Boundary wire vs. Grass sensors for robotic mowers

The vast majority of robotic mowers operate by means of a perimeter wire that defines a zone. An alternative are devices with a grass sensors that work without it. Both have their advantages and disadvantages.

Most robotic mowers work with a perimeter wire that serves as a boundary for a specific area to be mowed. After your purchase of a robotic mower, you will have to install a perimeter wire before the robot can make its rounds. Those who would like to do away with  this process have the option of purchasing a robotic mower without a perimeter wire. These models work with sensors that detect the lawn and are therefore ready for use immediately after purchase. Nonetheless, even these models are not equally suitable everywhere and have their advantages and disadvantages. Therefore, which is better? Robotic lawnmowers with or without perimeter wire? Here is a comparison of both systems.

How does a robot without a perimeter wire work?

A robot without a perimeter wire uses a grass sensor to orient itself around the lawn. It recognizes which area consists of grass and mows it. This means, for example, that the neighbor's lawn must be marked off by physical barriers, as the robot logically does not recognize any difference. Moreover, a robotic lawnmower does not always clearly recognize flower beds either. If you want to avoid the robotic lawnmower accidentally cutting small perennials or ground cover plants, you should also rely on a flower bed border.


Disadvantages of grass sensors

Despite the absence of a cable, it is advisable to designate flower beds or other no-go areas with the help of stones or other obstacles that cannot be driven over. Although robots work quite well with sensors, mistakes are certainly possible. It can be disappointing  if it ends with ground cover plants in the flower bed being mowed. So, in terms of garden design, there are definitely some restrictions for the robot to work properly without sensors. This means that all lawns must clearly be recognizable as such or that barriers mechanically assist the device to change direction. Unfortunately, frequent turning comes at the expense of speed, as turning maneuvers are time-consuming.

The risk of the robotic lawnmower overstepping the lawn boundary is particularly high when there are leaves on the ground. The device often perceives the leaves as lawn. Thus, if leaves cover flower beds, there is a real risk that the robotic lawnmower will also drive over them.

The lack of orientation for your  garden helper is also a problem when it comes to charging. The robot cannot find its way back to its station due to the lack of a perimeter wire or guide wire and has to be brought back manually to recharge each time. In large gardens, this can lead to having to search for the device first if the battery runs out while in operation.

Advantages of grass sensors

But there are, of course, advantages. The fact that there is no time spent laying wire proves to be particularly practical. The robot is ready for use immediately after purchase, without the need to define the area or set a timer beforehand. In gardens with many obstacles, this can especially be an advantage since the time-consuming demarcation of beds and the like is eliminated.

Rearranging the outlay of the garden also proves easy as there are no underground or overhead wires to watch out for. During annual dethatching, there is no wire to get in the way and even an ill-advised spade stroke cannot lead to a loss of function of the robot.

Since the robot isn’t able drive to the charging station on its own, it also doesn't matter where the nearest power outlet is. This means that the device can be used in an allotment garden, for example.



For robotic mowers with grass sensors, a clear demarcation of beds and other areas that should not be driven over is crucial. There is also the risk of slipping on slopes, so barriers are suitable (e.g. to help prevent the robotic lawnmower from landing in the pond). Pebbles are less suitable, as they can quickly damage the mower and the blades. The following materials are better suited:

  • Barriers made of wooden boards and stakes.
  • Pool noodles or aqua rings
  • Lawn edging stones, which protrude approximately 10 cm above the ground


In most robotic lawnmowers, the induction loop of the perimeter wire indicates the boundaries to the robot. With the help of a magnetic field, the robot thus detects when it crosses the boundary and automatically changes direction. The wire is laid from the charging station along the outer edge of the lawn and also includes any obstacles such as flower beds. The wire then comes back and connects to the charging station. There are a few things to keep in mind to help ensure that the signal is transmitted through the wire without interference.

Laying perimeter wire

In principle, the laying of a perimeter wire is done quickly and also works smoothly if you consider a few points.

