Prepared by W. Bryan Smith, Area Extension Agent, Agricultural Engineer, Clemson University. (New 5/08.)
In order to properly irrigate a yard (or crop) the homeowner or grower needs to know when to begin irrigation, how much water to apply, and how long to operate the system. Most irrigation systems will be “zoned” into several sections, with each section requiring a different operating time to apply the correct amount of water. Manually turning valves on and off can work nicely, but it does require attention to the project at hand. Become involved in another project and “oops,” suddenly time has slipped by and you have allowed a zone to irrigate for an extra thirty minutes.
Irrigation timers were developed to “automate” the irrigation process. Replace the manual valves with electric valves, add some wire, install the irrigation timer, and then simply set the appropriate times for each zone.
The first electric timers were electro-mechanical. A clock motor ran the entire timer, with a simple dial to set the current day and time, another dial to set the day and time the irrigation system should turn on, and one small dial for each zone to set the time to irrigate. These timers are dependable, simple to set up, rugged and long-lasting, and are still available from many irrigation manufacturers.
There are several limitations to an electro-mechanical timer. The timer is very inflexible. If the timer is programmed to begin irrigation on Tuesday at 9:00 p.m., it will do so – and every zone will irrigate that evening. There is no way to program two zones to irrigate on Tuesday and three more to irrigate on Friday. Also, the timer will not allow different irrigation times to be programmed for the same zone. It is not possible to irrigate zone one for 20 minutes on Tuesday and then again for 40 minutes on Friday with an electro-mechanical timer unless we manually re-set the time. Finally, each zone has a maximum irrigation time of 60 minutes. This can be a problem, especially with drip zones.
Figure 1. An electro-mechanical timer.
(Courtesy Rainbird Corporation)
The next advance in timers was the electronic timer. These timers (usually characterized by a L.E.D. readout) were very flexible, with three “programs” available for use. A homeowner might use “Program A” to set zones one, two and five to irrigate on Tuesdays, “Program B” to set zones three, four, and six to irrigate on Mondays and Fridays, and “Program C” to set zones three and six to irrigate again on Saturdays. These timers were much more complex to program, but their main fault was their susceptibility to voltage surges. If a thunderstorm occurred in the area, there was an excellent chance that your electronic timer needed repair.
The latest timer advance is the hybrid timer. This timer combines the ruggedness and ease of programming of the electro-mechanical timer with the flexibility of the electronic timer. Most hybrid timers have a combination of dials, buttons and a LCD readout to simplify programming. There are usually three different programs to allow irrigation flexibility. Hybrid timers have rapidly become the timer of choice for the landscape.
Figure 2. Hybrid timers.
(Courtesy Hunter Industries)
Timers are at once a blessing and a curse to the landscape. The blessing is obvious – irrigation zones are turned on and off automatically at the proper time.
The curse of the irrigation timer is less obvious and is based on plant water use. We typically will begin irrigating during April or May, when the humidity is relatively high and the days are cool and pleasant. We check the system, set the times for the zones on the timer, and then monitor the yard for a few weeks. Everything looks nice, so we congratulate ourselves for a job well done on irrigation scheduling and then forget about the timer for the rest of the year.
Enter the curse. Plants require more water when temperatures are higher and the humidity is lower as previously mentioned in HGIC 1802, Landscape Irrigation Management Part 3: How Much Water?. The plant transpires more water during these higher-stress times, so more is taken up by the roots. From this simple fact we can easily understand that the landscape will require more water when we reach the higher temperatures and lower humidity of July and August. Suddenly, the irrigation system isn’t providing enough water for the landscape.
The solution, of course, is to add time to the various irrigation zones to meet the higher water need. That works fine through July and August, but when September rolls around we’ll have to reduce the zone time again.
Most homeowners want a completely automatic system - the irrigation installation company installs the system, sets the timer, and no further attention is necessary. Timers automatically turn valves on and off, but they cannot “sense ” the need of the landscape (software and sensors are available to do this, but they are cost-prohibitive for the home landscape). Timers are helpers, not schedulers. We have to regularly monitor the landscape and change the irrigation zone times based on plant need.
There is a bright spot in this picture. Most new hybrid timers have a water budget feature. The water budget in the timer comes from the factory set for 100% - that is, the timer will irrigate each zone for 100% of the time we program into the timer. As the season progresses we may decide that the landscape needs more water. We can enter one place on the timer – the water budget section – and increase the water budget from 100% to, say, 120%. This adjustment increases the time each zone operates by 20%, regardless of how many zones are on the timer or when they are scheduled to operate. This is an extremely handy feature that saves a great deal of programming time. We can also decrease the time in a similar fashion as the summer progresses into late fall.
Irrigation timers provide a way to automate the irrigation system, which helps lower the labor required. They do not, however, “think” about the plant conditions or consider the soil moisture level (unless very expensive systems are installed). Timers are an aid, but they will never eliminate the attention and management of the homeowner.
Adapted from the 2007 South Carolina Master Gardener Training Manual.
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