We have just installed a Rachio 3 Controller and in looking to setup the zones, I have discovered that the local gardeners have (as I always suspected) been overwatering the grass. I got a resident to dig a sod and they discovered that our St. Augustine grass roots are basically only 5cm. They never gave the grass a chance for the roots to need to search for water. So I am starting off our watering with 8cm root depth.
Is 8cm an appropriate starting depth ?
How often should I increase the depth increments ?
Sources I’ve read say that St. Augustine should have roots at least 6" (15 cm) deep. And that (surprise) “Over-watering and watering shallowly contribute to poor shallow root systems.” A surprising number of people (even here) just don’t believe the soil should be allowed to dry out some before watering again.
Anyhow, if your roots are only 5 cm deep, then starting with 8 cm wouldn’t be too bad, or even 10. Some roots get so think and spidery, it’s hard to tell. You certainly wouldn’t want to go immediately to 15 cm, but should work your way up to it, probably over a season or so. Realize this will mean watering more time, but less often, which is what is best.
To develop deep roots, you need to water deeply and less frequently. Try once a week. Low tech method is to dig down 6 " to see how deeply the water has permeated to 4-6" depending on your soil type. That will tell you how long to irrigate.
Controller method in the ADVANCED section: Set the root depth for your grass at 4-6". St. Augustine is a warm season grass. Enter the precipitation rate of your irrigation Spray or Drip, inches per hour, in the Rachio. Also set slope, light conditions, etc. Set it on a fixed schedule, once a week. Watch it to see if it is getting deep water and how the grass does. Yes, the soil should dry out in between watering because plants need oxygen to enter the soil where the water was. They rot with out oxygen.
First you need to be sure of the texture of your soil. searh on line to learn how to test your soils texture. If it’s clay, you will have to break down the watering schedule into cycles or short periods of time with rest time in between. In the program, you will see cycles for clay. Clay soil is so dense that after one or two minutes of spray irrigation, the water runs off of puddles. It may take one hour for the water to be absorbed into the soil. If you water for a long time, it will run off and probably not reach a 6" depth. Many people just increase the irrigation time and have poor to results.
The sod company I used tells me that sandy soil has its issues too. Watering long can just waste water as the water will go through the sand way past the roots, so a cycle and soak is effective there too. I wish there was a probe that I could stick in the ground to measure moisture content as various depths even while the zone is on.