Determining the best irrigation schedule for a mixture of landscape plants is difficult enough but when salinity is involved, either in the soil or in the irrigation water, it complicates matters. Let’s cover some irrigation do’s and don’ts and see how salinity might affect the way we irrigate.
Avoid Daily Irrigations
Except for shallow rooted plants like lawns, annual flowers and vegetables in raised beds, daily irrigations should be avoided any time of the year. Many turfgrasses and annuals have root systems that extend into the soil 12 inches or less. During the heat of the summer and under desert conditions some of these plants may require daily irrigations.
|The deeper you should water. Lawns, annual flowers and annual vegetables have the shallowest roots and need to be watered the most often.|
The concept of irrigating nondesert landscape plants is focused on wetting the root system to its entire depth, allowing the soil to drain and re-wetting the soil again when half of this water has been used by the plant or evaporated.
The Amount of Water Applied Varies with the Size of the Plant
So we can see that the volume of water applied in a single application is directly related to the depth of the root systems of plants. When designing a landscape irrigation system we try, to the best of our abilities, to put plants with similar rooting depths on the same valve or station.
More often than not we are handed an irrigation system with a mixture of plants that have a variety of rooting depths. When deciding an irrigation schedule for a single valve or station we generally have two options; set the number of minutes based on the average rooting depth of all the plants or let the plants with deepest root systems dictate the number of minutes of station runtime.
Do We Conserve Water or Minimize Plant Problems?
This decision depends on whether to conserve water or minimize landscape problems. When we decide to under irrigate some plants so the majority receive the correct amount of water, we may see some plant damage. If the under irrigation is not severe, we may see the slowing of plant growth, a decline in density due to leaf drop, leaf tip or burning of leaf margins. When plants are severely under irrigated then we begin to see branch die back and in some cases death.
|This is a mixture of a mulberry tree with cactus growing underneath it. Water the mulberry tree and the cactus gets over irrigated. Water only for the cactus, as this was done, and the mulberry doesn't get enough water.|
Under irrigating, or applying less water than dictated by a plants rooting depth, can also impact safety issues. What happens if we under-irrigate large trees such as pines which have shallow roots to take up water but require deeper roots to anchor it in the soil?
Current irrigation technology is based upon time management and varying how water is applied to plants. This technology varies the amount of water applied to plants by changing the number of minutes valves are open, increasing or decreasing the points of water emission or changing the rate of water applied at the point of emission. This translates to increasing or decreasing the number of drip emitters, bubblers, nozzles or spray heads or substituting old points of emission for new ones that have different rates of application.
Making these changes to an irrigation system that was designed by a professional and focused on the uniform application of water, more than likely will make the system less uniform and less efficient. This will likely result in substantially higher water usage. These types of alterations to professionally designed systems must be done with care.
|These six station inexpensive irrigation controllers are fine for most small landscapes that don't require a lot of flexibility.|
When to Make Changes in the Irrigation System?
There are some obvious cases where changes must be made. For example, changes must be made when some plants are receiving excessive amounts of water or not enough water while others on the same valve appear to be watered adequately.
As plants get bigger, they need more water. When plants get bigger, their tops get bigger as well as their root system. Increasing plant size requires the application of higher volumes of water. Increased plant size dictates that the area irrigated under the plant also needs to be increased. Logic tells us we need to increase the amount of water by applying it to a larger area.
Just because a few plants on an irrigation valve have grown larger seldom requires increasing the number of minutes of runtime. Other plants on the same circuit that received adequate amounts of water would then be over-irrigated for the sake of a few.
Is Increasing the Number of Minutes the Right Decision?
Of course increasing the number of minutes is the easiest solution to the problem but is it the right one? The quick fix of bumping up the number of minutes creates no new revenue for the landscape maintenance company and is likely a poor solution to an irrigation problem. Is it possible that a discussion of the problem with the owner or supervisor might result in a better solution to the problem for everyone involved? Might this discussion generate revenue, or even a better looking landscape, and also result in water conservation?
The second decision in scheduling irrigations is determining when to apply water. This is a very different question than determining how much water to apply. Unlike the first question, the answer to the second question is implemented solely by determining when to turn the valve on or off.
No Technology Replaces a Good Irrigation Manager
Research has demonstrated that no irrigation controller, no matter how “smart” it is, can substitute for a knowledgeable irrigation manager. The principal reason is that knowledgeable irrigation managers focus on their irrigation schedules ahead of the irrigation curve, not behind it. Most “smart” irrigation controllers are irrigating behind the irrigation curve or relying on historical information to predict the future.
Knowing when to turn an irrigation valve on is half art and half science. The “science” part of it can be handled by many good irrigation controllers. The “art” part of it is staying aware of your irrigation system, how it’s operating, monitoring the landscape and paying close attention to current as well as projected future weather conditions, particularly during the summer months.
You can’t do a good job irrigating sitting behind a desk. Knowledgeable irrigation managers seldom if ever take vacations during the heat of the summer. That is their busiest time. This is the time they are most aware of their landscape environment, weather environment and irrigation “system” environment.
Monitor Three Weather Conditions
Three primary weather factors good irrigation managers are constantly monitoring, whether they realize it or not, current or potential changes in the temperature, wind and sunlight intensity.
In most environments, and in particular desert environments, our everyday irrigation predictions are based on normal, seasonal temperatures, normal wind speed and a clear sky. The single factor most likely create mayhem in a landscape during the summer months is wind. The second is above normal spikes in temperature. The absolute worst scenario is a spike in daytime temperature accompanied with strong winds and a clear sky. A good irrigation manager stays ahead of the curve by applying water in anticipation of mayhem, not during mayhem.
Salts Complicate Things
As if this was not difficult enough, salts and salinity add a dimension to the irrigation dilemma which test the abilities of the best irrigation managers. Salts are salts. Without getting too technical, any time a substance is added to water and dissolves you have the potential for increasing salinity. Any time you add something to the irrigation water or the soil you may affect salinity.
|Saltz, particularly those that contain sulfates, are corrosive to cement and steel.|
Two types of plant damage result from excess salts or salinity in the soil or irrigation water; the type of salt may be directly toxic to the plant (such is the case with plain old table salt which contains two very toxic chemicals to plants; sodium and chlorine or chlorides) and competition with the plant for water.
Any type of salt, table salt or even fertilizers, will dissolve in water adding to a general increase in salinity. Slow release fertilizers are less of a problem in this regard than fast or quick release fertilizers. When salts of any kind are added to the soil, these salts compete with the plant for soil moisture.
|Salts can leave behind deposits on drip emitters.|
In other words the salinity of the soil “pulls” or holds on to water, making water less available to the plant. This means that when salinity is a problem, we need to irrigate more frequently to reduce plant stress. This is particularly true in summer months.Two basic concepts when irrigating to avoid salinity problems are dilution and flushing. Keep the salts diluted and add enough water to flush these salts below the rootzone. This requires that the soil drains adequately.