 Brian Vinchesi |
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.
You also are creating the potential to leach nutrients, increasing the opportunity for water-borne diseases and allowing water-loving weeds to flourish.
By cutting back irrigation after July and using a specific strategy to group the irrigation events, you may gain more utilization of the water you apply.
For example, instead of a one-day-on, next-day-off schedule, consider irrigating on consecutive days followed by more than one day off. The same amount of water delivered on consecutive days as contrasted with every other day will move water deeper into the soil and create the opportunity for a deeper root zone.
More than one consecutive day off will allow air pores in the soil to (re)develop, bringing critically needed oxygen to the roots. So although you have reduced the overall runtime(s) for your system after July, you still can strive to develop a larger soil moisture reservoir.
Make sure you document (and date) what scheduling changes you make and periodically inspect the root zone to verify your results. Pulling a soil core out of the root zone is absolutely invaluable in making decisions about irrigation scheduling.
Encourage Root Depth
Cultural practices can also play a role in creating greater irrigation efficiency while cutting back run times. Focusing on turf root depth is primarily determined by the genetics of the plant. Bluegrass, for example, could be made more water-efficient in the fall by mowing it higher.
The routinely taller blade will trigger the plant to develop longer roots and a bigger soil moisture reservoir. The taller blade will also better shade the surface of the soil, thus slowing evaporation.
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.
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