Mowing your lawn properly: cutting height, rhythm and the one-third rule

Two days after mowing, the lawn looks like a threadbare carpet. Pale, strawy, bare soil showing through on every rise. The first instinct is to reach for the hose. Yet the cause almost always sits somewhere else entirely: the height adjuster on your mower.

Lawn grasses are built to be grazed. Their growing point does not sit at the tip, the way it does on a tomato, but down at the base of the blade, right above the soil. That is why grass tolerates something that would kill any other plant: having a third of its leaf mass taken off twenty times a year. But that tolerance has a lower limit. Cut below it and you leave the grass too little green leaf to pay for the regrowth.

One thing at a time: how high, how often, with what and what happens to the clippings.

Why cutting too short is the most common lawn mistake

The dream of the English carpet runs deep, and the height adjuster drops one notch so very easily. Straight after mowing it looks wonderful. The damage turns up ten days later.

Take off too much leaf in one go and the plant has to push out new growth from its reserves instead of from the leaf area it already has. That costs roots. And shallow rooted grasses struggle to reach the remaining moisture deeper in the soil once things dry out. The pretty short lawn of May becomes the brown short lawn of July.

Then there is the light. A dense sward shades its own soil. Once the height drops below about 3 cm, light reaches the soil surface, and seeds are waiting there: dandelion, white clover, daisy (Bellis perennis). If you would rather not have them, mowing too short is how you invite them in.

The one-third rule, worked through once

Of whatever height is standing on the lawn right now, at most a third may come off. The remaining two thirds are your target cutting height.

1. Set a target height and then measure it

   For a domestic lawn, 4 cm is the sensible number. Set the height adjuster to the matching notch, mow one strip and go back with a ruler. Almost every mower cuts differently from what its scale promises.

2. Work out your trigger height

   Target height times 1.5. With a 4 cm target that means you mow once the grass reaches 6 cm. Those 6 cm are your signal, not the day of the week.

3. Bring overgrown grass back in stages

   If the grass stands 12 cm high after your holiday, dropping straight to 4 cm would be a scalping. Cut to 8 cm first. Wait about a week, until a good 9 cm is standing again, then go to 6 cm. Only at the appointment after that do you reach 4 cm. Three passes, and not one of them breaks the one-third limit.

Which height for which patch

Most lawns are a mix of perennial ryegrass (Lolium perenne), smooth meadow grass (Poa pratensis) and red fescue (Festuca rubra). How much that mixture tolerates depends on where it grows.

The rhythm across the season

In central Europe the mowing season runs roughly from CW 15 to CW 44, a fortnight earlier in the mildest river valleys and later in upland areas. The signal is not the date but the soil temperature: from about 8 to 10 °C the grasses get going.

The first cut usually falls in CW 16 to 17 and should not be a deep one. Between mid-May and the end of June the lawn is back at 6 cm so quickly that a fixed weekly slot makes sense. Right now, at the end of July in CW 31, growth is at its slowest. In dry weather the lawn barely puts on height, and then the rule is simple: do not mow just because it happens to be Saturday. From September the autumn flush arrives, often with two or three appointments in October. The last cut falls in CW 41 to 43 at around 4 to 5 cm, and in mild areas as late as CW 44. Too short and the lawn goes into winter weakened; too long and it flattens under the snow.

Wet grass, midday heat and the time of day

Wet grass lies down in front of the blade instead of standing up to it. The blade catches it at an angle, and what stays behind is a bruised, unevenly cut sward. At the same time the damp clippings clog the discharge chute and cake onto the underside of the deck. When you mulch mow in the wet you get clumps instead of fine chippings, and those clumps smother the turf underneath.

So wait until the morning dew has dried off. Late morning or early evening works best. In full midday sun the fresh cut ends dry back further, and the lawn takes longer to close up again.

Looking after the blade without a workshop

Sharpening once a season is enough in a domestic garden. If your lawn carries a lot of sand, molehills or stones, make it twice.

1. Power or spark plug off, then tilt

   Take the battery out, pull the mains plug, or on a petrol machine disconnect the spark plug cap. Put on cut-resistant gloves, because the cutting edge is still sharp-edged even when it is blunt. On a petrol mower, run the tank as empty as you can first, then tilt it so that the spark plug and carburettor end up uppermost. Otherwise oil runs through the crankcase breather into the air filter and carburettor.

2. Scrape off the caked grass and mark the blade

   Loosen the layer under the deck with a wooden scraper. Before you unbolt anything, stick a piece of tape on the underside of the blade: mounted the wrong way round it does not cut at all, it just fans the air.