  • For example, there are certain distances that should be kept to edges, walls and other barriers so that the blades won‘t get damaged. These are usually specified by the manufacturer. If you choose a model with an edge-cutting function, the trimmer will not have to do much work on the edges of the lawn. Furthermore, if the edges and borders are designed accordingly, there may not   be a  need to do any work at all.
  • With level obstacles such as paths, the robot can simply drive over them without having to mark them separately. Paved paths that are flush with the top edge of the lawn, therefore, do not need to be cut out separately and can serve as passages between two lawn areas.
  • The wire must not cross at any point but must make a complete loop around the lawn area. This is especially important if you are laying the wire from the outside edge across an area to border a flower bed, for example. When returning to the outer edge, the wire must run parallel without crossing over. The distance should not be more than 10 cm so that the robot does not perceive the wire as a boundary but merely drives over the corresponding area.
  • Laying is possible up to 20 cm, but it can also be done above ground. This is convenient if you want to lay the wire under patio slabs and the like. Although laying the wire underground is a bit more time-consuming and any troubleshooting is more difficult, the wire is better protected against damage.
  • When laying the wire above ground, it is important to lay it flat to help avoid  the robot from cutting  it. For this purpose, make sure that the wire is taut and fasten it with lawn nails at regular distances.
  • If the lawn is to be scarified in the future, it is recommended to lay the wire underground. This helps avoid damage to the wire from the blades of the robotic lawnmower.

Disadvantages of perimeter wire

Compared to devices with grass sensors, robotic mowers with perimeter wire have many advantages. But there are also (supposed) disadvantages. First of all, there could be a considerable amount of time spent planning and laying the wire. In case of a defect, it may be difficult to find the point of breakage, especially if the wire has been laid underground. Moreover, special tools may be needed to repair the area.

There are also disadvantages in terms of flexibility with a boundary wire. If you plan on redesigning your garden, it may be necessary to completely reinstall the wire. This will be laborious especially if the cable is underground. This additional work is relative, however, since bed edgings and other barriers must be redesigned even for devices with sensors.

Advantages of perimeter wire

After the robot finds its way back to its charging station thanks to the wire, it can recharge itself quite autonomously when the battery is empty. This saves you the hassle of searching for the device and returning it by hand. In addition, the robotic lawnmower will continue its work even if you go on vacation or are gone for periods of time. Thanks to innovative control options via app, programming the device is also possible remotely.

In terms of the layout of the wire, there are restrictions because there are no obstacles necessary to protect the flower beds.  A lawn that has separate zones and can be reached via a paved path is also suitable for the lawn robot with a boundary wire.


Basically, both modes of operation perform their work in the same way. The devices are designed to provide a regular and even cut of the green area. Likewise, both devices may be equipped with other sensors such as tilt and lift sensors, which are designed to stop the blades if you lift the robot or it gets into a difficult incline position. Shock sensors, in turn, are designed to redirect the machine if the garden helper encounters an obstacle. Alternatively, practical 360° sensors help provide  efficiency, as they detect obstacles at an early stage and take wide-ranging evasive actions. The area output, maximum gradient angle or even the volume differ depending on the manufacturer and model. Other aspects that will surely play a role in your purchase decision include price, control options, battery power, multi-zone function, cutting width and/or the equipment with a GPS module.

There are also specific differences between robotic mowers with a grass sensor and with a boundary wire.


Perimeter wire:

  • installation takes some effort
  • possible new installation necessary in case of new garden redesign
  • autonomously locates charging station
  • caution is required when scarifying or digging
  • suitable for separate zones that can be reached via paths
  • troubleshooting may be time-consuming
  • helps to avoid bed edging
  • wall socket necessary for the charging station

Grass sensor:

  • faster installation
  • boundaries around beds necessary
  • manual return robot to its charging station
  • minimal  effort in redesigning the garden
  • easy digging and scarifying, as there is no wire to be damaged
  • no wire breakage
  • no settings such as a timer function or defining passages necessary
  • may be  more suitable for large slopes
  • well suited for simple gardens without major obstacles or with numerous, well-separated obstacles
  • separate zones cannot be reached
  • no wall socket in the immediate vicinity necessary
  • foliage can be problematic


In the meantime, there are first models that orient themselves with the help of GPS. This function usually supports the induction loop with a perimeter wire and is mainly used to optimize mowing times. Moreover, some come with additional theft protection. 

A few models actually navigate using GPS. However, the accuracy depends highly depends on the device and the location. If this principle works, however, the robotic lawnmower is able to optimize the routing over time and thus gain efficiency in the medium term.


What sounds tempting is not always as simple as it seems in practice. Lawn robots without boundary wires operate in an efficient manner when the lawn area is very clearly different from all other garden areas. Here, too, some planning is necessary. In addition, robots with sensors can be   expensive. Thus, the purchase of a device with a cable may be more practical, especially since mowing can be done autonomously. Moreover, you should only have to lay the boundary wire once. Overall, robotic lawnmowers with a boundary wire are  versatile according to the current state of the art and are also simply  practical in everyday use.