Extreme Heat Stress

The dog dawg days of August are upon us a little early this year with temperatures headed into the high 90-degree range with oppressive humidity later this week.

When temperatures get this high, lawns really do not like it too much. There are two different stresses that turfgrass plants endure in weather like this. They can occur together or separately.

The first of these phenomena is called heat stress. The grasses that we grow, with the exception of zoysiagrass, in the Northeast are referred to as cool season grasses. They grow optimally at air and soil temperatures that exist here during April, May, September and October. When temperatures increase, the type of photosynthesis that the cool season plants use and the type of transpiration (drawing up of water from the soil, through the plant and passing it through the leaves in order to cool the plant) becomes less and less efficient until it stops all together. When the plant reaches this point, it uses a defensive mechanism called dormancy - we call it a brown lawn. Instead of being a negative, it is really a blessing in disguise as it allows the plants to survive until the weather returns to more optimal conditions.

The second is called drought stress. It is a deficit in the amount of water required to keep the cells turgid (full of water). Either the environment is so hot that the plant cannot pull enough water from the soil or the soil has become so dry that no water is available. Either way, the plant wilts and turns brown.

So, what do we do?

First, don’t panic. Grasses have inhabited the Earth for 40 million years - just about as long as I have been doing lawn care. They have evolved to survive all manner of environmental stresses - all without our help.

Second, don’t make any sudden moves. Grasses are a lot like people - they don’t like sudden changes to their diet or their circumstances - they eschew change. If you haven’t been watering, don’t start. If you have been watering, you can make small changes, but not large ones. As far as the season is concerned, the die has already been cast and you have to play the hand you’ve been dealt.

The signs and symptoms of heat and drought stress are remarkably similar and if you are clued into what to look for and what it means, you can prepare yourself for what is to come.

This is the front lawn of a physician’s office. It is a Kentucky Bluegrass sod lawn that has an irrigation system that the landscape contractor perennially neglects to program properly. As soon as Mother Nature turns off the water spigot and the temperature starts to go up, this lawn becomes a bellwether of circumstances to come on other lawns in the area.

As you look at the lawn from left to right, you can see the progression of stress symptoms. The lawn on the left is still healthy and green. In the center, you can see a discoloration, a purple hue that joins the green color of the grass. Finally, on the right, you can see the brown of the grass that has gone into dormancy.

Since I took this photograph a couple of weeks ago, the landscape contractor has paid a visit to adjust the irrigation controller to apply more water and the lawn received a four inch dose of rainfall. The part of the lawn on the right is still in dormancy -still brown -but the lawn on the left has regained its green color.

Drought stress could just as easily as been heat stress. It all depends upon the circumstances.

But it is important to realize that, for the most part, these consequences of hot, dry weather are reversible, in no small part due to the care that we have taken during the spring and fall when the grass is growing like gangbusters.

Our fertilization efforts have encouraged the lawn to set down an ample root system. When soil temperatures get high, lawns lose a great deal of their root mass. However, since Lawn Dawg has been feeding your lawn, you have a much larger reserve of root mass from which to draw. You have given your lawn an advantage over a lawn that has not been following such a fertilization regime.

Watering the lawn

If you ask a hundred people for advice on how to water your lawn, you’ll get a hundred and one answers.

I have my own theories, based upon teachings of old school golf course superintendents, new school turfgrass research, and good old fashioned common sense.

First, lawns should be watered infrequently and deeply. An inch of water applied on one day is preferable to one third of an inch of water applied over three days. This recommendation realizes that roots respond to stimuli in the environment. If water is available right at the surface, that’s where the roots will be. If you cause your grass to drill deeper into the soil to find water you are encouraging a deeper and more robust root system.

Second, irrigate your lawn when the water applied has the least possibility of evaporating. Evaporated water from the surface never gets to the root system. Thus, you should irrigate late at night into early morning completeing the watering cycle by 5am. The water then has the opportunity to soak into the soil before the sun returns. But you’ve been told that watering at night promotes fungus, haven’t you? Consider for a moment that nearly every morning in the summer you can find dew condensed upon your lawn. That condensed water is more than adequate for proliferation of fungal diseases. Indeed, at the end of every grass blade there is a wound caused by mowing. The plant secretes juices, called guttation fluid that is full of carbohydrates that the fungus cannot manufacture for itself. The stage is set without our influence.

What we can and should be doing in timing our irrigation is to manage the duration that the lawn is wet. Let’s say that dew begins to form on the lawn at 9:00 at night and that it finally dries out at 9:00 the next morning. That is twelve hours. We should irrigate as not to elongate that period of time in which the lawn is naturally wet. If you start irrigating your lawn such that the cycle time finishes at 11:00 in the morning, you have given your fungus an additional two hours of favorable conditions to infect. A period of dryness interrupts the infection process.

A management practice on golf courses is to “whip the greens”. No, they haven’t been misbehaving but on mornings when the greens are not going to be mowed, workers will use either a garden hose or bamboo pole to knock the dew off of the grass plants. This interrupts the duration that the plant is wet, thus reducing the disease pressure on the grass.

Another practice you can employ is called syringing. This is the application of a small amount of water during the heat of the day to lower the temperature of the grass and give the plant a little extra water to pass off into the atmosphere - called transpiration. On golf courses, workers will be taught to recognize the purple hue that indicates heat/drought stress, as in the photographs above. In fact, you can buy special sunglasses that filter out certain parts of the light spectrum that make those purple areas stick out like a sore thumb. Once the worker has seen an area that has gone into stress, he takes out a hose with a soaker nozzle and gives the area a quick shot of water. They can also use the irrigation system to accomplish the same thing.

But you may have heard that applying water when its hot like that just boils the grass. Well, my response is to put it into human terms. When you’ve been outside on a hot day, sweating like crazy and someone turns a hose on you how does it feel? Pretty darned good, doesn’t it?

These suggestions are general management practices. They may not suit your specific circumstances at all. You may not have an irrigation system, my lawn doesn’t and it expects no water other than what Mother Nature gives it. Your town may have water restrictions at to when and how much water can be applied. You may work all day and not be around to watch for and react to observable stresses in the lawn. All of that is OK. Ultimately, it is Mother Nature who is in charge and it is she that will remove the environmental stress. In Southeast Massachusetts, you can expect the weather to turn during the last week of August. Early morning low temperatures begin to plummet, signaling the slow decent into winter. The grass recognizes this change and will begin to grow again.