3. Draw the edge back and balance it

   Work along the existing bevel, usually somewhere around 30 degrees, and take the same amount off both sides. Then rest the blade on a round screwdriver through its centre hole: if one side tips down, take a little more off there. There is more in sharpening garden tools.

Mulch mowing or grass box

A mulching mower has a closed deck and a blade with extra cutting edges that chops the clippings over and over, until they trickle down between the blades of grass as fine particles. Down there they rot away within a few days and, across the season, cover roughly 20 to 25 per cent of the annual nitrogen requirement. How to supply the rest is covered in feeding your lawn through the year.

The conditions are dry grass, no more than 2 to 3 cm taken off, and a steady rhythm. Mulch every fortnight and take 6 cm off each time and you produce a mat that suffocates the turf.

And thatch is not the same thing as mulch: thatch builds up from dead runners and root remnants, whereas properly mulched leaf material is gone within days. If a spongy layer forms anyway, the cause is usually too much nitrogen, along with soil compaction, waterlogging or an acidic pH. See scarifying against moss.

Putting the clippings to use instead of carting them off

Fresh clippings are wet and rich in nitrogen. Tip them onto the compost heap in a thick layer and they pack down, the air is squeezed out, and instead of rotting they go putrid. Mix them roughly one to one with shreddings, leaves or cardboard and they are the best accelerator a heap can get. See speeding up your compost. All of this applies only to clippings from untreated lawns, though.

As a mulch in the vegetable bed they are excellent, provided you let them dry for a day first and then spread them 2 to 3 cm deep between the rows. Tomatoes, courgettes and squash respond immediately, because the soil stays moist and no more soil splashes up onto the lower leaves. On strawberries the layer keeps the fruit clean. The one place it never belongs is right up against the stem. And because slugs love to gather under damp grass mulch, keep the layer thin, let it dry properly, and switch to straw when slug pressure is high.

If you have naturalised crocuses or daffodils in the lawn, those patches run on their own mowing calendar: they stay uncut until about six weeks after flowering, so the foliage can die back and the bulbs can store reserves for next year. And brown grass in high summer is not an emergency, it is summer dormancy. See lawns in summer heat and drought.

With lawn care, three years on you no longer remember what you did when, and every spring you start again from scratch. Note in your garden journal which week the first cut fell in, when you fed and overseeded, and when the blade was last sharp. After two seasons you will see your own pattern, and that is more accurate than any general table.

Common questions about mowing the lawn

How high should I cut my lawn?

For an ordinary domestic lawn, 4 cm is the safe number, and 5 to 6 cm in shade and during heatwaves. An ornamental lawn will go down to 2 to 3 cm, but then it needs fine-leaved cultivars, good soil and two appointments a week. An ordinary hard-wearing lawn, by contrast, becomes vulnerable to drought below 3 cm and lets light reach the soil, where weed seed germinates.

How often do you need to mow a lawn?

Go by height, not by the calendar: once the lawn stands half again as high as your target height, so target height times 1.5, which with a 4 cm target means 6 cm, it is due. In May and June that works out at every 5 to 7 days, in a dry high summer often only every 10 to 14 days, and weekly again in September. Across the season that adds up to around 20 to 25 appointments.

Can you mow a wet lawn?

Better not. Wet blades lie flat in front of the mower, the cut turns out uneven, and thatch cakes onto the underside of the deck. Then there is safety: wet grass is easy to slip on, especially on a slope, and mains-powered machines should only ever run through a residual current device anyway. Wait until the dew has dried off. If continuous rain lets the grass get too long, mow one notch higher rather than not at all, and use the grass box.

Which is better, mulch mowing or the grass box?

Both, at different times. Mulch mowing pays off from May to September on a weekly rhythm with dry grass, because it returns roughly a quarter of the annual nitrogen requirement. The grass box belongs on the first and last cut, after weeks of rain when the grass is overlong, and whenever there is fungal disease about.

How often do I need to sharpen the mower blade?

Once a season is enough in a domestic garden, ideally before the first cut in April, and twice on sandy soils. It is overdue when the lawn shows a pale grey haze the day after mowing: that means the tips are frayed, and those open wounds are the entry point for fungi.

When is the last cut of the year?

Usually between CW 41 and CW 43, and depending on the weather in mild areas as late as CW 44. Leave the lawn at around 4 to 5 cm and collect the clippings, because in autumn they no longer rot down on the surface.