Plant Water Use
How much water do your plants need?
It depends upon the plant type and where you live. For example, a cactus needs a lot less water than a lawn; and the same type of lawn will need a lot more water in Hemet compared to Newport Beach. Fortunately, researchers have determined the various water needs of plants based on the plant type along with your local climate.
FACTORS THAT AFFECT PLANT WATER USE - EVAPOTRANSPIRATION
Imagine yourself sitting in the sun on a hot day. Your body sweats to cool itself. Throw in some wind and high humidity and you are really going to sweat! This is very similar to what a plant experiences. However, instead of sweating, the plant loses water through a process called transpiration. Water enters a plant through its roots. It then gets transported through the plant up the stem and exits through the leaves as water vapor. (Interestingly, only about 1% of this water is retained by the plant!) Water also evaporates off the soil surface into the atmosphere through evaporation. The two processes combined are called evapotranspiration, or ET for short.
In California, cool-season grass (specifically tall fescue) is used as a benchmark in which to compare the water needs of other plants. Why cool-season grass? Because it uses more water than most other plants. Simply put, the maximum amount of water healthy tall fescue needs over a given period of time is thus referred to as “reference ET,” written as “ETo.” ETo is a rate that can be measured (with a bewildering equation), and is reported as inches over a given area, like inches of rain. As you might guess, the highest ETo in the state is in the deserts, and the lowest is along the foggy central coast. In July, during an average year, the average ETo is over 9 inches in Palm Springs but only slightly over 4 inches in San Francisco. About half way between the two is Riverside.
WHAT MAKES ET CHANGE?
* solar radiation (sunshine)
* vapor pressure (similar to relative humidity)
* wind speed
Solar radiation has the most influence. The higher the rate of ET, the faster a plant uses water, and the faster water evaporates from the soil. Simply put, your plants will require about half of their annual water needs in summer and a lot less water in the winter months; the actual amount depends on where you live. The good news is, established plants (that are at least a season or two old) do very well when irrigated below the maximum amount of water they would take up if unlimited water was available! In other words, there is no need to replace all the water lost through ET. Many landscape plants do well between 20% and 50% of ETo (0.2 and 0.5 of ETo).
Why do you need to know about ET?
1. You can use it to get a pretty good idea of how much water your plants will require.
2. More and more water districts are adopting water rate structures with budgets that vary throughout the year depending upon ET. If you can irrigate according to the rate of your local ET, you will be very efficient, and your yard will thank you.
3. “Smart” or “weather-based” irrigation controllers irrigate according to your local ET. Note: Many water districts provide rebates on these types of controllers.
What month of the year do most people overwater?
The ET drops significantly in the autumn due to less hours of daylight and less solar radiation. Don’t forget to adjust your controller to less watering days in the autumn.
HOW Do You DETERMINE HOW MUCH WATER YOUR PLANTS NEED?
The percentage of ETo required by different types of garden and landscape to perform well is shown in Table 1. Most plant types will survive with somewhat less water than these estimates but they will not meet yield, growth or appearance expectations. It is also important to understand that most plants will use more than they need to perform acceptably if it is available, even those plants deemed low-water using or drought tolerant. Thus, it is easy to waste water and overwater many plants without realizing it.
Keep in mind that one of the best ways to monitor the water needs of your plants is to watch for these signs of drought and take action before irreversible damage occurs!
- Wilting or drooping leaves that do not return to normal (without adding additional water) by morning
- Curled or yellow leaves that may fold or drop, along with potential twig drop
- Leaves that lose luster and become grayish or bluish
Table 1. Percentage of reference ET (ETo) for established landscape plants, turfgrasses, and home garden crops to provide acceptable performance in California (Footnote 1, 4)
By using the table you can see that trees need only half as much water as a cool-season grass lawn. But there is more to it than that!
Most plants, other than turfgrass, annuals, and most edible crops, do best with less frequent watering days, not less frequent watering time. For example: If a lawn needs water four days a week, we would assume a typical tree will need water two days a week. However, a tree typically has deeper roots than a lawn, so it would actually be better to water the tree once a week at double the amount of water