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Maintain Lawns to Reduce Summer Ozone

By Carl Wilson, Horticulturist, Colorado State University Cooperative Extension, Denver County

If you're in charge of maintaining a lawn, you have a role in minimizing Denver's summer ozone pollution and its human health effects.

Ground level ozone forms under warm temperatures when a pollutant emitted by lawn mower engines, VOC, is present with another pollutant, NOX, emitted by automobiles and industrial sources. Ozone that is formed at ground level irritates breathing passageways unlike beneficial stratospheric ozone that filters out skin-damaging UV rays high above the earth.

The small engines found in gasoline-powered lawnmowers, leafblowers, string trimmers and other yard care equipment emit as much as 25 percent raw, unburned gasoline. An hour of lawnmower operation produces as many emissions as a 1997 mid-sized car driven 125 miles. During warmer summer weather, advisories will be called in the six county Denver Metro area when ozone levels are high. Consider the following lawn care steps to minimize the ozone health risks for everyone.

  • Water lawns moderately but not excessively
        Lawns respond with ample growth when over-irrigated. In addition to more mowing and therefore more air pollution, excessive watering  creates thatch buildup and other landscape problems. Wait to water until you see "footprinting" in the lawn. Footprinting refers to grass that takes a long time to spring upright after walking through the lawn. Reset sprinkler clocks from season to season to save water and preserve air quality. Watering for the entire growing season based on clocks set for peak, summer hot-spells creates excessive mowing during other periods.
  • Fertilize lawns in the fall -
        Late season fertilization in fall eliminates excessive spring lawn growth and minimizes the need for mowing. It delivers all the benefits of spring fertilization including early green-up, good color and thickness without creating excessive spring mowing chores. Less mowing means less air pollution. Fall fertilization generally lasts through early summer.
  • Use slow release fertilizer products in spring and summer
        In late spring and summer, use slow-release fertilizer products. This minimizes mowing by eliminating the excessive top growth seen with fast acting fertilizers. Lawn thickness and quality can be maintained to minimize weed invasion while also minimizing mowing chores and air pollution.
  • Delay lawn mowing by 24 or perhaps 48 hrs. during summer advisories
        People are still urged to keep in mind the "one-third mowing rule." That is, never remove more than 1/3 of the grass blade in a single mowing. For example, 3 inches is an average recommended height to maintain lawn grass today. Mowing to return a lawn to 3 inch height is no problem when the lawn grows to 4 or 4.5 inches. If a lawn reaches 5 inches, mowing to a 3 inch height stresses the turf. This is in addition to the concern over what to do with the long clippings.
        In general, lawns grow slower in summer due to the heat. Kentucky bluegrass is a cool season plant and tends to languish in hot weather. It should be possible in most cases to delay mowing with no concerns for lawn health. Plan mowing based on consideration of smog advisories plus the one-third rule, not a calendar schedule.
  • Mow in the evening or cooler part of the day when it is cloudy and there are breezes
        This minimizes ozone formation by dispersing VOC and moves mowing to a time of day when temperatures are less favorable for ozone formation.
  • Reduce lawn areas when renovating landscapes
        Less lawn means less mowing. Minimizing turf areas may be part of a plan to renovate an overgrown landscape, save on water costs or add a deck or other outdoor living area.


Other mowing options such as hand-pushed reel mowers, or electric mowers are excellent alternatives that are not a concern during smog advisories. Sweeping with a broom is a manual alternative to leaf blowers, as are hand shears instead of gasoline powered trimmers. Note that EPA and CARB-certified mowers manufactured after 1999 are cleaner burning and cut down on emissions.

Additional tips to minimize pollution include: avoid spilling fuel during filling operations, fill tanks only 3/4 full to allow for expansion, tightly screw on the gas caps of the mower and fuel can, place gas containers in a cool place out of the sun, and reduce mower operating time to that essential to cutting the lawn.

Maintaining mower engines is key to burning fuel as cleanly as possible. Change oil after 25 hours of use and use 30W oil instead of a multi-weight type in warm weather. Clean or replace the air cleaner element regularly, at least every 3 months or when the oil is changed. Clean, re-gap or replace spark plugs. Keep the engine and underside of the mower deck clean of grass buildup. Sharpen the mower blade regularly for cleaner cuts that reduce pollution by requiring less fuel.

Thanks for doing your part to keep Denver's air clean and landscapes green.

Photograph courtesy of Judy Sedbrook.

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Date last revised: 01/05/2010