple start times you can use this to your advantage. Most modern controllers offer the option of 2 or three start times. When this feature is implemented your system will run through the entire cycle of zones more than once per day also called cycle irrigation. Here’s an example: Instead of watering zone 1 for 20 minutes, use the multiple start time feature and set up three start times. You can then water zone 1 for 6 minutes three times for a total of 18 minutes. The shorter run times will reduce runoff and water losses and will reduce deep infiltration below the root zone. Table 1 shows a sample watering schedule using multiple start times.