Water wasters

The time between July and fall is when landscapers typically over-irrigate the most. Or stated another way, this is the time we typically have the most potential to save water through more efficient scheduling. It may be counterintuitive to think that plants would need less water during August and September, for example, than in July. After all, in many places in the continental U.S. this is a time we may consider as our “best” weather.
Hot summer days – cut back the water. Huh?

ET Factors
Let’s look at some facts. Plant evapotranspiration (ET) is primarily determined by the climatic conditions of temperature, humidity, wind and light. So in general terms, as the days get shorter after July, overall plant water requirements decrease in direct relationship to the shortening days.

To be more specific, using 30-year average data gathered by the National Oceanic and Atmospheric Administration for the following random sampling of cities across the United States, the ET in all areas decreased by the following percentages from July through September:

• Palo Alto, Calif. – 18 percent
• San Diego – 14 percent
• Phoenix – 14 percent
• Dallas – 21 percent
• Atlanta – 15 percent

As you might expect, the decline in ET is much steeper in October as we approach winter.


Scheduling Strategies
This means if you are using the schedule that is successful for you in July, and use it any other time of the year, you are wasting water – maybe a lot of water.

Conclusion
Reducing irrigation  and the amount of water applied to a landscape requires a contractor to have knowledge of root depth, soil texture and the water holding capacity of the soil.

These factors all work in conjunction. The cues for the landscape contractor who wants to better manage schedules are to irrigate to fill the root zone and allow a reasonable water depletion, i.e. allow the (re)formation of air pores before refilling the root zone.