The Myth of Lawn Burning

A common misperception when it comes to lawn care is that it is "unsafe" to apply lawn fertilizer in the summer.

"We hear that a lot from consumers," said Jim Larkin, former Director of Consumer Research at Scotts. "Consumers won't apply fertilizer in the summer for fear that doing so will burn the lawn."

Typically people want to know: "Will applying a fertilizer in the summer burn my lawn?"

According to Ron Boylan, Manager of Scotts Training Institute, that question has been around for years. "In the old days, people used all water-soluble fertilizers, and spreaders that were poorly calibrated," said Boylan. "And water-soluble fertilizers are more likely to burn the lawn as the temperatures increase."

He said that many of the fertilizers on the market, and especially the cheaper fertilizers, are basically water-soluble.

The problem with water-soluble lawn fertilizers is that they provide too much nitrogen, at one time. For example, once the grass is given a quick shot of highly-concentrated nitrogen or phosphorus, the grass responds by "giving water back to the soil" because it senses a problem in the soil balance.

"It is a law of nature called reverse osmosis, and that is what causes the burning of the lawn," Boylan said. "The grass doesn't actually burn, it dries-up because it is giving-up moisture." Osmosis is the process by which plant roots take in moisture, and osmotic balance is essential to a healthy lawn.

So now we know the origin of that myth. But is it safe to fertilize in the summer? "Yes, as long as you use a high quality slow-release fertilizer, and follow the label directions," Boylan said.

"Slow-release fertilizers don't overload the soil with too much nitrogen, at too fast of a rate."

However, Boylan cautioned that not all slow-release fertilizers are created equal. Fertilizer manufacturers often accomplish slow-release of nitrogen by several means, but the most common method is to add coated urea to the fertilizer mix in some fashion. These coatings can vary greatly in accomplishing the slow release goal.

Urea is a source of nitrogen that is manufactured for use in agricultural fertilizers (about 90 percent of all manufactured urea is used by farms, and the other 10 percent for lawn fertilizer). It is also an element commonly found in mammalian urine (which explains why dog urine burns the lawn). To convert the nitrogen in the urea to a form the grass can use, it needs moisture. Thus, urea is essentially a water-soluble nitrogen fertilizer.

"Great for corn, but not for grass," Boylan said. "The nitrogen is released too quickly."

To make it safe for the grass, manufacturers attempt to slow down the release of nitrogen by coating it with non-water soluble materials, such as wax or sulfur, which is used to coat the urea. It's similar to the coating on M&M candies. The problem with that method, is that the nitrogen release is too unpredictable.

In the case of Scotts® lawn fertilizers, the urea is remanufactured into a methylene-urea compound with a much more controlled release rate. When applied to the lawn, the bacteria in the soil react with the methylene urea (instead of just needing moisture) to break down the fertilizer particles, thus releasing the nutrients to the lawn. This process will last for six to eight weeks.

Keep in mind that before applying any fertilizer, consumers should always carefully read the directions on the bag, and owners of cool-season grass lawns should never apply fertilizer when the temperatures are over 90° F, because the grass is under too much stress. Remember, any fertilizer can burn a lawn if it isn't applied properly.

"Grass is no different than any other living thing, it can only take so much heat and stress," said Boylan. "Even walking on the lawn when it is that hot can be stressful to it. In fact you can tell when a lawn needs watered if your foot prints do not spring back as you walk."

A rule of thumb to remember about fertilizing in the summer: make sure your lawn is well watered the week before you apply fertilizer. A lawn should get about one inch of water every week—water one inch, about every five days. Also, it is best to water in the morning when it is cool and won't evaporate too quickly.

Which brings us to another "myth" about lawns: watering in the hot weather will burn the grass, which is another question that lawn owners often ask.

"Years ago, fertilizers had to be watered-in because these all water-soluble fertilizers could easily burn the lawn; if a burn occurred people assumed it was the water that did it, and that was not the case," said Boylan. "The tendency is to think, 'The last thing I did is what caused it, and I watered the lawn last.'"

A study conducted by Scotts revealed that consumers are reluctant to water their lawns when the temperatures reach high levels. Homeowners think that water magnifies the sun's intensity, and that watering the lawn in summer will damage it.

"The sun's rays are not magnified by the drops of water. In fact the evaporation of the water from the grass actually cools the grass," Boylan said.

Bottom line: use a slow-release fertilizer, read label directions carefully, water at least ½" per week, and try to keep foot traffic on the lawn to a minimum during excessively hot weather. Your grass will thank you for it in the long run.

Here are some other lawn tips for this summer:

• Mow high,( the hotter, the dryer the higher), mow often( about every five days). The longer grass blades will help shade the grass and protect the roots.
• Avoid foot traffic in times of high stress, if possible.
• Thoroughly water the area where your pets urinate.
• Keep your mower blade sharp and balanced.
• Fertilize only with a high quality, slow-release fertilizer.
• Properly apply fertilizer to your lawn at the recommended rate on the bag, and check your spreader to ensure that it is working properly.
• Mow in the evening when the temperatures drop.
• Water in the morning, if possible, and avoid watering at night.