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Irrigation Management: Watering Lawns & Gardens

Many consider watering their lawns and gardens to be a quick fix for plants that are wilting to sustain them until the next rain. Others think of it as providing plants with all the water needed daily. Neither is correct. Irrigation is really a balancing act whereby we try to manage the soil moisture content keeping it at a level optimum for plant growth.

Applying too much water causes the soil to be saturated. Oxygen in the root zone of the soil is displaced as it is replaced by water, preventing plant roots which require oxygen from growing properly. If this condition persists too long, the plants will drown.

When plants have too little water, they will often begin to hoard their water by closing their stomata. However, stomata need to be open for photosynthesis to occur because carbon dioxide needed for photosynthesis enters the plant through stomata. If the plant remains drought stressed for a prolonged period, it will eventually wilt and possibly die.

Irrigation is only one way of replacing water in the soil necessary for plants to grow. There are other sources of water additions to the soil. All these water sources must be considered in irrigation management to strike the proper balance where there is enough moisture for optimal plant growth, but not enough to saturate the soil and lead to the plant drowning.

Sources of Water Additions to the Soil

As mentioned previously, there are several ways that water in the soil can be replenished. Obviously, irrigation is one. But rain, snow, sleet, hail, and run-on are other sources.

Rain is typically the largest natural contributor of water to our landscapes in the US. Most of the water from gentle to moderate rain showers will make its way into the soil. However, soils often do not get the full benefit of storms, where a lot of rain drops in a short period of time. When this occurs, much of the water is lost to run-off and ends up in streams, creeks, rivers, lakes, and ultimately the ocean as a result.

Snow, sleet, and hail often fall during times of the year when plants do not require a lot of water. Wet snow that drops when the temperature is close to freezing only yields about 1 inch of water for every 6 inches of snowfall. Dry snow that falls when temperatures are much colder may only contribute 1 inch of water for every 12 inches of snowfall. Sleet and hail can also contribute water, though generally in very small quantities.

Run-on occurs when water from neighboring properties with higher elevations run down onto your property. If the water remains on your property, it can contribute to the moisture content of your soil which may be beneficial if your property’s soil needs additional water. However, it could have negative effects if it occurs often and leads to prolonged saturation.

Causes of Water Subtraction from the Soil

There are many things that can lead to moisture being loss from your soil. Some of these include evaporation, transpiration, runoff, and leaching.

Water can be lost from the soil’s surface through evaporation. Heat causes the water to convert to water vapor at which point it escapes into the air. While this typically only affects the top inch or so of soil, it can be significant in a drought situation. Evaporation can be mitigated by using mulch to keep the surface of the soil cool and hold in moisture.

Transpiration is the process where water vapor escapes into the atmosphere from the leaves of plants through their stomata, small openings mostly on the underside of leaves. Water escaping from the leaves creates tension which causes water to be absorbed from the soil and pulled up from the roots through the stems to replace the escaped water in the leaves.

Runoff occurs when water is applied to the landscape faster than the soil can absorb it. The speed at which water can be accepted by the soil is the infiltration rate. When the rate water is applied exceeds the infiltration rate, the excess water runs across the landscape’s surface exiting the property rather than down into the soil. Not only is that excess water not going to be made available to your lawn and plants, but it can also lead to erosion. Hills in the landscape will typically have have lower infiltration rates leading to higher runoff.

Leaching occurs when your soil is course or sandy. Such soils have little ability to retain water (or nutrients). Water that is accepted by such soils will typically continue to drain down through the soil profile past the root zone making it too deep to be useful to plants. When this happens, the water has left your landscape just as if it had evaporated or runoff. Also, when water leaches through course or sandy soil past the root zone, it will often take with it much of the nutrients, fertilizers, etc. that might be in the root zone and needed by plants.

Determining When to Irrigate

Most people do not know when they should begin irrigating. Some wait for visual cues like wilting. Others simply water all the time. However, there are ways to determine whether you should be utilizing your irrigation system.

