Gardeners have been urged to follow the 'one third' rule when mowing their lawns this summer.

The ‘one third’ rule is designed to boost grass growth and help your garden stay as healthy as possible. 

It’s a job that comes round every summer, and once No Mow May is out of the way, the question every gardener struggles with is: just how often should you cut the lawn?

Everyone out there has different arguments, with different advice, taking in everything from killing weeds to boosting insect and bird populations.

Some say the grass should be kept football pitch short, others say you should leave longer patches of grass round the sides.

But most garden experts swear by the ‘one third’ rule.

The rule means you should never cut more than one-third of the grass blade on any one lawn mowing.

As Lawnsmith.co.uk says: “If your lawn mower is set for a 1” cut, then you must mow at or before the grass gets to 1 ½”, whether that’s 4 days or 8 days between cuts. That means you only remove one-third of the grass blade. You will not do any harm if you cut more frequently so long as your lawn mower and blades are in good condition.

“If this rule means you have to do your grass cutting more often than you would like you can raise the lawn mowing height which will slow growth down. For example, you could set your lawn mower to cut at 1½”; you must then mow at or before the grass gets to about 2 ¼”. This may now mean mowing every 7 to 10 days depending on temperature and water availability.”

Premier Lawns has the same advice, in metric. 

It warns gardeners that mowing the grass from very long to very short in one go can damage the lawn, weakening grass growth.

It adds: “When you mow an established lawn, you should never remove more than one third of the grass blade.  So, if your lawn is 6cm long, one third of 6cm is 2cm, 6cm – 2cm = 4cm.  You should set your mower blades at 4cm before you start off.

“Don’t be tempted to let your lawn grow long and then suddenly mow it very short. That’s the way to weaken the grass plants and allow moss and weeds to infiltrate. It can also result in a pale, unevenly coloured patch of grass rather than a rich green, velvety carpet.”

It goes on to explain that grass uses its blades to take in sunlight, and convert this sunlight energy into chlorophyll, which gives lawns their green colour.

But suddenly slashing the length of the grass will significantly reduce the amount of chlorophyll the grass produces, putting the lawn on a ‘crash diet’ which could harm its health

It continues: “By sticking to the 1/3 rule for mowing your lawn, you are avoiding that extreme diet situation where the plants get weaker. Instead you are only reducing the plants’ calorie intake by a manageable amount.”