When asked to name a grounds manager in Upstate New York who was practicing water conservation, Cornell University turfgrass professor Marty Petrovic burst out laughing. Most eastern managers just don’t think about such things. Many water the same way they spray – out of paranoia, to insure that the lawns stay green enough to satisfy golfers or corporate boards. Water is so cheap and plentiful that they take it for granted. Water conservation is a problem only for their colleagues on the west coast, they think.
It may not always be that way. In Nassau County, Long Island, booming suburban population growth has caught up with the water supply, and water use is strictly regulated. Among other restrictions, watering may be done only in the early morning or late evening, and anyone caught watering in the rain is subject to a fine. Nassau is symptomatic of problems throughout the New York City metropolitan area, where it’s estimated the cost of solving water supply problems may approach $15 billion. The New York State legislature is considering a statewide requirement that automatic sprinkler systems have in-ground moisture sensors wired in. Sooner or later, every city and state with growing metropolitan areas will face similar restrictions.
Perhaps these problems will never come to rural areas, but there are more immediate reasons to avoid overwatering. Ironically, plants that are heavily watered come to depend on the water. You could almost say that plants find water addictive: give them too much, and they need even more. As a result, plants receiving too much water are often less healthy and less able to resist period of drought.
Excessive watering can also leach nitrogen from the soil, which not only increases fertilizing costs but also pollutes groundwater. (It has been suggested that overwatering also leaches pesticides into the groundwater, but there’s no experimental evidence for this according to Dr. Petrovic. Studies that have looked for pesticide leaching haven’t found any, he says, perhaps because modern pesticides break down quickly or become chemically bound to the soil.) Finally, keeping the soil too damp may encourage fungal diseases, which will either damage the plants or damage your budget as you apply unneeded fungicides.
KEEPING IT GREEN
You can keep your grass green without creating any of these problems. Studies have shown that water applied as needed, based on careful monitoring of soil moisture, can reduce water use by as much as 50 percent compared with routine scheduled watering, without any decline in turf quality.
That means that water conservation can pay off at the bank. OK, water is a lot cheaper in the east. Dr. Petrovic estimates that a golf course that might spend $500,000 a year for water in California will spend only about $15,000 here. But if you could cut that in half, you’d still have $7,500 to spend on something else, plus whatever you might save on fertilizer and fungicides. Fortunately, turfgrass researchers are learning more about how plants use water, and new technology is available to provide better watering control.
Water enters plants mainly through the roots, is carried upward into the leaf structure and then “transpired” into the atmosphere. The main function of this moving water is to carry nutrients through the plant, and to give the plant rigidity. Transpiration also helps cool the plants in Summer. Only a tiny portion of the water that passes through the plant is used up in the chemical reactions of photosynthesis, in which solar energy combines water and carbon dioxide to make plant tissue.
Not all water poured on the ground is taken up by plants. Some evaporates from the surface of the ground, and some drains away below the surface. Some of what remains in the root area is inaccessible to plants.
HOW WATER IS USED
A number of factors – many of them interrelated – influence the amount of water used by plants, and the amount you must apply to meet their needs. Here are a few:
Plant species. Each species has its own unique need for water. Zoysia grass uses about half as much, on average, as tall fescue. (Don’t confuse a low water use rate with drought tolerance. The latter refers to how well a plant deals with whatever it considers a shortage of water, and is based on such characteristics as how well the plant can close off transpiration and keep the water it has taken up.) If the availability of water is going to be a problem, landscape designers should choose grasses appropriate to the climate that use the least water.
Root growth. The deeper and more developed a plant’s root system is, the more access it has to what water is there. If plants are given an unlimited supply of water they will develop shallow roots, just as a person who never has to work for food will develop few muscles. Then, the shallow-rooted plants will need more water throughout the season to maintain themselves. Root growth in warm season grasses continues throughout the Summer, so controlled watering will encourage more extensive root growth. Cool season grasses experience most of their root growth in the early Spring when Mother Nature is usually supplying plenty of rain, then lose root structure through Summer, so managers have little opportunity to encourage root growth through watering practices. However, anything else that encourages luxurious root growth, including aeration and control of diseases, will increase the turf’s ability to use available water. Avoid anything that encourages excessive shoot growth, such as overuse of nitrogen fertilizer, since shoot growth competes with root growth for the plant’s energy. Some pesticides, including pre-emergence crabgrass controls, can injure root systems, so they should be used only if absolutely necessary.
Mowing height and practice. This is a confusing area. Higher cut turf grows a deeper root system, and so can make better use of available water; but higher cut turf also uses more water, due to lower resistance to evaporation. Dr. Petrovic proposes an approach that takes advantage of both of these factors: mowing high in the Spring to develop a better root system, then mowing close in the Summer to reduce water demand. He will try out the idea this Summer on test plots of Kentucky bluegrass. It may also help to maintain mowing equipment carefully; some studies show that mowing with a dull blade shreds the leaves and causes greater water loss.
Nearby trees. Tree roots are voracious in pulling water from the soil, and must be figured in your calculations.
