Robot Lawn Mower Edges: Why Some Trimming Remains

Illustration of an unbranded robot mower on a garden lawn

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Robot lawn mowers are designed to automate the bulk of grass maintenance, but many users find that a strip of uncut grass remains along the perimeter. This occurs because the main cutting disc is typically located centrally under the mower chassis, meaning the mower’s physical body prevents the blades from reaching the absolute edge of the lawn. While some advanced models include dedicated edge-cutting hardware, factors such as boundary wire placement, mowing patterns, and software settings often determine whether manual trimming is still required.

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The physical limitation of central cutting discs

Most robotic mowers use a central cutting disc. For the blades to reach the grass, the mower must be positioned over it. Because the mower has a physical width, it cannot drive its centre point directly over the boundary edge without the chassis crossing over the edge (e.g., onto a flower bed or pavement). Consequently, a gap is left between the furthest reach of the blades and the boundary.

To mitigate this, some manufacturers have introduced dedicated edge-cutting systems. For example, certain Husqvarna Automower NERA models feature an “EdgeCut” disc located at the rear of the mower. This secondary disc is positioned at a fixed height of 45 mm to target the perimeter grass specifically. When the mower detects a boundary, it performs a sweeping turn to allow this rear disc to trim the edges.

How boundary wire placement affects edges

The precision of the boundary installation is a primary reason why some trimming remains. For boundary-wire and mapped virtual-boundary systems, the installed boundary controls where the mower turns. Other navigation systems may use different methods.

Wire placed too far from the edge: If the boundary wire is installed significantly inside the lawn’s edge, the mower will turn around before the blades can reach the perimeter, leaving a wider strip of uncut grass.
Wire placed too close to obstacles: If the wire is too close to a physical wall or fence, the mower may bounce off the obstacle before completing its turn, which can lead to inconsistent cutting.
The “drive past wire” function: Some systems allow the mower to slightly exceed the boundary wire limit to reduce the remaining gap, though this depends on the specific model’s capabilities.

The impact of mowing patterns

The path the mower takes influences how effectively it handles the edges. Mowers generally operate in either irregular (random) or systematic patterns.

Irregular mowing

In irregular mode, the mower moves in random directions. Coverage builds over repeated passes; the finish and operating time depend on the mower and layout.

Systematic mowing

Some mowers support systematic parallel tracks. For compatible Husqvarna EdgeCut models, the manufacturer gives these pattern considerations:

Parallel patterns: These may leave edges uncut if the mower only approaches the boundary from one direction.
Checkerboard or triangle patterns: These are more effective for edges because the mower alternates directions between sessions, attacking the perimeter from multiple angles.

Mapping technology and “stay-out” zones

Modern mowers use various mapping technologies that can inadvertently leave edges uncut if not configured correctly. For instance, when using AIM (Automower Intelligent Mapping) technology with a physical boundary wire, the accuracy is approximately 1 m. If a work area’s waypoints are set too close to the boundary wire, the mower might turn based on the work area border rather than the boundary wire itself.

For this specific AIM setup with a physical boundary wire, Husqvarna recommends placing the mapped work-area border at least 1.5m outside the lawn. Keep the physical boundary wire and required barriers in their correct positions; this is not an instruction to move a virtual perimeter outside a safe mowing area. This ensures the mower reaches the boundary wire before turning, minimising the uncut strip.

Managing edges on slopes

Sloping terrain introduces stability and traction issues that can affect edge quality. If a mower slips while attempting a sweeping turn at the edge of a slope, it will leave a patch of uncut grass.

Husqvarna gives the following options for compatible EdgeCut installations, subject to the model’s slope limits:

Direction: Mow straight up the slope whenever possible.
Angle: Setting a systematic mowing pattern at a 45° angle to the slope can improve incline handling and edge cutting.
Alternative: If systematic patterns cause the mower to slip while crossing the slope side-to-side, switching to a one-direction pattern or an irregular pattern for that specific area may be more effective.

Maintenance of edge-cutting hardware

For mowers equipped with dedicated edge-cutting discs, hardware maintenance is essential. Because these discs operate close to the ground and boundaries, they are prone to clogging.

Seasonal buildup: Wet, fast-growing grass in spring and wet leaves in autumn can block the disc guard.
Cleaning: The disc guard should be inspected regularly. Follow the model manual: switch off and isolate the mower as instructed, wear protective gloves and use the permitted cleaning method. Use a hose only where that model allows it, and never a pressure washer.
Blade consistency: When replacing blades on an edge-cutting disc, all blades on that specific disc must be of the same type to ensure a balanced cut.

Do not reduce mandatory boundary setbacks to reach more grass. A level, supported mowing strip can help only where the installation manual permits the mower to overrun it safely. Otherwise finish the remaining edge with shears or a suitable trimmer, checking for wildlife and keeping clear of boundary cables.

Useful products for this job

The links below open Amazon UK product searches. Check the exact item, seller, current price and compatibility before buying.

String trimmer: Necessary for trimming the final strip of grass that robot mowers cannot physically reach. Check for battery compatibility if buying from the same brand as other garden tools. Search Amazon for String trimmer.
Replacement robot mower blades: Maintaining sharp blades is essential for clean cuts, especially on the perimeter where grass may be denser. Ensure blades are the exact type recommended by the manufacturer; do not mix blade types on a single disc. Search Amazon for Replacement robot mower blades.

Related garden guides

Sources and further guidance

Guidance researched in September 2026. The feature image is an AI-generated illustration. Product guidance is based on published information, not hands-on testing.