Robot Mowers for Uneven Terrain
There is no published rating for bumpy ground. Here is what to look at instead, and what you can fix in the lawn itself.
Manufacturers publish a slope rating. None of them publish a roughness rating — and yet uneven ground causes a great deal of trouble: scalped patches, mowers beached on a hump, wheels spinning in a dip.
Because there is no direct specification, you have to reason from the ones that exist.
Why uneven ground is difficult
A robot mower is small and light, with a fixed cutting deck a set distance above its wheels. It has no operator to lift it over a bump.
On uneven ground, three things go wrong:
- Scalping. Over a hump, the deck contacts the crown of the bump and shaves it to bare soil.
- Missed grass. In a dip, the deck is too high and the grass is left long. The result is a patchy, uneven finish even though the mower is working correctly.
- Grounding. A hump between the wheels can lift them clear of the turf, at which point the mower spins helplessly until someone rescues it.
The proxies to buy on
Since roughness is not published, use these instead:
Slope rating. The best available proxy. A high slope rating implies a drive system with real traction and torque, which is what rough ground demands. A model rated for 35% or more is likely to cope better than one rated for 15%.
All-wheel drive. Four driven wheels are far less likely to lose traction when one wheel is in the air, which is exactly what happens on a bumpy lawn.
Wheel size. Larger wheels roll over an obstacle that stops smaller ones. Bigger, deeper-treaded wheels are a strong sign a model is built for rough ground.
Ground clearance. Rarely published as a figure, but visible in photographs. A machine that sits higher is harder to beach.
Cutting height range. A model that can cut high has more room to clear bumps.
What to fix in the lawn
Some of this is not a mower problem at all.
- Top-dress the dips. A thin layer of sandy loam in autumn, repeated over a couple of seasons, gradually levels minor undulations and is much cheaper than a better mower.
- Deal with molehills promptly. Fresh soil heaps stop mowers and blunt blades.
- Remove or bury exposed roots where you can, or exclude the area and mulch it.
- Fill rabbit scrapes and holes before they become traps.
- Raise the cutting height. Cutting a centimetre higher removes a surprising number of scalping problems immediately.
Realistic expectations
A robot mower will not produce a striped bowling green on a lumpy field, and no specification will change that. What it will do is keep rough grass consistently short with no effort from you, which for a paddock-like lawn is usually the actual goal.
If a fine, level finish matters to you, level the ground first. The mower is not the limiting factor.
What to check before you buy
- How bumpy is the ground really — minor undulations, or genuine holes?
- Does the model have all-wheel drive?
- What is its slope rating, as a proxy for drive capability?
- Do the wheels look substantial in the photographs?
- Could you raise the cutting height, or level the worst areas, instead?