Acreage resources

Mowing wet grass: dew versus rain

“Don’t mow wet grass” is common advice, but it treats two different situations as one. Grass wet from overnight dew and grass wet from rain behave differently under a mower, and the research on each points in different directions.

Two kinds of wet

Dew forms overnight on the leaf surface and sits on top of dry or merely moist soil. It is usually gone by mid-morning. Rain wets the leaf as well, but it also soaks the soil, loads the grass with more water, and can take a day or more to clear on clay or in shaded gullies. Most of the problems people associate with mowing wet grass come from the second kind.

The case for cutting through dew

Most of the published research on mowing wet grass comes from golf course turf, where dew removal is a routine part of disease control. Several university studies have found that knocking dew off the grass early in the morning, whether by mowing, rolling or dragging a hose, reduces dollar spot, a common fungal disease. A University of Kentucky study found mowing off the dew reduced dollar spot by 78 percent, and work at Ohio State found mowing was the most effective of the removal methods tested.

The reason is leaf wetness. Many turf diseases need the leaf to stay wet for a number of hours before they can infect it. Removing the dew shortens that window. One study (Ellram, Horgan and Hulke, 2007) also found that dull blades did not increase disease, which runs against the usual assumption that a torn leaf on a wet morning invites infection.

Two caveats apply. The studies are on short, cool-season golf turf rather than the coarse warm-season grasses common on western Brisbane acreage, and the benefit is mainly about disease rather than the quality of the cut. Even so, there is no good evidence that a dew-wet morning harms the grass, and a fair amount suggesting the opposite.

The problems with cutting after rain

Clumped clippings

Wet clippings stick together and drop in clumps or windrows instead of scattering. Evenly spread clippings are good for the grass: long-term studies at Rutgers and elsewhere found returned clippings maintain or improve turf colour and recycle nutrients into the soil. Concentrated clippings are a different matter. A Virginia Tech study that deliberately loaded extra clippings onto turf found that at high rates they smothered it, causing thinning, yellowing and slower growth. A wet clump left on the surface is a small-scale version of the same thing.

An uneven cut

Long grass carrying rainwater bends under the weight and tends to lie down in mats. A rotary deck relies on the airflow from the blades to lift grass before cutting it, and heavy, wet grass resists that lift. As wet clippings pack under the deck, airflow drops further. The flattened grass is passed over rather than cut, then stands back up as it dries, leaving a ragged finish and tufts above the set height.

Both our decks (the Ventrac HQ682 Tough Cut and the MS720 finish mower) are rotary decks mounted out front, so the tractor’s tyres travel over grass that has already been cut and do not flatten it first. That helps, but it does not overcome the weight of the water or the loss of lift.

Compaction and ruts

This is the effect that lasts longest. Wet soil compacts far more readily than dry soil, and compaction reduces the air and water reaching the roots and restricts root growth. On saturated ground, wheels also cut ruts and scuff the turf, particularly when turning or working across a slope. Unlike a poor finish, which grows out in a week or two, compaction and rutting can affect the ground for seasons.

Safety on slopes

Wet grass, especially over clay, sharply reduces traction. On a slope that is comfortably within the machine’s rating when dry, the same ground after rain can produce a slide that is difficult to recover. On steep acreage this is the deciding factor, ahead of anything to do with the grass. See mowing safety on slopes for our wet-weather operating policy.

How we approach it

  1. We will usually cut through morning dew, adjusting speed and cut height as needed;
  2. After rain, we wait until the soil has firmed and the grass has dried enough to cut cleanly, and on steep or clay ground that can mean an extra day; and,
  3. If ground conditions on the day are unsafe for the slope, we will reschedule rather than work it.

Where we do cut grass that is still damp, we slow down, raise the cut height or take a second pass, and clean the deck afterwards.

What the research does and does not cover

There is solid research on dew removal and disease, on clipping loads, and on soil compaction under traffic. We are not aware of a controlled study that mows rain-wet grass and measures the outcome directly. The advice on rain-wet mowing is therefore built from those adjacent findings and operator experience, rather than from a single study.

Sources