Proper irrigation scheduling is a tricky skill that surprisingly few landscape professionals have mastered. By far the largest loss of plant materials on new landscape projects is the direct result of improper irrigation scheduling. You may be surprised to learn that the most common irrigation scheduling problem is not too little water, or even too much water, it is watering 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.
Smart Controllers: The Arm Chair Method.
Before getting into the details of irrigation scheduling, it should be mentioned that there is an easier way. More and more irrigation control systems will do most of the work for you. These irrigation controllers (sprinkler timers) are commonly called “Smart Controllers”. After the initial set up, these Smart Controllers automatically adjust the watering times depending on the water needs of the plants. I highly recommend you look into purchasing a Smart Controller. If you live in a drought area contact your water supplier to see if they have any programs that will assist you in the purchase price of a Smart Controller. Some water suppliers will pay for part or all of the controller cost!
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. Irrigation scheduling for water-loving plants is easy, you just give them as much water as you can find or afford. Since most problems related to irrigation scheduling involve irrigation of the more drought-tolerant plants the rest of this discussion will focus on these plants. Fortunately, almost all plants will perform well under these irrigation scheduling guidelines. As with anything, there are some exceptions. If in doubt, check with your favorite nursery, or a landscape plant encyclopedia.
At this point it may be helpful 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 between irrigations 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. Sort of ironic isn't it?) Wilting can also be caused by any number of other diseases or even insect damage. 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) to a uniform depth of at least 6 inches. 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 to a 6 inch depth. 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 to a 6 inch depth. 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 6 inches deep 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 to a 6 inch depth 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.