Irrigation Information: Common Questions

With this recent ‘flash drought’ (as some meteorologists are calling it) many customers and lawn enthusiasts have been emailing and calling me with questions about their lawn and irrigation. First let’s start with some simple tips that can apply to any lawn, irrigated or not.

First, reduce your mowing frequency….increase the number of days between mowing.  Mowing is a stress and the less stress we can put on the grass the better. The rule for mowing is called the 1/3 rule.  Strive to remove NO MORE than 1/3 of the total leave at any one mowing.  That is to say, if you are trying to maintain your tall fescue lawn at 3 inches, don’t let the grass grow taller than 4 inches….mow it before it reaches 4 inches tall.  Now that it is hot, the grass is not growing nearly as fast, so it will take more days for the lawn to add the extra 1/3 growth.  (In April-May, I recommend mowing the lawn every 4-5 days.  Now it might be every 7-10 days, depending upon the growth rate of your lawn.)

Speaking of height…raise your mower a notch or two.  Raising the mower up a notch or two (usually 1/2-3/4 of an inch) will give the grass blade some extra length.  That extra length will help shade and cool the soil and help the grass combat the intense sunlight. Raise your mower a notch or two and increase the number of days between mowing to reduce stress on the lawn.

Now for some specific questions:

“Dr. Rodney, I don’t have a sprinkler system and my lawn is completely brown.  Is it dead?”

That is a great question.  Assuming the lawn was healthy before it browned and assuming it turned brown only from heat and drought and not disease or other issues, your lawn is not dead.  More than likely, your lawn is dormant.  Lawns go dormant when the stress becomes too great for them.  Just like trees drop their leaves in the fall and go dormant for the winter….lawns can turn brown and go to sleep, a.k.a dormant, to avoid the stress of summer.  So a lawn that turns brown in the summer from heat and drought is not dead and can survive like that for several weeks.  BUT it can die if the drought and heat is too long.

The big question is how long can the lawn stay brown and dormant before it dies. Unfortunately, that is not easy to answer. I will say scientifically speaking Kentucky bluegrass has been shown to survive the summer without water for more than 60 days.  And generally speaking most turf scientists will say tall fescue can survive about 5 weeks of drought before the lawn starts to die.  BUT all of those conditions are under ideal situations.  If the lawn was unhealthy (short roots, compacted soils, active disease progression, etc.) before it entered dormancy, the length of dormancy that the lawn can tolerate will be much less.

So what to do? Well one solution is to watch and wait. If some areas die, reseed them in the fall. Another option is to try and give the lawn just enough water to prevent death, but not bring it out of dormancy.  I usually recommend lawns receive an inch of water every 2-3 weeks from rainfall or irrigation.  This small amount will usually keep the lawn from dying.  It will be brown, but it helps prevent death.

“Dr. Rodney, how often and how long should I run my sprinkler system or my hose-end sprinkler?”

In a nutshell for cool season lawns like tall fescue and Kentucky bluegrass, I recommend irrigating every 2-3 days in late June, July, and August: every 3-5 days in late May and September: every 5-7 days in early May and early October and every 7-10 days in April and late October.  And those recommendations will change depending on the local weather patterns.

The second question people ask is how long? I recommended to irrigate an inch. But what is that.  Everyone asks me, ‘Just tell me how long, 20min? 30min? an hour?’ The run time is going to change based on your sprinkler type, pressure, nozzle type, spacing, etc. The easiest way to get an idea of how long to run your sprinkler system or hose-end sprinkler is to use a rain gauge. Place a rain gauge in the area to be irrigated and then run it for a set time, usually 30 min.  Then measure the rain gauge.  If there is an inch in the gauge, you know you need to run that zone or that sprinkler for 30 min.  If there is only 0.5 inches in the gauge you need to run it for an hour to achieve the 1inch goal.  Get out and measure it.  Usually you need to run your sprinkler much longer than you thought.

The goal with irrigation is to replace the amount of water that has been removed from the soil.  Water is removed from the soil in 2 ways; lost to the air by evaporation, and used by the plant for growth and cooling itself off. The scientific term for this water loss is called evapo-transpiration or ET.  So the amount of water we apply will be based on the ET rate–and ET is based upon the weather.  The hotter, drier, sunnier, and windier it is, the greater the ET rate is.  The cooler, cloudier, calmer and more humid it is, the lower the ET rate is.  Usually, the goal with ET based irrigation is to add 1inch of water after the daily ET rates have totaled 0.75-1inch of water lost. Typically, in KS and MO the ET rate in late April-early May is about 0.1 inch per day.  That means in early May, we will lose about 0.7inches of moisture in one week.  Therefore, after 1 week without rainfall, we should try to irrigate and apply about .75-1inch of water. Typically in July the ET rate is around 0.33inches per day, totaling to about 1inch of moisture every 3 days. Which is why I recommend to irrigate every 2-3 days.

“My irrigation system is working, but there are areas that are brown and crispy.  What should I do?”

No irrigation system is 100% perfect, but I would advise you to check the sprinkler coverage to make sure the entire area is getting irrigated.  If you can’t inspect the patterns or you don’t see the problem, maybe it would be best to have an irrigation professional come and inspect the system.  Sometimes the system can be adjusted to improve efficiency, but unfortunately, some systems can’t be improved without an overhaul.  I’ve seen it too many times, and it is sad.  Someone decided to save money and go with the cheapest installation proposal offered.  Most of the time that install was so cheap because the installer cut corners; using inferior heads, using too wide of head spacing, incorrect pipe sizing etc.  And unfortunately, you can’t easily upgrade these errors….many times you have to abandon that bad systems and start over.  You get what you pay for.  Quality costs more.  But I digress.  Back to the original question.  If you still have those dry spots, get a hose and sprinkler and supplement that area if you’d like to prevent the grass from going dormant.

“How can I tell the difference between dormant grass, dead grass, and diseased grass?”

Tough question.  Diseased grass will usually have lesions or spots on the leaves, or it will be greasy and matted down.  Dormant grass will usually stand upright but with a buff/tan color.  The best indicator of a dead lawn is to wait until it cools and rainfall/irrigation return.  If it doesn’t green up, you have your answer.

Dr. Rodney St. John 

Dr. Rodney has a Ph. D. in Horticulture, plus more than 20 years’ experience in the lawn service and garden industry. He loves to help people grow grass and teach them how to properly care for their landscape.