St. Augustine lawn watering requirements

U_P_turf_newby – posted 01 August 2006 14:06

Im taking a survey (to kill my curiousity). Im wondering how often you all are watering your St. Augustine in Southern Florida (on average). I understand it depends on weekly rainfall, but Im asking for a generalization during the summer.

Thanks in advance!

TexanOne – posted 05 August 2006 23:51

I don’t know about south FL, but I water my St Augustine in Texas by published ET (evapotranspiration) levels. From my experience, SA grass seems to do best at about 60% ET values.

Right now, I’m watering mine at 50% ET values because of watering restrictions.

U_P_turf_newby – posted 06 August 2006 05:35

First of all, thanks for the reply, I appreciate it.

I understand the concept of ET but I’m not 100% sure of what you are meaning about 50% ET, 60% ET…. How do you calculate percentages of ET?

The link below gives ET information of various stations in Florida…I live near Ft. Lauderdale…

http://fawn.ifas.ufl.edu/tools/et/et.asp

Thanks in advance for your help

TexanOne – posted 06 August 2006 23:19

ET values were originally developed for the Ag industry to determine how much water was needed for full-growth crops like wheat. Years ago, it was found that most turfgrasses don’t need the amount of water that crops do. The obvious questions though were, “How much water does my turfgrass species need?” and “What is the minimum amount of water needed for healthy turf?”. Generally, warm season turfgrasses (like St Augustine) need about 60% of ET values. The ET value can range from about 45% ET for minimal turfgrass survival and up to 80% ET for lush, fast growth. So, assuming 60% ET and if the daily ET rate were 0.30”, the St Augustine would theoretically need 0.18” to replace water loss that day.

(0.30 x 0.60 = 0.18” irrigation)

Of course, no one will water their grass 0.18” each day. However, if you had 4 days of 0.30” ET, you then would have this formula:

(0.30 x 4 days) x 0.60 = 0.72” irrigation – or about ¾” of water every 4 days.

All of that is fairly simple and straightforward, but you also have to determine how much water your soil will hold, and how much allowable soil-water depletion will take place before you can build an irrigation schedule. If you are really lucky, your city may do an irrigation audit for you for free or at a nominal charge.

The topics of evapotranspiration, irrigation scheduling, system efficiency, and estimated turfgrass water usage are stuff that books are written about. However, there are several good websites that explain how to do your own irrigation audit and this is one of the best:

http://www.wateright.org/References.asp

Even though this site is geared for the western states and for automatic irrigation systems, the information provided can be useful for everyone. The information may seem overwhelming on how to conduct an irrigation audit, but it’s all simple math. Start with first things first – click on “Evaluating Sprinkler Irrigation Uniformity” and go down the list from there.

If you have an irrigation system and decide to do an audit, I found the best catchment containers to be plastic champaign glasses with the bases removed. They are pointed on the bottom and you can stick them in the turf by making a small pilot hole using a screwdriver – they won’t tip over and ruin your testing. You can also buy vast quantities of them at a discount store for just a few bucks.

ET-based irrigation scheduling has been the norm over the water-restricted western states for many years but it’s finally catching on places like Florida and the Southeast. If money isn’t a big issue, most of the irrigation manufactures like Toro and Weathermatic now offer ET-based controllers that compute ET and self-adjust from downloaded satellite data. That’s a little rich for me, but definitely cool stuff!

Hope this helps you and sets you on the right course.

TexanOne – posted 07 August 2006 00:28

But to answer your question in the original post:

I have 3 of my 9 irrigation zones watering St Augustine turfgrass in conditions from full sun to mostly shade (the remaining 6 zones are watering shrubs and drip irrigation shrub zones). The timing varies on each St Augustine zone because of differences in water application rates, but generally, I am watering my St Augustine 0.60” of water every 4 days in August. The highest irrigation level is July when I’m watering it at 0.55” every 3 days and the lowest is when I’m watering it about 1” every 20 days in January.

This is based on irrigation scheduling that assumes St Augustine needs an annual average of 52% of historic ET values.

If you want the technical details, the irrigation cycle is set to replace water when the soil-water depletion is 60%. In other words, the evapotranspiration rate has used 40% of the water the soil can hold. I am also irrigating to a depth of 9” (assuming the soil can hold 0.18” of water / inch of soil). Because rainfall is almost a non-factor here, the irrigation system is doing the same job as providing 38” of annual rainfall – spread out evenly over the year.

These figures may not apply to you because West Texas is nothing like South Florida. It is much hotter, colder, and drier and we have hardpan alkaline, desert soils here.

Temperatures are routinely well over 105 degrees with very low humidity levels and we get the occasional polar cold fronts where temperatures will get near zero degress. St Augustine is one of few turfgrasses that do well here. In fact, St Augustine is better than any other turfgrass here, including Zoysia, Bermuda, and Buffalograss. Forget any cool season grasses like Fescue! I am always amused when I read that St Augustine “needs large amounts of water”. That is nonsense – St Augustine is very tough and can take desert conditions in stride. The only limiting factor for St Augustine here is cold weather, but on the other hand, we don’t have the insect or fungal problems of Florida.

[This message has been edited by TexanOne (edited 07 August 2006).]

U_P_turf_newby – posted 08 August 2006 04:25

Wow, thanks a million for the reply! I truly appreciate the AWESOME, AWESOME info! You definitly got me looking in the correct direction.

Once again, thank you!