How to Mow Slopes Safely: Slope Limits, Mower Types and Direction

Slope, or grade, is the rise divided by the run. A lawn that rises 2 feet over a 10-foot horizontal distance is a 20 percent slope, which is 11.3 degrees. As a slope steepens, more of a mower's weight shifts onto the downhill wheels, the uphill wheels lose grip, and the machine becomes easier to slide or tip.

Three things make it worse: wet grass, which cuts tire traction sharply; sudden turns, stops or speed changes, which shift weight abruptly; and edges such as ditches, retaining walls, ponds and drop-offs, where a slide becomes a rollover. Riding mowers carry the added risk of the machine landing on the operator, and walk-behinds the risk of slipping into the blade path.

Measure the steepest section you would mow, not the average across the yard. Three easy methods:

• Level and board: lay a straight 4-foot board on the slope with its uphill end on the grass, set a level on top, and lift the downhill end until it reads level. Measure the gap in inches, divide by 48 and multiply by 100 for percent. A 9.6-inch gap is a 20 percent slope.
• Phone inclinometer: set the phone on the same straight board (not directly on bumpy grass) and read the angle in degrees.
• String and line level: stake a string at the top, run it 10 feet downhill, level it with a line level, and measure from the string down to the ground. A 24-inch drop over 10 feet is a 20 percent slope.

Many riding mower manuals include a printed slope gauge you can fold and sight along a tree or pole. Use it; it is set to that manual's limit.

Mower manuals usually give slope limits in degrees, while robot mower makers usually rate slopes in percent. The two are not the same: a 45 percent slope is only about 24 degrees.

The maximum is whatever your operator manual says, and it applies to dry, firm turf. Many riding mower and zero-turn manuals set the limit somewhere between 10 and 15 degrees (roughly 18 to 27 percent), some are lower, and most reduce it further when you tow an attachment, carry a bagger or mow wet grass. A zero-turn's limit is often at the low end because of how it steers.

Do not treat the number as a target. Uneven ground, soft spots, a rut or a hidden hole can make a slope within the rating behave like a steeper one, and a mower that feels stable going straight can tip in a turn. If you cannot find the manual, download it from the manufacturer by model number before you mow a grade.

Riding mowers, including lawn tractors and zero-turns: up and down, never across. This is the direction in CPSC and OPEI guidance and in most riding mower manuals. Driving across a slope puts the uphill wheels light and lets the mower roll sideways, the most common rollover scenario. Driving straight up and down keeps the weight between the wheels. Go slowly, avoid sudden stops, starts and turns, and make turns on level ground at the top or bottom rather than on the face of the slope.

Walk-behind mowers: across, never up and down. If you slip while mowing across, you fall beside the mower rather than into it. Mowing downhill, the mower can run away from you; mowing uphill, it can roll back onto your feet. Never pull a walk-behind backward toward you on a slope, and wear shoes with real tread.

Robot mowers: follow the maker's setup for slopes, which may include zone settings, slower speed or a specific mowing direction on steep sections.

A zero-turn steers and brakes through its two rear drive tires. The front wheels are free-swiveling casters that do not steer at all. When those rear tires lose grip on wet grass or a steep grade, you lose steering and braking at the same instant, and pulling back on the lap bars does not stop the slide. Heading downhill, weight shifts forward onto the casters and away from the drive tires, which makes it worse.

That is why zero-turn manuals set conservative slope limits and why lawn tractors, which steer with the front wheels and brake through the transmission, generally handle grades better. Some electric models are designed with slopes in mind, such as the wide frame on the EGO POWER+ Z6 ZT4214L 42 in. Electric Zero Turn Mower, and some larger zero-turns such as the Husqvarna MZ61 61 in. Zero Turn Mower with ROPS come with a rollover protection structure (ROPS). Where ROPS is fitted, keep it raised and wear the seat belt. Neither feature raises the manual's slope limit.

No. Wet grass is the single biggest traction killer on slopes, and a grade you mow comfortably when dry can become unsafe after rain, heavy dew or an irrigation cycle. Riding mowers slide, zero-turns lose steering, and walk-behind operators slip.

Wait until the grass is dry to the touch, usually late morning or afternoon after dew, and turn off sprinklers the night before you mow a bank. If the soil is soft enough for tires to leave ruts, it is also soft enough for a wheel to sink and tip the mower. On a lawn that is always damp, mow slopes with a string trimmer or a robot mower rated for the grade.

They improve traction, but they do not change the slope limit in your manual. Turf tires with a deeper tread pattern grip better than worn or smooth tires, and correct, equal tire pressure on both sides keeps the mower from pulling. Some manufacturers specify wheel weights or counterweights for certain attachments; the EGO POWER+ ABK4200 Bagger Kit for 42 in. Z6 Zero Turn for the EGO ZT4204L, for example, includes counterweights to balance the bagger. Add weights only as your manual directs, since weight in the wrong place can make a mower less stable.

A differential lock, like the one on the Husqvarna TS 375XD 54 in. Lawn Tractor, helps a lawn tractor keep driving when one wheel slips on damp ground. Again, it is a traction aid, not a license to mow steeper grades.

Stop mowing and switch tools when any of these are true:

• The slope is steeper than your manual allows.
• You cannot stand comfortably on it, or your feet slip when it is dry.
• The slope ends at a drop-off, ditch, retaining wall, road or water. Keep riding mowers well away from edges, at least the distance the manual specifies.
• The ground is rough, rutted or soft.

A string trimmer lets you cut a bank from secure footing at the top or bottom. A battery model such as the EGO Power+ ST1623T 16 in. String Trimmer Kit with 4.0Ah Battery is light enough to swing for a long bank, and the gas ECHO SRM-2620T 25.4cc High Torque Gas String Trimmer handles thick, tall growth. Wear eye protection, long pants and boots. For a bank you fight every week, consider replacing the grass with a low ground cover or mulched plantings, which removes the hazard for good. See the best string trimmers.

Robot mowers are often the best answer for slopes, because no one is on or behind the machine. Check the rated slope in percent against the steepest section you measured:

• Husqvarna Automower 410iQ Robot Mower, Husqvarna Automower 420iQ Robot Mower and Husqvarna Automower 440iQ Robot Mower: 45 percent (about 24 degrees).
• Segway Navimow i210 AWD Robot Mower: 45 percent with all-wheel drive, for about a quarter acre.
• Mammotion LUBA 2 AWD 3000X Robot Mower and Mammotion LUBA 3 AWD 5000 Robot Mower: 80 percent (about 39 degrees) with four-wheel drive.
• WORX Landroid Vision Cloud WR342 4WD Robot Mower: 84 percent (40 degrees) with 4WD, for about half an acre.

Leave margin below the rating, because wet grass affects robots too. Keep children and pets off the lawn while a robot mower runs. The robot mower size for steep yards page matches models to grade and area.