Robot mower edge-trimming companions: cordless string trimmers and edgers for the strip the mower leaves uncut

Why a robot mower leaves an uncut strip

A robot mower spins a blade disc that sits underneath the chassis, set back from the outline of the body. The wheels and the plastic shell reach the fence, wall or bed border first, so the blade tips never get to the last stretch of grass. Anything that makes the mower stop short of an obstacle widens that strip. Collision bumpers, lift sensors and the clearance the navigation system keeps around a hard edge all add to it.

The cause depends on how the mower finds its boundary. A boundary wire mower follows a loop you laid out, and the manual specifies how far inside a fence or bed the wire should sit. A wire-free RTK mower works from satellite-corrected positioning and keeps a safety margin because its position estimate has an error tolerance. A vision-based mower detects obstacles with cameras and may slow or turn before it touches them. All three leave some uncut grass along the perimeter. The size of the gap varies by model, so read the cutting-width and edge-distance details in the spec sheet rather than assuming.

Where the gap shows up and how much it matters

The gap is worst where the mower has to keep a safe distance: fence lines, house walls, tree trunks, raised flower bed borders, posts, and the ring around the charging station and docking path. Inside the lawn it is smaller, and open edges next to a flat path or curb can come out nearly clean if a wheel can ride on the hardscape.

Because the mower cuts often and keeps the lawn at one cutting height, the uncut strip stands out. The main lawn stays short and even while the border grows tall and seeds, and the contrast looks worse than the same strip would on a lawn you mow weekly. Most owners find the strip is narrow but runs the whole perimeter. For a yard with many mowing zones, islands and obstacles, the perimeter length adds up quickly, which is what decides whether a light trimmer will do or whether you also want a stick edger or a physical fix.

Edge-cutting mower, string trimmer, stick edger or a hardscape fix

Four routes close the gap, and they solve different parts of it. Use the table to match the route to your yard.

For most robot mower owners a cordless string trimmer is the first purchase, since it handles every obstacle type. Add a stick edger only if you have long runs along concrete or bed borders. Treat hardscape fixes as a one-time project that cuts the trimming you do later.

What decides a good cordless string trimmer for this job

Trimming a robot mower’s leftover strip is light work. You are cutting short grass in a narrow band, not clearing overgrowth, so weight and handling matter more than raw power. Check these specifications:

• Head type. A bump-feed head advances line when you tap it on the ground. An auto-feed head advances line on its own as the spool slows. A fixed-line head uses pre-cut lengths and avoids feed jams. Auto-feed saves time on a long perimeter, but any head is fine for short sessions.
• Cutting swath. The swath is the diameter of the circle the line cuts. A narrower swath gives you control around posts and bed borders. A wider one speeds up long fence runs.
• Line diameter. Thicker line handles tougher growth and weeds but demands more from the motor and drains the battery sooner. Thin line suits regular maintenance trimming.
• Motor. Brushless motors tend to run longer per charge and need less service than brushed motors. For occasional edge work a brushed motor can still be adequate.
• Shaft and balance. A straight shaft reaches under fences and shrubs. A curved shaft is lighter and easier to handle. Look for an adjustable or telescoping shaft if you are tall or short.
• Edger conversion. Some trimmers rotate the head 90 degrees and add a wheel guide so you can run it along a path edge. This is useful, but it cuts a rougher line than a dedicated blade.

Edge-only jobs are short, so a smaller battery capacity is usually enough. Size up the amp-hours only if your perimeter is long or you also use the same trimmer for general yard work. Brands like WORX and BLACK+DECKER sell 20V and 40V cordless kits aimed at homeowners, which suit this job.

When a stick edger is worth adding

A stick edger uses a vertical blade that cuts down into the turf along a path, curb or bed. It produces a defined lip that a string trimmer cannot, and it keeps grass from creeping over concrete. If your robot mower leaves a strip along a driveway, sidewalk or paver patio, that clean cut is what makes the lawn look finished.

