How High to Mow a Lawn in Washington

Washington lawns are mowed at the height that suits the grass, and the two grass families here sit far apart. Tall fescue wants 3 to 3.5 inches. Kentucky bluegrass wants 2 to 3 inches. Perennial ryegrass wants 1.5 to 2.5 inches. Zoysiagrass and bermudagrass want only 1 to 2 inches. Set the deck for the wrong species and you can thin out the lawn within a season.

The mowing height for Washington lawns

Start by identifying what you actually mow, because the heights below are not interchangeable. A bermudagrass lawn cut at 3 inches turns stemmy and leggy. A tall fescue lawn cut at 1.5 inches loses most of its leaf area and struggles through its first dry spell.

These are the ranges extension programs publish for each species, and Washington gardeners plant all five. No single one wins here. The state sits in a trade-off that gardeners in the transition zone know well. A cool-season lawn stays green through the mild, damp months but suffers in July. A warm-season lawn handles summer without complaint, then sits brown from October to April.

Height matters in both families for the same reason. The blade is the part of the plant that feeds the roots, and a taller blade means more feeding area. Taller grass also shades the soil surface, which keeps it cooler and makes it harder for weed seeds to germinate. That is why tall fescue, the coarsest of the cool-season grasses, is held highest of all.

If you are not sure what you have, look at the lawn in January. Grass that is still green is a cool-season type. A patch that has gone straw-colored and stays that way until spring is warm-season. For a mixed lawn, the safest height is the one that suits the dominant grass without hurting the others.

Everything here applies most directly to western Washington, the mild maritime side of the Cascades. East of the mountains the climate is high desert, with colder winters and hotter, drier summers. The species heights stay the same, but the calendar and the watering demands change, which the next sections cover. For the general technique behind all of this, see Mowing a Lawn.

Raising the deck when the heat comes

July is the driest month on the western side of the state, at just 0.6 inches of rain. NOAA’s 1991-2020 climate normals for the western Washington reference station show June at 1.45 inches, July at 0.6 and August at 0.97. That adds up to about 3.0 inches across the whole summer, out of 39.3 inches for the year. Most of the year’s rain falls well outside the growing season.

For a cool-season lawn, that is the moment to move to the top of its range. Tall fescue goes to 3.5 inches, Kentucky bluegrass to 3 inches and perennial ryegrass to 2.5 inches. The extra blade length shades the crown and slows the loss of moisture from the soil. Cutting short in July is the most common way a cool-season lawn goes from tired to dead.

How much does summer rain actually help?

Less than the number suggests, and it depends on your ground. On sandy soil, rain passes through within the day, so even a wetter summer can leave a lawn needing water every second day. Clay holds an inch of rain for a week or more, but it also sheds a lot of a quick downpour before it can soak in.

Days at or above 90°F pull water out of the soil and the leaf faster than light summer showers can replace it. These normals describe what the region can usually expect over three decades. They are not a forecast, and a rain shadow, a hillside or a city lot can all shift them. A plain rain gauge on your own lawn is what settles the question.

Warm-season lawns behave differently. Zoysiagrass and bermudagrass grow hardest in the heat and tolerate a low cut. They stay at 1 to 2 inches all summer, and the main summer job is mowing often enough that you never remove too much at once. The next section explains how to judge that.

Dry weather slows growth, so let the grass set the schedule. Brown, crunchy grass that has gone dormant needs no scalping to look tidy.

The one-third rule, and why it is not arbitrary

Take only a little off the blade in a single mowing. That is the rule most extension services repeat, and it rests on how the plant works. Roots and new growth run on the sugars made in the leaves. Cut away a big share of the leaf and the plant spends stored energy to regrow it, while the roots stop growing until the leaf area recovers.

The rule turns into a simple schedule once you know your target height. Tall fescue kept at 3.5 inches should be mowed before it gets much taller. Kentucky bluegrass kept at 3 inches gets cut before it gets much taller. Zoysiagrass kept at 2 inches gets cut before it gets much taller. A cool-season lawn growing fast in a wet spring may need mowing more often, while the same lawn in a dry July can go longer between cuts.

Hauling out the mower after a long gap is where people go wrong. After a rainy week or a trip away, the grass can be well past the threshold. Cutting it straight to the target height removes far more than a third, and the lawn turns yellow-brown for a while. Mow in two passes instead. Cut it to roughly the top of the range first, wait a few days for the grass to recover, then drop to the target height.

Clippings from a one-third cut are short, break down fast and return nitrogen to the soil. Bag or rake the long, wet clippings from an overgrown lawn, because they mat on the surface and block light. Keep the blade sharp too, because a dull one tears the grass tips and leaves a ragged cut that dries out faster.

The first and last cut of the year

On the western side of the state, the last spring frost usually falls between February 27 and March 18, and the first fall frost between November 13 and November 30. Those windows come from NOAA NCEI’s 1991-2020 normals, and they describe the mild maritime lowlands. They do not describe eastern Washington, where the high desert has a much shorter season, and they should not be carried across the Cascades.

A mild frost date also does not mean an early start. Soil in a western Washington spring stays cold and wet well past the last freeze, and low light keeps growth slow. Wait for the grass itself to tell you. When a cool-season lawn has put on enough new growth to pass the one-third threshold, make the first cut at the normal height for the species. Do not drop the deck low to clean up winter debris. Mowing a saturated lawn does more harm than waiting, because wheels leave ruts and compress the soil.

Warm-season lawns start later. Zoysiagrass and bermudagrass stay brown until soil warms, so the first cut comes after the grass greens up, often well after cool-season lawns have been mowed several times. Gardeners in the Deep South get a much longer window for these grasses, as covered in How High to Mow a Lawn in Mississippi.

The last cut follows the same logic. Cool-season grass in the lowlands can keep growing into late fall, so keep mowing at the regular height for as long as it needs it. Do not cut extra short for winter, because a very short lawn is more exposed to cold, wet conditions and moss. Fallen leaves can be mulched in with the mower while the grass is still short enough to take them. Warm-season lawns stop earlier, since they go dormant and brown from October to April. Cool, damp places elsewhere follow similar rhythms, as in How High to Mow a Lawn in Newfoundland and Labrador.

For eastern Washington dates and local grass recommendations, Washington State University Extension and your county office can give figures for your elevation and soil.

Common questions

What height should I mow tall fescue in Washington?

Mow tall fescue at 3 to 3.5 inches. Use the upper end in July and August, when the lawn is under the most stress.

Can I mow lower in summer to cut less often?

No. A shorter cut leaves less leaf to feed the roots and exposes the soil to heat, so the lawn needs more water and is more likely to thin. Cutting less often is better done by letting the grass grow slower in dry weather.

Should I bag the clippings?

Usually not. If you remove no more than one-third of the blade, the clippings are short and break down quickly. Bag or rake them only when the grass was overgrown and the clippings are long and wet.

Do the same heights work east of the Cascades?

The species heights are the same, but the season is shorter and summers are drier. Watering needs are greater, and frost dates differ from the western lowlands. Check with Washington State University Extension or your county office for local timing.

Related guides

• How High to Mow a Lawn in Oregon
• How High to Mow a Lawn in Idaho