How to Mow a Lawn: Mowing Height by Grass Type and the One-Third Rule

The one-third rule says never remove more than one-third of the grass blade's height in a single mowing. University extension services recommend it for every common lawn grass because the leaf blade is where grass makes its food. Removing more than a third at once stresses the plant, slows root growth and leaves brown, scalped patches.

To use it, work backward from your target height. If you keep tall fescue at 3.5 inches, mow when it reaches about 5.25 inches (3.5 x 1.5). If the lawn has gotten away from you after a wet week or a vacation, do not cut it down in one pass: raise the deck, take off a third, and lower it again over the next two or three mowings a few days apart.

These are common university extension mowing ranges. Use the higher end in summer heat, drought and shade, and the lower end in cool, moist growing weather. The "mow when it reaches" column applies the one-third rule. Full profiles are on each grass type page and in the mowing height chart.

Deck height settings are approximate, and the numbers on the lever do not always match the real cut. To check, park the mower on a flat driveway, shut it off and disconnect the spark plug wire or remove the battery and safety key, then measure from the pavement to the lowest point of the blade tip. On turf the grass will stand slightly taller than that number because the wheels sink a little.

Make sure the mower's range covers your grass. The EGO POWER+ LM2135SP 21 in. Select Cut Self-Propelled Mower Kit adjusts from 1 to 4 inches, the Honda HRX-BE 21 in. Battery Self-Propelled Mower from 0.75 to 4 inches, and the EGO POWER+ Z6 ZT4204L 42 in. Electric Zero Turn Mower rider from 1.5 to 4.5 inches. Robot mowers vary more: the Segway Navimow i210 AWD Robot Mower cuts 2 to 3.6 inches, which suits cool-season lawns but is too high for bermudagrass kept at 1 inch, while the Husqvarna Automower 520H EPOS Robot Mower goes down to 0.8 inch for low-cut turf.

Mow by growth, not by the calendar. The one-third rule decides: whenever the grass reaches one and a half times your target height, it is time.

• Cool-season lawns (fescue, bluegrass, ryegrass) grow fastest in spring and again in fall, often needing a mow every 4 to 5 days during the spring flush, and slow down in summer heat to once a week or less.
• Warm-season lawns (bermudagrass, zoysiagrass, St. Augustinegrass, centipedegrass) peak in summer, when bermudagrass kept at 1 to 1.5 inches may need mowing twice a week.
• Dormant lawns in drought or winter need little or no mowing.

Frequent, light mowing is exactly what robot mowers do. A robot such as the Husqvarna Automower 420iQ Robot Mower trims a little every day, which keeps the lawn within the one-third rule without anyone tracking growth. See the best robot lawn mowers.

Mulch most of the time. Extension services widely recommend leaving clippings on the lawn ("grasscycling") because they break down quickly and return nitrogen and other nutrients, which can reduce how much fertilizer you need. Clippings do not cause thatch; thatch builds from stems, roots and runners, not the leafy, water-rich clippings.

Bag instead when:

• The grass got too tall and clippings would sit in clumps that smother the turf.
• Weeds are going to seed or the lawn has an active disease.
• Fall leaves are heavy, or you are preparing to overseed or aerate.

Mulching needs the right blade and deck. On many riders, a mulch kit such as the Husqvarna HU42 42 in. Mulch Kit with Blades (for Husqvarna 42-inch stamped decks) or mulching blades such as the Husqvarna 48 in. Mulching Blades (3 Pack) make the difference, while Husqvarna 48 in. High-Lift Bagging Blades (3 Pack) improve suction when bagging. When you side discharge, aim clippings toward the lawn, never toward the street, beds, people or cars, and blow any clippings off pavement so they do not wash into storm drains.

Change direction every time you mow. Mowing the same lines every week wears ruts from the wheels and makes the grass lean one way, which leaves an uneven cut. Rotate through four patterns: lengthwise, crosswise, and the two diagonals.

For a clean, efficient job, mow two laps around the outside edge first. That "headland" gives you room to turn at the ends of each straight pass without scuffing the turf or leaving uncut arcs. Overlap each pass by a few inches, about 15 percent of the deck width, so no strips are missed. On slopes, the direction is set by safety rather than looks: walk-behinds go across, riding mowers go up and down (see mowing on slopes safely).

Lawn stripes are just light reflecting off grass blades bent in different directions. Grass bent away from you looks lighter, and grass bent toward you looks darker. A mower with a rear roller, or an add-on striping kit with a roller or flap, bends the blades more firmly for bolder stripes.

1. Mow the headland laps around the edge first.
2. Mow a straight line along the longest edge, using a fence or driveway as a guide.
3. Turn on the headland and come back directly beside the first pass, overlapping slightly.
4. For a checkerboard, repeat at 90 degrees.

Cool-season grasses like Kentucky bluegrass and ryegrass stripe best, and stripes show more at the upper end of the mowing range. Change the stripe direction regularly to avoid ruts.

Sharpen at least once or twice a season, and more often on large lawns; a common guideline is every 20 to 25 hours of mowing. A sharp blade slices the grass cleanly. A dull one tears it, leaving ragged tips that turn brown or whitish a day or two later and give disease an easy entry.

Look at the grass the day after mowing. If the tips look shredded, the blade is due. Sandy soil, sticks and acorns dull blades faster. Always balance a blade after sharpening, because an unbalanced blade vibrates and wears the engine bearings. The lawn mower maintenance guide covers sharpening and balancing step by step.

• Raise the height to the top of your grass's range. Taller grass shades the soil, keeps roots cooler and loses less water.
• Mow less often. Growth slows in heat; let the one-third rule tell you when.
• Mow in the cooler part of the day, early evening or mid-morning once dew is gone, not in the heat of the afternoon.
• Leave the clippings to hold moisture at the soil surface.
• Keep the blade sharp; torn tips lose more water.
• Do not mow dormant brown grass that is not growing. Mowing and traffic on dormant turf cause damage that shows once rain returns.

Cool-season grasses like tall fescue and bluegrass often go dormant in long summer droughts and recover with fall rain. Follow local watering restrictions.

Mid-morning, once the dew has dried, or late afternoon into early evening. Wet grass clumps, clogs the deck, cuts unevenly and makes slopes slippery. Midday heat stresses both the lawn and you. Avoid mowing right before dark, when it is hard to see holes, toys and pets, and check local noise ordinances for gas mowers early in the morning.

Before you start, walk the lawn and pick up sticks, rocks, dog toys, pine cones and anything else a blade could throw. A mower blade can launch a small stone fast enough to break a window or injure someone standing nearby.