It is a narrower tool. It does nothing for grass around a tree, under a fence or against a wall. Blades dull on soil and stones, so you will sharpen or replace them. Most edgers are heavier and need a steadier hand, and a rotary blade can throw stones, so wear eye protection. If your yard has only a few short hardscape edges, a string trimmer with an edger mode may be enough. Choose a dedicated stick edger when you have long straight runs and want the cut to hold up through the season.

Battery platform note for owners of existing cordless tools

If you already own cordless tool batteries, check them before buying a kit. Batteries are rarely interchangeable across brands. What matters is the platform: the voltage family, the connector shape, and whether the manufacturer sells a bare-tool version of the trimmer.

If the trimmer you want runs on your existing platform, buy the bare tool and skip the battery and charger. This saves money and avoids storing another charger. If it does not, a kit is simpler than a mismatched bare tool, and the new battery may be useful later for other tools in that family. Match voltage claims carefully, since a battery sold at one nominal voltage does not fit a tool built for another. Larger amp-hour packs run longer and weigh more. Spare batteries and chargers are sold separately for most platforms, and a second pack is worth having only if you trim a long perimeter or use the same platform for other yard work. Avoid third-party packs unless the tool maker lists them as compatible.

Trimming around a robot mower without causing damage

Trimming near a robot mower creates a few specific failure modes.

• Cut boundary wire. A surface-pegged or shallow wire loop is exposed along the very strip you trim. Trimmer line can nick or sever it. Keep the head up and away from the wire, or bury it deeper when you can. A cut wire usually makes the mower stop with an error until you repair the break.
• Scalping the border. Trimming lower than the mower’s cutting height leaves a visible step and can bare the soil. Set your trimming height to match the mowed lawn, not below it.
• Debris on the mower. Clippings and grit land on the charging contacts, sensors and wheels. Wipe the charging station contacts and check the rain sensor, lift sensor and bumpers after trimming.
• Mower running during trimming. Pause the mower or send it to the dock first. A mower that starts while you work is a hazard to hands and feet, and its blade can throw a string trimmer’s dropped debris.

Trimming does not have to happen every mowing session. Because the uncut strip grows slowly, trimming every few weeks during the growing season is usually enough, and that is the main reason a light cordless tool is a good fit. Replace worn trimmer line or spools as needed, and keep the head free of tangled grass so the line feeds properly.

Shop cordless string trimmer

Shop cordless string trimmer and edger

Shop WORX cordless string trimmer

Shop BLACK+DECKER cordless string trimmer

Shop stick edger for robot mower edges

Shop string trimmer replacement line and spools

Related guides

• Best Robot Lawn Mowers (2026): Wire-Free RTK, Vision, and Budget Picks
• Best Robot Mower for Half an Acre
• Best Robot Mower With No Perimeter Wire
• Best Robot Mower for a Small Yard

FAQ

Do I still need a string trimmer if my robot mower has edge cutting?

Usually yes. Edge-cutting designs shrink the uncut strip along open edges, but they still stop short of fences, walls, trees and raised beds. A cordless string trimmer remains the tool that finishes those spots.

Can a string trimmer damage my robot mower’s boundary wire?

Yes. Trimmer line can nick or cut a wire that is pegged on the surface or buried shallowly along the lawn edge. Keep the head raised near the wire and check the mower for errors after trimming. If the loop is broken, repair it at the break.

Should I buy a string trimmer or a stick edger for the strip my mower misses?

Start with a cordless string trimmer, since it can reach fence lines, tree bases and corners. Add a stick edger only if you have long runs along a sidewalk, driveway or bed border where you want a crisp vertical cut.

Can I use batteries I already own for a new trimmer?

Only if the trimmer is built for the same battery platform and voltage family as your existing packs. If it is, buy the bare tool and skip the extra battery and charger. If not, a kit with its own battery is the simpler choice.