One method for determining moisture levels in the soil is to use a regular screwdriver to test the soil. Walk around your landscape one or two times per week sampling the soil in various places. Push the screwdriver tip 4-6 inches into the soil and pull up a bit of soil. Hold the soil sample in your hand and feel how moist it is. If it is dry, then you should turn on your irrigation system. If it is relatively moist, then irrigation is not needed at this time. If it is excessively moist, then you may have a drainage issue that needs to be addressed.

Another method for determining moisture levels in the soil uses foot printing. Simply walk across your lawn, turn, and look to see you left footprints. If footprints are readily visible in the grass, then you should begin using your irrigation system. In areas such as flower beds where the foot printing method is not practical, use the screwdriver method.

It is possible to estimate the actual plant water use for your landscape using methods like Penman-Monteith - however these are quite time consuming and not practical for the typical homeowner. Such methods require the daily tracking of information like temperature, relative humidity, wind speed, and solar radiation. The amount of amount of water used by the landscape is calculated daily. And once a certain amount of water has been used, the landscape is irrigated. Methods of calculating actual plant water usage are not practical for most uses, but you should be aware of their existence.

It is possible to measure soil moisture. There are all sorts of devices available for monitoring soil moisture, from inexpensive, less accurate devices costing a few dollars to fully automated computerized systems costing thousands of dollars that communicate with the irrigation system to turn them on when needed. A good option for most homeowners is to purchase and use a tensiometer.

A tensiometer is a glass or plastic tube with a porous ceramic tip on one end, a small water reservoir on the other end, and a vacuum gauge usually on the side to measure the pressure inside the tube. The tube and reservoir are filled with water and sealed. The tensiometer is pushed into the soil until the ceramic tip is at a depth of the center of the root zone. As water wicks out of the tensiometer through the pores in the ceramic tip, it creates a vacuum inside. The pressure continues to build inside as the soil outside becomes dryer and pulls more water from the tensiometer. Eventually, when the pressure climbs to a certain level indicating a minimal amount of moisture in the soil, the irrigation system is turned on. The price for such a device runs around $100.

How Much Water Do Plants Need?

Different plants require different amounts of water. How much water they need should be considered when coming up with a landscape design. Plants with similar water requirements should be grouped together.

While there is some information available regarding the water requirements of some plants, there are many plants for which such water requirements do not exist. When no such information is available, start by applying 1 inch of water per week as a starting point. Then monitor the landscape weekly and adjust the amount of water applied accordingly.

To determine how many inches of water are currently being applied in a particular zone or section, place 6 to 8 straight sided cans in that zone or section and let the irrigation system run a complete cycle in that zone. Measure the amount of water in each can and calculate the average. If it is less than one inch, then increase the time it runs. If it is more than an inch, then decrease the time it runs.

It should be noted that water requirements in spring and fall are typically much less than those during the heat of summer, so you cannot simply set it and forget it in the spring. It is important that you manage your irrigation system. Monitor the landscape every week or two using methods discussed previously like the screwdriver method or foot printing method to determine whether the times on the various zones should be increased or decreased.

What Time Should You Irrigate?

When one should irrigate is often a topic of debate. Some will say it is best to water in the afternoon, while others will say it is best to water in the early morning. But few will recommend watering during the nighttime hours. The correct answer likely depends on factors like the climate where you live.

From strictly a water conservation perspective, night is the best time to water because the temperatures, winds, solar radiation, and therefore evaporation rates are much lower. It is estimated that only 10-15% of water is lost when irrigating at night. Estimated water loss during the daytime watering ranges from 20-30%. Water loss during morning watering likely lies somewhere between those for night and daytime watering.

