Setting and Reading Height of Cut on a Reel Mower

A height of cut on a reel mower is the gap between the bedknife edge and the reel blades, measured in thousandths of an inch. It is not the height of the grass. The grass stands taller than that gap, and how much taller depends on the turf, the rollers, the weight of the unit and the direction of the pass. A fairway mower and a green mower can both be set to 0.125 inch and cut two different surfaces. A wetland delineation on a course follows the same logic as a prism gauge reading, in that the boundary is measured on the ground and then written down, and the written number is what the next person works from.

What does a height of cut actually measure on a reel mower

On a reel mower the number on the adjustment dial describes the clearance between the bedknife and the reel. The bedknife is the stationary bar at the bottom of the cutting unit. The reel is the cylinder of spiral blades above it. Grass is trapped between the two and sheared. The gap is the height of cut.

That gap is not the height of the leaf. A walk-behind green mower set to 0.100 inch leaves a canopy that measures 0.110 to 0.140 inch with a prism gauge, because the rollers press the surface down and the grass rebounds after the pass. A triplex fairway unit set to 0.500 inch leaves a canopy close to 0.500 inch, because the rollers are lighter relative to the cut and the grass is stiffer. The dial is a machine setting. The canopy is a turf measurement. They are related, not identical.

The same distinction matters off the course. A property with a stream corridor, a wet meadow or a pond edge carries the same question in a different form: what is actually there, and what does the record say is there.

How is a height of cut checked on the course, and with what tool

The check is done with a prism gauge, also called a grass height gauge. It is a small flat plate with a graduated slot. The plate is laid on the surface, a tuft of grass is lifted into the slot, and the height is read against the scale. Ten to twenty readings per green are typical, taken across the surface rather than in one spot.

The second check is a physical one. A strip of turf is cut and the clippings are weighed or the stubble is measured with a ruler against the bedknife. A third check is the paper test: a strip of newsprint is fed into the reel. If the reel cuts it cleanly across the full width, the bedknife is sharp and the clearance is even. If it tears or folds, the bedknife is dull or the reel is out of adjustment.

Mechanics record the dial setting, the prism reading and the date. The two numbers drift apart as the bedknife wears. A bedknife that has lost its edge will not shear cleanly at the set gap, and the turf responds by standing taller and by showing a frayed leaf tip. The dial has not moved. The cut has.

How do bentgrass and bermudagrass greens differ in cutting height through the season

Creeping bentgrass is a cool-season grass. It is mowed low in spring and fall, when growth is active and the plant can recover. Summer heat pushes the plant into stress, and the height of cut is raised, often by 0.010 to 0.020 inch, to carry more leaf area through the day. Winter dormancy in the North means the mowers come off the greens or the height is raised further and the surface is protected.

Bermudagrass is a warm-season grass. It is mowed low in summer, when it is growing, and raised in fall as it goes into dormancy. In the transition zone and the Upper South, bermudagrass greens are often overseeded with ryegrass for winter color, and the height of cut is set for the ryegrass, not the bermudagrass, until the overseed is killed back in spring.

The two grasses also differ in how they respond to a low cut. Bentgrass spreads by stolons and tolerates a tight cut if fertility and water are managed. Bermudagrass spreads by stolons and rhizomes and tolerates an even tighter cut in full growth, but it will not tolerate a tight cut when it is cold. The same dial setting means two different things in July and in January.

Why a green is not a fairway

A fairway is mowed for the lie of the ball. A green is mowed for the roll of the ball. The fairway height of cut is set so the ball sits up enough to be struck cleanly. The green height of cut is set so the ball rolls true. That is the whole difference, and it drives every other decision.

A fairway mower is a wide, heavy unit. It covers ground fast and it is set between 0.400 and 0.600 inch on most courses. A green mower is a narrow, light unit, often a walk-behind or a lightweight triplex, set between 0.080 and 0.150 inch. The green mower makes more passes over less ground, and it is floated or cushioned to avoid marking the surface.

The rollers differ too. A green mower carries a smooth front roller that presses the surface and helps the ball roll. A fairway mower carries a grooved or spiral roller that handles thatch and grain. The roller is part of the height of cut, because it changes how the grass is held before the blade reaches it.

What changes when the grass changes

Species, season, fertility and water all move the number. A bentgrass green on a sand-based rootzone in New England is typically mowed at 0.100 to 0.125 inch in the growing season. A bermudagrass green in the transition zone is typically mowed at 0.100 to 0.150 inch in summer, and higher in dormancy. A Poa annua population in a bentgrass green will push the height up, because Poa does not tolerate the same tight cut and will scalp.

Thatch depth moves the number. A green with more than 0.250 inch of thatch holds moisture at the surface and scalps at a low cut. The fix is topdressing, verticutting and aeration, not a higher mower. The height of cut is a symptom as often as it is a cause.

Irrigation moves the number. A green that is dry and firm can be cut lower than a green that is soft and wet, because the plant is not pushing leaf growth to cool itself. A green that is overwatered will not tolerate a low cut, no matter what the dial says.

Reading the number on the course

The dial is the starting point. The prism gauge is the check. The clippings and the leaf tip are the verdict. A green that is cut too low shows a brown cast in the afternoon, a frayed tip, and a ball that rolls fast for a day and then slows as the plant thins. A green that is cut too high shows a slow ball and a thatch layer that builds.

The mechanic sets the gap. The superintendent sets the height. The two are not the same job, and on a course that is mowed well, the difference is visible in the first ten feet of the first green.