The one-third rule: the mowing mistake that stops you getting a great lawn

Most people think the way to a better lawn is a bag of feed. It isn't. It's the height you cut it at.

I've walked on to hundreds of lawns over 30 years, and the same thing is going wrong on most of them. The grass is being cut too short, too much at once. The lawn looks tidy for a day and then spends the rest of the week trying to recover.

This guide is about the one rule that fixes that. It's called the one-third rule, it's been around since the 1950s, and once you understand why it works you'll never scalp a lawn again.

The video covers the science and a bit of history. Watch it, or read on, it's all below.

What's in this guide

• What is the one-third rule?
• The iceberg effect: why the leaf matters more than you think
• What happens when you cut too much off
• Where the rule came from: the Dust Bowl and Franklin Crider
• What height should you cut your lawn at?
• How to check what height you're actually cutting at
• Little and often: how to keep to the rule
• Mowing in dry weather
• Rotary, cylinder or robot?
• The quick guide

What is the one-third rule?

Never take more than a third of the grass leaf off in one cut.

That's it. If your grass is 60mm tall, the lowest you should cut it is 40mm. If you want it at 30mm, you cut it to 40mm first, leave it a few days, then cut again.

It sounds like a small thing. It's the difference between a lawn that gets stronger every time you mow it and one that gets weaker.

The iceberg effect: why the leaf matters more than you think

Think of the grass plant as an iceberg. The bit you see, the green leaf, is the small part. Underneath is a root system that can be twice the size of what's on top.

The leaf is the factory. It takes sunlight, water and carbon dioxide and turns them into carbohydrates through photosynthesis. That's the plant's food.

The roots are the miners. They're underground pulling up water and nutrients, and they can't make their own food. Every bit of energy they use has to be sent down from the factory above.

In plain English: the roots live off what the leaf makes. More leaf, more food, bigger roots. Less leaf, less food, and the roots shrink to match.

What happens when you cut too much off

When you take half the leaf off in one go, you've just shut down half the factory.

The plant goes into survival mode. It stops feeding the roots and throws everything it has into regrowing leaf, because without leaf it dies. The roots go hungry. Some of them die back.

Do that every week through the summer and you end up with a lawn that has no depth underneath it. Shallow roots can't reach water, so it browns off the minute it gets dry. Thin grass lets light hit the soil, and that's exactly what moss and weed seeds are waiting for.

The scalped lawn isn't tidy. It's a lawn that's been put on the back foot and is being kept there.

Where the rule came from: the Dust Bowl and Franklin Crider

This isn't something I made up. It goes back to the 1930s.

During the Dust Bowl in America, farmers had ploughed and overgrazed the prairie grasses until there was nothing holding the soil together. The topsoil blew away in storms you could see from space, if there'd been anything up there to see it.

Out of that came a lot of research into how grass actually survives. In 1955 a scientist called Franklin Crider published a study on what happens to grass roots when you cut the top off.

He found that if you removed up to a third of the leaf, root growth carried on as normal. Go past a third and root growth stopped. Take half off and the roots stopped growing for over a week. Take more than that and they stopped for weeks.

The plant was conserving energy. It had to choose between roots and leaf, and it chose leaf every time.

That study is where the one-third rule comes from. Seventy years on, every greenkeeper in the world still works to it.

What height should you cut your lawn at?

Higher than you think.

Rotary mower: 30 to 50mm. That's most lawns. If your lawn takes a hammering from kids, dogs or shade, go towards the top of that range.

Cylinder mower: 10 to 30mm. A cylinder cuts like scissors, so it can go lower without tearing. Only go to the bottom of that range if you're feeding, watering and cutting two or three times a week.

If you're cutting at 20mm with a rotary, which a lot of people are without knowing it, the grass has almost no leaf left to feed itself with. Go up a couple of notches and give it a fortnight. You'll see the colour come back.

How to check what height you're actually cutting at

The numbers on the mower dial are a guess. Setting 3 on one mower is 25mm. On another it's 40mm. And it changes as the blade wears.

So measure it. After you've cut, push a ruler or tape measure down through the grass until it hits the soil. Read off where the tips of the leaves sit. That's your real cutting height.

I do this with people on their lawns and they're nearly always shocked. They think they're at 35mm and they're at 18mm.

Do it once and you'll know what your dial actually means.

Little and often: how to keep to the rule

The only way to keep to the one-third rule is to cut often enough that you never have to take a lot off.

In summer that's once a week, twice if it's growing hard. In spring and autumn, weekly or every ten days. In winter, whenever it's growing and the ground is firm enough to walk on. There's no such thing as a last cut of the year, there's just a cut when the grass needs one.

Come back from a fortnight's holiday to a jungle? Don't drop it to your normal height in one go. Cut it high, leave it three or four days, then come down. Two cuts, not one.

The grass barely notices a light cut. It notices a heavy one for a week.

Mowing in dry weather

When it's dry, two things change.

First, go up a notch. The extra leaf shades the soil and slows the water loss. A lawn cut at 50mm holds on for days longer than one at 30mm.

Second, if your mower has a mulching plug, use it. In dry weather the clippings are small and they break down fast. Dropping them back on the lawn returns moisture and nutrients to the soil instead of carting them off to the bin.

And if the grass has stopped growing, stop cutting. You're just stressing it for no reason.

This is also when I feed. A summer feed at this point isn't about pushing growth, it's about keeping the roots fed while the plant is under stress. Seaweed on top of that helps the plant hold on to water and cope with the heat. The two together are what get a lawn through a dry August without browning off.

Rotary, cylinder or robot?

People ask me this every week on the channel. Short version:

Rotary. The one most people have. A spinning blade that chops the grass. Forgiving, handles long and uneven grass, cuts at 30 to 50mm. Keep the blade sharp or it tears the leaf and leaves brown tips.

Cylinder. A reel of blades that scissors the grass against a fixed plate. The cleanest cut you can get and the only way to go properly low. Needs a flat lawn and regular cutting, and it's no use once the grass gets away from you.

Robot. Cuts every day, takes a few millimetres off, drops the clippings. It is the one-third rule on autopilot. If the budget stretches, and your lawn suits one, it's hard to beat for lawn health.

Whatever you've got, the rule is the same. Sharp blade, right height, a third off at most.

The quick guide

Cutting height: rotary 30 to 50mm, cylinder 10 to 30mm.

How much to take off: never more than a third of the leaf in one cut.

How often: weekly in summer, twice if it's flying. Weekly to ten days in spring and autumn. Winter when it's growing and the ground is firm.

Check your height: ruler down through the grass to the soil, read where the leaf tips sit.

Dry spell: go up a notch, mulch the clippings, stop if it stops growing.

Why this matters: the leaf feeds the roots. Leave enough leaf and the roots go deep. Deep roots find water, hold colour and crowd out weeds. Everything else you do for your lawn works better on top of that.

Want the full beginner's guide to mowing, with the mower comparison and the winter and wet-grass stuff? That's its own video: This one thing stops people getting the best lawn.

Where to go next

Want to see what a different cutting height does side by side? The Test Patch series is exactly that, one lawn split into patches with different treatments so you can see the difference for yourself.

If you want to go deeper on how the pros keep grass alive at 4mm, the Golf Course Masterclass is the greenkeeping side of all this.

And if you're not sure how much feed your lawn needs, measure it from your phone. Draw round it on the map and it works out the size and what to put down for the season.

Came here from the Start Here page? Head back to it, or have a look round the Learning Hub. It's all free.