Proper irrigation is very important for plant health. Too much or too little can both be damaging to plants.
I’ll begin with a description of a process called evapotranspiration. This process combines evaporation and transpiration. Evaporation is when water changes from a liquid to a gas and moves into the atmosphere from the surface of bodies of water, wet soil or wet plants. Transpiration is a process the plant uses to cool itself. Water is taken up by the roots from the soil or into the leaves from rain or irrigation and translocated through the plant and exits from the stomata in the leaves cooling the plant as it exits. The process is much like our own perspiration for cooling our bodies.
Evapotranspiration rates vary with the type of plants as well as the weather. And irrigation should be adjusted accordingly to avoid the dreaded over and under watering I mentioned. The hot, dry and windy spring months have the highest evaporation rates. But the hottest months of summer are also have some of the highest evaporation rates. It is a common practice to turn irrigation systems off during the summer rainy season but this causes the death of many lawns in the summer. Rain can be sporadic to nonexistent at times and it doesn’t take long for our sandy soils to experience drought conditions. And that means the plants have no water to take up from the soil or absorb from rain and no way to cool off during months with the highest transpiration need. It’s like a person being stuck in the desert with no water. The cooler months of late fall and winter are the months to save on your water bill since the evapotranspiration rate decreases greatly during those months. And many people over water in the cooler months because there is less rain and they feel the plants need more irrigation which brings on disease. And then if disease does appear it causes the cells to collapse and so they cannot translocate the water and the plants turn brown which people mistake for a need for more water. In fact they are feeding the fungus and causing death.
The main objective is to grow plants that will survive with little to no irrigation. You want a deep extensive root system which can be achieved using proper management practices. And proper irrigation is the first step.
Frequent light watering can cause a shallow root system. To develop a deep root system you should only water when plants show a sign of need. Like wilting. This is usually only twice a week at most and preferably only once a week. And when you do water only apply enough to wet the root zone, approximately ½ to ¾ of an inch with each watering.
The next step is to figure out how long to irrigate to apply ½ inch to your lawn and plants. Most people run the sprinklers for a given number of minutes without knowing how much water they are really applying. Determining the rate of water your system applies is a pretty easy job.
- Obtain five to ten straight sided containers to catch the irrigation water. Tuna cans or something similar in size are perfect.
- Place the containers in one zone at a time scattering them at random within the zone. Repeat this in every zone because there may be differences in the irrigation rates.
- Run the water for fifteen minutes.
- Use a ruler to measure the depth of water in each container. Measurements to the nearest eighth of an inch are adequate.
- Find the average depth by adding the depths and then divide by the number of containers.
- To determine the irrigation rate in inches per hour multiply the average depth of water times four.
Try to calibrate the system at the same time of day that it is run so that water pressures are similar. Low water pressure can reduce the amount and coverage of water applied by the system.
Never mix sprinkler head types. Mist heads apply more water than stream and rotor heads. You need to match sprinkler heads for uniform coverage. A mist head can apply the needed ½ inch in 15 minutes where a rotor head will need 30 to 60 minutes depending on whether it is a 180 degree radius or a 360 degree radius. If you mix these in the same zone you will be applying too much or too little depending on how long you program the zone to run. Match sprinkler heads for uniform coverage. It can take a month or two to condition your yard to survive several days between irrigation or rainfall. During this time the root system is developing and growing deeper into the soil.
You need to check your system frequently, especially if you see dry spots appearing on the lawn or shrubs and flowers wilting. You may have broken heads, clogged nozzles or need to adjust the direction of the spray. Also check that shrubs have not grown over the heads and are now blocking the water from reaching areas. You should not run the sprinklers longer trying to turn the brown spots green because most likely there is no water reaching those areas and running it longer will not change that.
Watering your flower beds should be a special consideration. Overhead watering can cause disease and so drip irrigation is the way to go. It is best to tap into a rotor head zone if possible since they will typically run for 45 to 60 minutes which is ideal for a drip irrigated flower bed.
Lawn zones should be separate from shrub zones since they need to run longer and tend to have rotor rather than mist heads. It depends on where the lawn zone is located. Is it in the full sun or shade? On a hill or flat surface? Do the heads need to turn 180 or 360 degrees. You need to time the zone according to the different conditions.
Shrubs may need little to no irrigation once they are established. Shrubs have a more extensive root system than either lawns or flowers and so can go longer between watering than either of those.