Robot lawn mowers

For much of the year the lawn needs cutting every week.

A robot mower takes that task over. It trims the grass a little at a time and goes back to its dock to charge.

These notes help garden owners work out whether one suits their lawn, and how to run it with wildlife in mind.

How a robot mower works

Once it is set up, a robot mower runs at times you choose. It cuts the grass and then returns to its charging dock. The fine clippings drop back into the lawn, so there is nothing to collect.

The dock needs mains power. It has to sit within reach of a socket.

Boundary wire or wire-free

Many models follow a boundary wire pegged around the edge of the lawn. The wire keeps the mower out of the borders and guides it back to its dock. Some dealers will bury the wire so it is out of sight. Changing the shape of the mowing area later means moving the wire.

Wire-free models map the lawn and work to virtual boundaries instead. Which?, the consumer organisation, notes that they usually cost more. Some use satellite positioning with a fixed reference station, known as RTK. These can lose the signal under heavy trees or beside tall walls. Others find their way with cameras, which need clear lawn edges, or with spinning lasers, known as LiDAR.

Will it suit your lawn?

Every robot mower has a largest lawn it can manage, given in square metres. Which? has tested models for lawns from 250 to 1,800 square metres. It advises against assuming that a smaller model can cover a bigger lawn in two trips.

Slopes need a check too. Each model has a maximum incline, and the models in Which?'s list range from 25% to 80%.

A lawn split by paths or fences may need a mower that handles several zones. Otherwise someone has to carry the mower from one area to the next. Pivoting blades give way when they hit something hard, such as a toy left on the grass.

Hedgehogs and other wildlife

Hedgehogs come out at night, and many of them now live close to people. In 2023 a team led by Dr Sophie Lund Rasmussen tested 19 robot mower models against dead hedgehogs.

Some models could injure a hedgehog, and others did not harm them. In all but one case, the mower had to touch the hedgehog before it noticed it. Smaller hedgehogs were more likely to be hurt.

The researchers point out that the risk grows when owners run their mowers after sunset. Which? suggests mowing in the daytime. It also suggests setting aside safe zones for wildlife with the guide wire, and trimming by hand at the base of hedges and small trees.

Before the first cut

Long, overgrown grass is a likely place for a hedgehog's nest. The British Hedgehog Preservation Society advises checking these patches for hedgehogs and other wildlife before cutting. It suggests cutting to about a foot first, then checking again before going any lower.

The Royal Horticultural Society suggests never taking off more than a third of the grass height at once. For an ordinary garden lawn it suggests 13 to 25mm in summer and up to 40mm in spring and autumn.

The battery

A robot mower runs on a lithium-ion battery, and these wear out over time. Some models let you change the battery yourself, and some do not. Which? gives its Best Buy only to mowers with a battery you can replace.

When an old battery reaches the end of its life, your local recycling centre can usually take it.

Useful links