Robot Mowers for Gardens With Trees

A garden with mature trees is one of the situations where the most advanced-sounding mower can be the wrong choice. It is worth understanding precisely why.

What a tree does to a satellite signal

Satellite positioning depends on receiving faint signals from satellites overhead. Leaves and wet wood absorb and scatter those signals. Under a dense canopy, a receiver may see too few satellites to fix its position, or — worse — receive signals that have bounced off branches and arrive late, producing a confident but wrong position.

An RTK mower under a large tree therefore typically slows, pauses and waits for reception to return. In a garden with one specimen tree, that is a brief interruption. In an orchard or a mature woodland-edge garden, the mower may spend more time waiting than cutting.

Conifers are worse than deciduous trees, and any tree is worse when wet. A garden that works fine in February can become difficult in July when the canopy fills in.

Approaches that cope with tree cover

Boundary wire. Completely unaffected. The mower follows a physical cable and never looks up. For a heavily wooded garden this remains the most reliable technology available.

LiDAR. Also unaffected, because it maps its surroundings rather than locating itself by satellite. Under a canopy it simply carries on.

Camera vision. Works under trees, though dappled light and deep shade make the task harder than in open sun.

Combined systems. A mower that uses satellite positioning in the open and switches to on-board sensing under trees gets the best of both. For a mixed garden — some open lawn, some shade — this is usually the most robust answer.

The other tree problems

Signal is not the only issue.

Surface roots create bumps that can lift and scalp the mower, or catch its underside. Raising the cutting height over rooted areas helps considerably.

Trunks need protecting. A mower that repeatedly bumps a young tree will damage the bark. Options are a physical ring or guard around the base, a no-go zone drawn in the app, or a loop of boundary wire around the trunk.

Fallen fruit and nuts jam blades and drive wheels. Under an apple or walnut tree, expect to clear windfall before mowing in autumn.

Leaf fall is heavy work for a small machine. Many owners simply pause mowing during the worst weeks and rake instead.

Deep shade produces thin, sparse grass that does not need cutting as often. Excluding permanently shaded ground from the mowing area is often better for the lawn as well as the mower.

Practical setup for a wooded garden

• Raise the cutting height under trees to clear roots.
• Protect young trunks before the first mow, not after the first scrape.
• Clear windfall in autumn.
• Consider excluding the most heavily shaded areas entirely.
• If you are choosing between two models and one has on-board sensing, that is the one to take.

What to check before you buy

• How much of the lawn is under canopy in summer, not now?
• Does the model have any navigation method that does not depend on the sky?
• What does it do when positioning is lost — stop, or continue on another system?
• If the model needs a local RTK antenna rather than a network service, where would it go so that it has clear sky?
• Are there surface roots that need a higher cutting height?