The horizontal distance between the outermost edge of the robot mower body and the outermost cutting arc of the blade disc. This varies from 2 cm (Mammotion Luba 2) to 15+ cm (some Husqvarna models). The blade offset determines the minimum width of uncut grass strip along obstacles.
Understanding the Problem
Every robot mower leaves an uncut strip of grass along hard obstacles — fences, walls, retaining walls, raised garden beds, and driveways. This strip exists because:
- The mower body is wider than the blade disc. The outer chassis will hit the obstacle before the blades can reach.
- Collision sensors (bumper or ultrasonic) trigger a retreat before the body touches the obstacle — adding another 2–5 cm of clearance.
- Virtual or wire boundaries are set with a safety margin from obstacles.
The total uncut strip width = blade offset + collision avoidance distance + boundary margin. For many mowers, this totals 10–20 cm (4–8 inches) of grass that a robot will never touch.
Strategy 1: Install Mowing Strips (Best Long-Term Solution)
A mowing strip is a hard, flat surface installed flush with the lawn along obstacles. The mower's wheels ride onto the strip, allowing the blade disc to extend past the original grass edge and cut right up to the obstacle.
| Material | Width Needed | Cost per Linear Foot | Pros / Cons |
|---|---|---|---|
| Concrete pavers | 10–15 cm | $3–$5 | Durable, clean look. Requires leveling. |
| Natural flagstone | 15–20 cm | $5–$8 | Aesthetically superior. More expensive, irregular shapes. |
| Poured concrete | 10–15 cm | $4–$6 | Seamless. Requires forms and mixing. Best for straight lines. |
| Rubber edging | 8–10 cm | $2–$3 | Flexible, easy to install. May shift over time. Less aesthetic. |
| Steel edging | 5–8 cm | $3–$5 | Thin profile, professional look. Sharp edges — install carefully. |
Installation Tip
The strip surface must be flush with the lawn surface — not raised above it. A raised strip creates a ledge that the mower cannot climb. Dig a trench, add a 2–3 cm layer of compacted sand for leveling, set the pavers, and backfill with soil. The grass will grow to the strip edge naturally over 2–3 weeks.
Strategy 2: Optimize Boundary Placement
For boundary-wire mowers, moving the wire closer to the fence reduces the uncut strip. The trade-off: too close, and the mower collides with the fence.
For RTK/GPS mowers like the Mammotion Luba 2, virtual boundaries can be adjusted in the app with centimeter precision. The optimal approach:
- Set the boundary line along the fence at the default distance.
- Run a test mow and measure the uncut strip width.
- Move the virtual boundary inward by the measured strip width minus 3 cm (safety margin).
- Test again. If the mower bumps the fence, move the boundary back 2 cm.
- Repeat until the uncut strip is 2–4 cm — acceptable for most homeowners.
Strategy 3: Landscape Redesign
If mowing strips and boundary tuning don't fully solve the problem, consider replacing the problematic grass strips with non-grass ground cover:
- Gravel or mulch borders — 15–30 cm wide along fences. Zero maintenance.
- Low-growing ground cover — Creeping thyme, clover, or sedum. Grows 2–5 cm tall and never needs trimming.
- Garden bed expansion — Extend existing garden beds to meet the fence line, eliminating the narrow grass strip entirely.
Cost Comparison: 50 Linear Meters of Fence
| Solution | One-Time Cost | Annual Maintenance | Trimming Eliminated |
|---|---|---|---|
| Do nothing (string trim weekly) | $0 | ~10 hrs/year at $0 (DIY) or $300–$500 (hired) | 0% |
| Boundary optimization only | $0 | $0 | 50–70% |
| Concrete paver mowing strip | $500–$825 | $0 | 95–100% |
| Gravel/mulch border | $200–$400 | $50–$100 (replenish mulch) | 100% |
| Ground cover replacement | $150–$300 (plants) | $0 after establishment | 100% |
Frequently Asked Questions
All robot mowers have a "blade offset" — the distance between the outer edge of the mower body and the outermost cutting arc of the blades. This ranges from 2–15 cm depending on the model. Since the mower body cannot touch the fence without triggering a collision sensor, a strip of grass equal to the blade offset remains uncut along every hard edge.
The Mammotion Luba 2 has one of the smallest edge gaps at approximately 2–3 cm from the front and sides, thanks to its offset blade disc design. Husqvarna models with the "close-to-boundary" setting (like the 450X NERA) can cut to within 4–5 cm of the wire. Worx Landroid models offer a "cut-to-edge" feature that gets within 3–4 cm. No current model achieves 0 cm.
Yes — a mowing strip (also called a mowing edge) is the most effective solution. Install a 10–15 cm wide strip of pavers, flagstone, or concrete flush with the lawn surface along the fence line. The mower wheel rides onto the strip, allowing the blades to reach the grass right up to the fence. This eliminates trimming entirely along that edge.
Yes, significantly. For boundary-wire mowers, placing the wire closer to the fence reduces the gap (but risks collisions). Most manufacturers recommend 25–35 cm from obstacles. RTK/GPS mowers like the Luba 2 allow you to digitally adjust the virtual boundary placement down to centimeter precision — giving you more control over the edge gap than wire systems.
For most homeowners, no. A dedicated robotic edger (like the Worx Landroid edging accessory) costs $200–$400 and requires separate maintenance. A one-time investment in mowing strips ($3–$8 per linear foot, DIY-installable) provides a permanent, zero-maintenance solution. However, if you have extensive garden bed borders that curve irregularly, an edger may be more practical than strips.