People often say watering at night increases the chance forming diseases from fungi or bacteria. While this could be true in some parts of the country, it is not necessarily true in all parts. For example, in the southeast there is a long period of time each night until the following morning when lawns and the leaves of plants are already wet because of dew forming. So as long as irrigation occurs after dewfall each night and does not extend the time it takes for the dew to evaporate the next morning, then the likelihood of a nighttime watering schedule leading to disease is relatively small. It is worth noting that the time that dew appears and the amount of time it remains the next morning varies by season, so watering times will need to be adjusted accordingly as spring changes to summer and as summer changes to fall.

How Does Soil Type Affect Irrigation?

The soil type in a landscape is an often-overlooked factor when considering the design and operation of an irrigation system. While most focus on the amount of water that should be supplied, the slope of the landscape, and its associated runoff, little though is typically given to how soil type affects irrigation.

Different types of soil possess different water retention capabilities. Sandy soils are made of large soil particles and therefore have large spaces between particles. Because of this, sandy soils drain quickly and cannot hold a lot of water.

Clay soils, however, are made of fine, microscopic soil particles. For this reason, the spaces between soil particles are miniscule. Thus, clay soils drain slowly and retain water very well (in many cases, too well).

Coarse sands have the capacity to hold about 0.05 inches of water for each inch of soil depth. Loamy soils can hold about 0.18 inches of water per inch of soil depth. And clay soils can hold about 0.17 inches of water per inch of soil depth. This is useful to know when calculating how often to water a lawn.

If you assume the rooting depth of most plants is about 10 inches deep and you know that your soil type is coarse, sandy soil with a water holding capacity of approximately 0.05 inches of water per inch of depth, then 0.5 inches (10 x 0.05) of water is about all the water that soil can hold. Any additional water applied to that landscape would simply drain or leach through the 10-inch root zone of the soil and be wasted. This leaching of excess water also results in the removal or leaching of nutrients from the soil. To apply an inch of water to the soil each week as previous suggested, the irrigation system should be programmed to run twice per week and apply 0.5 inches each time.

Clay soils hold about 0.17 inches of water per inch of soil depth. Assuming the same 10-inch deep root zone, clay can hole 1.7 inches of water. Therefore, the required 1-inch of water could be applied in a single application each week.

So, there are two rules of thumb that can be inferred from the above analysis. In general, sandy soils should be irrigated twice per week with about 0.5 inch of water. Clay and loamy soils require only a single application of about 1 inch of water each week. While it may be tempting to water daily with a much smaller amount still resulting in 1-inch per week, resist the urge. Doing so promotes shallow roots which reduces the plants’ tolerance to drought and makes them less stable and less hardy. It drastically reduces the amount of soil from which the plants can pull nutrients. Daily watering also promotes the growth of many weeds like nutsedge. There are a few exceptions where daily irrigation might be advised.

New sod, for example, should typically receive 0.2 inches per day for a period of 10-14 days to allow the roots to establish themselves. After this period, gradually cut back on water to once or twice per week, depending on the soil type.

Farmers growing commercial vegetables in sandy soil will often water daily. The irrigation water also typically contains daily plant fertilizer. Since the crops’ life cycle is limited to one growing season, farmers are more concerned with ensuring the plants are not stressed due to lack of water than they are with the depth of the plants’ roots.

Some potted or container plants may use very porous soil with limited capability to hold water. In these cases, you may need to water daily.

Water infiltration capacity is the ability of a soil to absorb water. Sandy soils typically have a high infiltration capacity, meaning they can absorb water quickly due to the large pore spaces between soil particles. Sandy soils rarely have runoff problems. Clay, on the other hand, has a low infiltration capacity due to the microscopic pore spaces between clay particles, preventing the soil from accepting water at a high rate. For this reason, care must be taken not to over irrigate clay soils as runoff can become a problem.

What is Polyacrylamide (PAM)?

Polyacrylamide or PAM is a synthetic soil additive typically sold in soft, granule or bead form that can increase the water retention capacity of the soil. When the soil gets wet, the PAM soaks up moisture. Then when the soil around it becomes dry, the PAM granules release the moisture back into the soil.