Soil conditions. In the soil, water molecules are attracted to the surface of soil particles and wrap themselves around the particles in a thin film. If the film is too thin, roots will not be able to pull the water away from the soil. If it’s too thick in relation to the spaces between soil particles, there will be no room for air to get in between, and plants will suffer from a lack of oxygen. A soil made up of very fine particles, such as clay, will hold more water than, say, coarse sand, because the total surface area of the particles is greater. Similarly, water will usually flow downward more slowly through fine soil than coarse.
Drainage design. It seems a paradox to build in drainage that pulls water away form the plants almost as soon as we apply it, but this is essential for healthy plant growth. As water flows downward, pore spaces are opened between soil particles that allow air to enter, giving the roots access to both air and water. However, once spaces are opened, water actually flows more slowly through coarse sand or gravel than through fine soil. As a result, where a layer of fine soil is placed above a layer of coarser material, water will build up at the interface. This also applies to the area around drain tiles. Modern drainage design takes advantage of this effect to create a reservoir of water from which topsoil moisture can be replenished through capillary action. But to prevent fungus infections of the roots it’s important that this area should be below the primary root system, so the root depth of the species of grass to be planted should be taken into account when designing drainage. In planning irrigation schedules, managers should be aware of the drainage characteristics of the site and its capacity for storing water.
Runoff. Healthy plants keep the soil porous, ready to absorb water. Mat and thatch act as a barrier until they become thoroughly wet, and on a slope will cause water to run off without soaking in. Compaction on heavily traveled areas has the same effect. Aeration and careful removal of thatch will reduce the amount of water you must apply to satisfy plant needs. Extremely fine soils also will absorb water more slowly, also leading to runoff. Irrigation rates should be adjusted so water isn’t applied faster than the ground can absorb it. Modern sprinkler control systems allow for repeated light applications in areas where water is absorbed slowly.
Sprinkler design. Sprinklers that apply water unevenly may require overwatering in some areas in order to supply adequate water in others. Rotating sprinkler heads apply water more slowly and uniformly than fixed heads. Heads serving similar types of turf should be zoned together. Automatic sprinkling systems should include inground moisture sensors and rain sensors to discontinue irrigation when the soil is sufficiently wet. Once a conservative irrigation system has been installed it must be maintained regularly to insure continued efficiency. As heads become worn, for example, they may pass more water than when new.
Weather. In times of low humidity, more water transpires form plant leaves and more evaporates from the surface of the ground. Wind also speeds these processes. In very hot weather, some managers water lightly in order to cool the grass by evaporation.
CHANGING YOUR METHODS
There are three major approaches for turfgrass managers who want to get away from watering by the clock.
Most managers operate by “instinct,” which really means watching for signs of trouble. When grass is under water stress it will first lose its stiffness – footprints and tire tracks will remain visible longer than usual. In some cases it will turn a dark blue-green; grass that is overwatered may turn yellow, possibly from a mineral shortage. Watching for wilt is not the ideal way to control irrigation, because turf that is allowed to reach the wilting point will lose some quality; however, some managers learn to identify out-of-the-way areas where damage appears first and use these as indicators.
A handful of soil that is properly moist should compact smoothly in you hand without crumbling. If it’s too dry it will crumble when squeezed; if it’s too wet it will drip water. Some managers can tell a great deal by shoving a stick into the ground; others use a piece of tubing to take up a thin core.
The second approach is to measure soil moisture directly. A number of instruments exist that will measure the amount of moisture in the soil; the most common are known as tensiometers. These instruments vary widely in accuracy, and most are limited to working in the “wet range”
of soil moisture. All require regular maintenance to operate correctly. In any case they can only measure the moisture at a few spots. At present, off-the-shelf technology does not allow these sensors to be connected into a computer controlled system; however, they can override automatic controls and shut off local valves; at least they will stop sprinklers from coming on during a rainstorm.
A third approach presently under study is to use atmospheric conditions to predict plant water needs. The bottom line is that plants suffer water stress when the losses due to transpiration are greater than the amount of water taken up by the roots. Transpiration increases, as noted above, when the air is dry or in windy conditions. So, information on evaporation rates can be plugged into complex formulas that also take into account soil conditions, plant species and others of the factors mentioned above to produce a water application recommendation.
There are three common ways to find evaporation rates: by using weather data, by evaporating water from pans, and by weighing actual plants in pots to see how much water they gain or lose. Few institutions are likely to want to buy the necessary instruments, but Dr. Petrovic foresees a time when some central agency, such as cooperative extension, might offer daily broadcasts of the information managers need. This might not provide exact information for every location in a service area, but “It would probably be a lot better than what they’re doing now,” he says.
Since this article was written there has been a great deal of progress in the second and third approaches listed above. The third approach is being used widely in certain areas of the country, not Western NY, where local weather stations relay weather data to secondary locations which in turn send the information to irrigation controllers. The irrigation controllers then increase or decrease their output accordingly on the next scheduled run. Most major irrigation manufacturers are making controllers to make use of this new technology. In my opinion, the flaw in this system is this; lets say the weather station that your controller uses is at Oak Hill Country Club. Oak Hill got deluged with rain yesterday but you live in Brockport where there was no rain. Your system will now not run for two or three days. That’s really not a great system.
I see the future in the second approach, moisture sensors installed at every irrigation system. Your moisture sensor is accurately communicating to your controller the conditions of your soil. What could be better than that? Your system will be allowed to run, or not run based on the real time need of your soil. Read more about this method. Soil Moisture Sensing