Mowing: setting the height and holding the one-third rule

Cutting height is the single parameter that decides which species can live in your lawn. Set it wrong and no fertiliser programme or irrigation schedule will compensate. The method is a fixed target height per species plus the one-third rule, and the whole job is holding both when growth accelerates.

The one-third rule, stated precisely

Never remove more than a third of the standing leaf blade in a single cut (per Illinois Extension). On a lawn kept at 30 mm that means cutting before the canopy reaches 45 mm, so 15 mm of growth is the trigger. On a 60 mm fescue lawn it means cutting before 90 mm, so 30 mm of growth is the trigger.

The rule exists because the leaf blade is the plant's entire sugar factory. Remove half of it and the plant spends two to three weeks rebuilding leaf area from root reserves instead of deepening roots and storing carbohydrate. Remove a third or less and the plant barely notices.

In the spring flush, which is when this rule is actually tested, a cool-season lawn can add 18 to 20 mm a day. A 30 mm lawn therefore goes from compliant to scalped in three days. The answer is not to raise the cut and forget it; it is to cut every third day for the four weeks the flush lasts.

Setting the height for the species you have

The working band is species-specific and narrow. Reel-cut bentgrass lives at 10 to 20 mm. Hybrid bermudagrass and zoysia sit at 15 to 30 mm. Kentucky bluegrass and perennial ryegrass are comfortable between 25 and 50 mm. Tall fescue and St. Augustinegrass need 50 mm and up. Fine fescues tolerate 40 to 70 mm.

Below the band, the stand thins to bare soil within one or two seasons. Above the band, the canopy shades its own lower leaves, which die and add to the thatch layer, and the lawn becomes spongy and disease-prone.

If you must lower a cutting height, do it in steps of 5 mm a week during active growth, never more than 5 mm at a time, and only in the season when the grass is growing hard.

Clippings, thatch and the mulching question

Clippings are 75 to 85 percent water and break down in two to three weeks. They do not cause thatch; thatch is the layer of stems, stolons and roots that accumulates at the soil surface, and those tissues are high in lignin and decompose slowly.

Mulching the clippings returns 1 to 2 g of nitrogen per square metre per year, which on a 6 g fescue programme is a fifth of the annual requirement. The exception is a lawn with an existing thatch layer over 15 mm: there the clippings sit on top of an already slow-decomposing mat, and bagging for one season while the mat is reduced is the right call.

Never leave clumps. Clumps smother the turf underneath and leave yellow patches that take three weeks to grow out. If a cut produces clumps, mow over them again after they dry, or cut more often.

Edging and the finishing parameters

Edging along paths, kerbs and bed edges is done with a half-moon edger or a mechanical edger, cutting a clean vertical face 25 to 40 mm deep. That depth is enough to hold a crisp line for a season and shallow enough not to sever the roots of adjacent shrubs.

The edging line is also where stolons escape into beds. On bermudagrass and zoysia this is a monthly job in the growing season, not an annual one; a 100 mm deep edging spade line, re-cut every four to six weeks, is what stops a warm-season lawn from walking into a border.

Change the mowing direction each cut. Repeated mowing in one direction lays the leaf over and produces a visible grain; alternating by 45 to 90 degrees keeps the canopy upright.

Questions that come up on this page

How often should a lawn be cut?

As often as the one-third rule requires, which in the spring flush is every third or fourth day and at steady growth is every 7 to 14 days. A weekly schedule is only correct at moderate growth rates.

Does cutting the lawn short mean less mowing?

No. Lower turf grows faster per unit area and accumulates more thatch, and the same one-third rule then forces more frequent cuts. Cutting low increases mowing hours, not reduces them.

Do I need to remove the clippings?

Only if there is already a thatch layer over 15 mm, or if the clippings are forming clumps. Otherwise leave them: they return 1 to 2 g of nitrogen per square metre per year and up to 25 percent of the lawn's water requirement.

Sources

The public reference material the figures on this page were checked against:

• Illinois Extension
• New Mexico State University
• Purdue Extension
• University of Delaware Cooperative Extension