Brown spots reveal summer environmental heat stress on lawn

How to Deal with Summer Lawn Stress

Summer Environmental Lawn Stress


What’s happening to your yard? Your lawn looked great starting in early April and all the way through May, but now you might be wondering why your lush, dark-green lawn is suddenly turning brown and crispy, despite all your fertilization and watering efforts. Before you reach for a bottle of fertilizer in an attempt to perk up your browning lawn, consider the information below, before deciding whether fertilizer is an appropriate response. 

heat stress on lawn, brown patches in grass


What is the real reason my lawn is turning brown?

You may think that your lawn has gone off-color due to a nutrient deficiency, lack of nitrogen perhaps? Well, if it’s currently anytime from approximately June through September, your lawn is more likely to be turning off color due to environmental stress from drought, heat, or a combination of both. All of the fertilizer in the world will be of no help to a grass that is stressed from lack of water, or intense heat above its optimum growing conditions.

In order to continue to grow and maintain cells that are healthy and functioning properly, your lawn needs to create food, as much or more food than it consumes. This comes down to two processes: photosynthesis and respiration. These two processes can be thought of as opposite reactions. Photosynthesis creates carbohydrates by combining carbon dioxide and water with the energy of the sun and creating oxygen as a waste product. The oxidation of these carbohydrates, called respiration, creates useable energy for plants to grow. When photosynthesis is greater than respiration plants create a surplus of carbohydrates that they can store and utilize in the future. A great time for plants to tap into these stored carbohydrates is during periods of reduced photosynthetic capacity, such as extreme heat or drought. 

Plants are efficient and pragmatic organisms. Rather than photosynthesize in an extremely inefficient manner and completely use up all stored carbohydrates to continue actively growing, your lawn may enter a state of dormancy. Dormancy allows your lawn to tolerate heat and drought by ceasing growth until more favorable conditions arise. Let’s take a closer look at how heat and drought can stress your lawn and send it into dormancy during the summer months.  

Drought Stress

Water is essential for plants, as they’re composed of about 80-95% water. It’s estimated that as much as 99.5% of water is lost by transpiration and guttation. For the purpose of this article, we’ll focus on the process of transpiration. Transpiration refers to the process of water moving through a plant and its subsequent evaporation from aerial plant parts, like stems and leaves. This evaporation is what cools plants during the heat of summer and drives the transport of nutrients from the soil into the plant. This is why water is so essential for a healthy, thriving lawn. 

Plants respond to drought in different ways. Plants can avoid drought with extensive root systems capable of reaching soil water that would be otherwise unavailable to a more shallow-rooted plant. Plants can escape drought by setting seed before plant death, allowing the seed to carry on the family name when a more favorable weather pattern presents itself. One very common method for lawn grasses to avoid drought is by entering dormancy. This dramatically reduces the amount of water necessary to sustain life and allows the plant to wait out dry conditions until rain or irrigation becomes available. Just because a grass is labeled as “drought-tolerant” doesn’t mean it’s going to stay lush and green year-round. Dormancy is an important process that can help cool- and warm-season grasses survive extended periods of drought.

dry cracked desert sand


Is dormancy bad for your grass? 

Your lawn going off-color during the summer isn’t a bad thing and doesn’t require fertilizer to fix. Your lawn is just doing what millions of years of evolution have programmed it to do in order to survive. Fertilizing a drought-stressed lawn can compound the problem and cause more injury to your turf than if it had been left alone. Remember, if it has been dry and you haven’t irrigated, lack of water is the most likely cause of your lawn going brown during the summer months. See what happens to your lawn after the next good rainfall or some heavy irrigation, chances are, it might just morph back into its lusher, greener self.   

Heat Stress 

Cool-season grasses like Kentucky bluegrass, tall fescue, perennial ryegrass, and fine fescues have a set of air and soil temperatures at which their growth is maximized. This is true for warm-season grasses like bermudagrass, zoysiagrass, centipedegrass, and St. Augustinegrass, as well. 


optimum temperatures for warm-season grass and cool-season grass

For warm-season grasses, injury due to temperature extremes is far more likely on the opposite end of the spectrum when temperatures fall below optimum during winter months. Cool-season grasses, on the other hand, are far more likely to experience heat stress during the summer months. Even under well-watered conditions, cool-season grasses can become weakened due to high-temperature stress, thanks to an inefficient process called photorespiration. Remember that carbon dioxide molecule your lawn needs in order to conduct photosynthesis? Well, it turns out that the molecule responsible for fixing carbon dioxide (RuBP) also fixes oxygen (O2) molecules, in an inefficient process called photorespiration. The affinity of RuBP for oxygen is even further enhanced by high temperatures. Couple the inefficient photosynthesis with an increase in respiration (read: increased demand for food) due to increased temperatures and we have a two-headed monster ready to prey on your cool-season lawn. 


photosynthesis chart

Summer air temperatures are often well above optimum temperatures for cool-season photosynthesis and soil temperatures may remain above optimum for root growth for extended periods. When this happens the ability to photosynthesize is reduced dramatically and new root growth may cease. If this is allowed to continue for an extended period, your lawn may need to tap into its energy reserves, which can be depleted rather quickly. If it can’t create enough food through photosynthesis and root growth is hindered, going dormant makes a ton of sense to ride out the storm until cooler temperatures arrive. 


When to Fertilize?

A good fertilization program involves a proactive approach to lawn health so that your yard is in as good of shape as possible when it is confronted with environmental stressors like heat and drought. The time to fertilize is in anticipation of stress not as a reaction to stress.

root hume and hose end sprayer

Consider that good advice for non-fertilizer products, like seaweed extract and humic acid, as well. Condition your soil year-round, rather than as a “Hail Mary” to try and save your lawn during an extended period of heat and drought. Remember, your lawn is most likely not dying, it’s going dormant for a while, and will be back as soon as Mother Nature becomes more inviting.

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