How High to Mow a Lawn in Western Montana

How high should I mow my lawn in Montana?

About 2.5 to 3 inches for a typical lawn. MSU Extension's MontGuide on maintaining lawns gives 2.5 to 3 inches as the optimum mowing height for Kentucky bluegrass, fescue and ryegrass mixes, which describes most irrigated lawns in Missoula and the Bitterroot. It adds that Kentucky bluegrass tolerates lower mowing, but fescues and ryegrasses thin when mowed short.

Colorado State University Extension, writing for a similar climate, gives a preferred range of 2 to 3.5 inches for the turf species grown there and says mowing below 2 inches can reduce drought and heat tolerance and raise insect, disease and weed problems. CSU says scalping is never recommended. Set the deck, then check the actual height with a ruler on a flat patch of lawn.

Why does a taller cut make such a difference?

MSU says higher mowing heights, especially in the warmer summer months, allow turf to develop deeper root systems and have fewer weed problems. Iowa State University Extension explains the mechanism: extra leaf area shades and cools the crowns of the grass plants, and extremely high temperatures at crown level can kill turf. Taller grass also shades the soil surface, which keeps weed seeds that need light from germinating.

The weed effect is real enough that extension weed guides lead with it. South Dakota State University Extension's dandelion advice lists mowing at a higher setting (approximately 3 inches) and never taking more than a third of the blade among the first steps against dandelions. Our guide to dandelion control in Montana lawns picks up from there.

What is the one-third rule?

Never remove more than a third of the grass blade in one mowing. MSU states it directly: mow frequently enough to remove only a third of the blade. Iowa State turns it into arithmetic: a lawn kept at 3 inches should be cut when it reaches 4.5 inches. Cutting off more stresses the plant, which has to spend stored energy regrowing leaf instead of roots.

The rule sets the schedule, not the calendar. CSU says this means adjusting how often you mow to how fast the grass is growing, which can range from four to ten days between mowings. In western Montana that means frequent cuts in May and June, fewer in a dry July and August, and a pickup again in September when cool nights return.

Should I leave the clippings on the lawn?

Yes, in most cases. MSU says not to catch the clippings but to let them decompose on the lawn to return nitrogen to the soil, and states plainly that clippings do not cause thatch. CSU's PlantTalk on clippings agrees that clippings do not cause thatch and break down quickly because they are about 70 percent water, and says leaving them can reduce how often you need to fertilize.

The exception is clumps. CSU says large clumps form when the lawn is not mowed often enough or is mowed while very wet from dew or watering, and those clumps should be removed so they do not damage the grass. Our thatch and dethatching guide explains what does build thatch, since it is not clippings.

How sharp does the mower blade need to be?

Sharper than most are. MSU says to keep mower blades sharp to avoid tearing grass and giving a whitish cast to the lawn; that frayed, pale look a day after mowing is torn leaf tips drying out. CSU recommends sharpening after every third mowing or at least once a month. CSU's turfgrass selection guide notes leaf shredding is more common with dull blades on turf-type tall fescue and perennial ryegrass, so lawns with those grasses show a dull blade fastest.

Should I mow higher in summer?

The sources differ slightly, and the difference is useful. CSU says there is no reason to change mowing height during the year. Iowa State recommends mowing bluegrass at 3 to 3.5 inches in summer and 2.5 to 3 inches in the cooler spring and fall, because the extra leaf shades and cools the crowns. MSU's own guidance (that higher heights help especially in warmer months) leans the same way as Iowa State.

Our practical reading for western Montana: keep the lawn at the top of MSU's 2.5 to 3 inch range through July and August, especially on south and west exposures and on the gravelly soils of the Bitterroot floor. Iowa State also suggests mowing in the cool of morning or evening rather than midday, and not mowing a dormant, brown lawn at all, because mower and foot traffic can damage it.

What height for shade, dry areas and low-water grasses?

Shade: CSU recommends raising the mower to about 3 inches in shady areas to give the grass more leaf surface for photosynthesis, and mowing less often if growth is slower. MSU's shade mix (fescue-dominant) is maintained at about 2.5 to 3 inches.

Dryland grasses: MSU says crested wheatgrass, streambank wheatgrass and the bromegrasses brown during drought and should not be mowed below 3 inches; it gives about 2.5 inches for buffalograss and blue grama. If you are not sure what grass you have, our guide to Kentucky bluegrass versus fescue describes how to tell them apart.

How low should the last mow of the year be?

Not a scalp. MSU's advice on gray snow mold, the matted gray patches common in Montana lawns after snowmelt, is to mow turf at recommended heights until it is no longer actively growing, to rake up leaves and to avoid over-fertilizing in fall. That means keep mowing at your normal height as long as the grass keeps growing in October, rather than dropping the deck dramatically for a final cut. CSU's phrasing fits: keep mowing until the lawn stops growing.

Our guide to what happens to a lawn under snow covers snow mold and winter kill in more depth, and leaf removal before the snow is the other half of that prevention.

Does mowing height change how much I need to water?

Indirectly, yes. MSU says frequent light watering leads to shallow roots and less drought tolerance, and that higher mowing, especially in summer, gives deeper roots. MSU estimates that a well-maintained bluegrass lawn in most of Montana uses about 1 to 1.5 inches of water a week early and late in the season and as much as 2.5 inches a week in midsummer. Deeper roots let the lawn draw on more of the soil between waterings, which matters on the fast-draining gravel common in the Bitterroot.

CSU's guidance on weedy grasses makes the same connection from the other direction: frequent shallow watering and mowing too short both decrease the vigor of the lawn and open the door to weedy grasses. MSU notes that annual bluegrass and rough stalk bluegrass thrive under frequent, light watering.

Is a mulching mower worth it?

It helps. CSU says mulching or recycling mowers chop clippings into smaller pieces that break down faster, and that for best performance the lawn should be mowed often in spring, when growth is fastest. CSU also reports that studies show it takes less time to mow more often and leave clippings than to mow less often and bag them. A standard mower with the bag off does most of the same job if you mow on the one-third schedule.

Mowing mistakes we see most often

Cutting short to stretch the time between mowings, which removes far more than a third at once. Bagging every clipping out of habit. Running a dull blade all season. Mowing a drought-dormant lawn that has nothing to cut. And mowing wet grass, which clumps and tears. Each one shows up later as thin turf, more weeds or brown tips, and each one is free to fix.

MSU's broader point is that mowing works with watering and feeding. A lawn cut at the right height still struggles if it is watered lightly every day; see our summer watering guide and the guide to fertilizer rates and timing.

Recurring mowing from HJ

Our lawn care and mowing service cuts at the right height for Montana cool-season grasses with sharp blades, adjusts frequency to how fast the lawn is actually growing, and coordinates with our licensed fertilizer and weed control program and seasonal clean-ups. This guide is part of our Montana Landscaping, Lawn & Irrigation Guide. Call (406) 215-7319 or request a free estimate.

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