Black & Decker 20V MAX* 12" String Trimmer/Edger

Why I reached for this trimmer

I keep a few string trimmers in rotation, but I’ve been spending the past few weeks with Black & Decker’s LST320C, a compact, cordless 12-inch trimmer that also converts to an edger. I brought it into my daily yard routine for a simple reason: I wanted something light, easy to start, and fuss-free for weekly maintenance around beds, fences, and the curb. After multiple charge cycles and a mix of tidy lawns and a few neglected corners, here’s how it fared.

Setup and ergonomics

Out of the box, the LST320C is straightforward. The guard snaps on, the auxiliary handle clamps down easily, and the telescoping shaft adjusts without drama. At 7.7 pounds, it’s genuinely light, and the balance is friendly—most of the weight sits near the grip and battery, so the head doesn’t feel “lever heavy” like some budget trimmers. The adjustable handle and height positions make it easy to dial in; I could hand it to a shorter family member with a quick tweak and they were comfortable within seconds.

The tool’s footprint is compact, and the wall‑mountable charger earns a bonus point in small garages or sheds. Being able to hang the charger near the door makes swapping and topping off a battery part of the post‑mow routine instead of a chore.

Power and cutting performance

This is a 12-inch trimmer with a high-torque transmission that focuses power at the string. In practice, that means it’s tuned for lawn maintenance, not brush clearing. It chews through typical residential grass, crabgrass runners, and dandelion stems without complaint, and it holds speed respectably when you feather the trigger through denser clumps.

Where it begins to show its limits is in tall, wet growth or thick, woody weeds—situations where a wider-cut or higher-voltage unit (or a gas trimmer) has obvious advantages. I could work through those patches with patience, but I had to slow my pace to let the motor keep its RPMs stable. If your yard regularly enters rough-cut territory, this isn’t the right tool.

For normal weekly trimming along fences, swing sets, mailbox posts, and around planters, the LST320C is easy to manage and plenty effective. The 12-inch cut swath is actually helpful for precision near fragile plants; I found I made fewer accidental nicks in mulch edging compared with larger heads.

Automatic line feed

The Automatic Feed Spool (AFS) is one of the reasons the trimmer feels so stress-free. It advances line as needed without any bumping. On short, well-kept grass, it feeds sparingly and accurately. In heavier patches, it will sometimes advance a touch more than I’d prefer, which is common with auto-feed systems when the head speed fluctuates. I fixed that by keeping a steady pace and avoiding abrupt throttle blips in tough spots.

Line replacement is simple, and an AF-100 spool is included. If you trim frequently, stock an extra spool so you’re not sidelined mid-session. You can rewind your own line if you prefer, but pre-wound spools are convenient and keep feed consistent.

Edging mode

Converting to an edger takes seconds. Twist the head, click it into the vertical orientation, and you’re set. The trimmer edges cleanly along sidewalks and driveway gaps, especially if you’ve been maintaining an edge regularly. If the edge has gone wild or is packed with compacted soil, plan on a couple of passes. The tool’s lightweight nature actually helps here—you can guide it with a relaxed grip and keep the cut straight without wrestling.

I wouldn’t call it a substitute for a dedicated heavy-duty edger if you’re rehabilitating a neglected lawn, but for weekly touch-ups, it’s more than capable and much faster than swapping tools.

Battery life and charging

The kit includes two 20V MAX lithium-ion batteries at 1.5 Ah each. Only one is required to run the trimmer, which means you can hot-swap and keep going. Charge time is roughly an hour per pack on the included wall-mountable base.

In my use, each battery delivered around 20–30 minutes of mixed trimming and edging, depending on how heavy the vegetation was and how aggressive I was with the trigger. On a typical suburban lot, I could do all the trimming with the first battery and finish the edging on the second, with a bit left in reserve. If your yard is small and tidy, you’ll often finish on a single battery; if you’re catching up after a rainy week, expect to use both.

One note: lightweight trimmers are at their best when you don’t treat them like brush hogs. Maintain a consistent rhythm, avoid burying the head into dense clumps, and let the line do the work. It’s better for runtime and for the motor.

Noise, vibration, and user comfort

Noise is modest for a string trimmer—certainly neighbor-friendly compared with gas—and vibration is low. The handle and shaft adjustments translate to less wrist fatigue, and the balance helps keep your back neutral. For extended sessions, I still recommend gloves and eye protection, but I didn’t feel beat up afterward, which isn’t always true with heavier or poorly balanced units.

Build and durability

Fit and finish are what I expect at this price point. The plastics are sturdy enough and the guards are not flimsy. The head rotation mechanism for edging has a positive, repeatable lock. I did a handful of unglamorous tasks—lightly tapping along chain-link, trimming near stone borders—and the head housing came away none the worse. As with any light trimmer, be sensible around sharp edges and keep the guard in place.

The 2‑year limited warranty is reassuring, and while I can’t speak to years-long longevity in this short window, nothing in my testing raised red flags.

Maintenance and ownership

• Keep the spool clean and free of dirt; it improves feed consistency.
• Swap to a fresh spool before it’s completely empty to avoid a mid-trim rethread.
• Store batteries indoors in moderate temperatures; they’ll hold capacity better.
• Hang the charger where it’s easy to use—the wall mount makes a difference.

What I’d change

• More runtime per pack: The included 1.5 Ah batteries keep the weight down, but a higher‑capacity option would extend sessions. If you have larger trimming needs, consider adding a higher‑Ah battery to your kit.
• Slightly stronger recovery in thick patches: The motor holds speed well for its class, but a bit more headroom would reduce bogging in heavy growth.
• Line feed in dense grass: The AFS works well most of the time, but it can overfeed when the motor labors. Smoother throttle input minimizes this, yet a more adaptive feed algorithm would be welcome.

Who it’s for

• Homeowners maintaining small to medium yards who prioritize light weight and convenience.
• Anyone who wants an easy switch between trimming and edging without buying two tools.
• Users who value quick charging and the ability to finish a weekly maintenance pass without managing gas, cords, or primer bulbs.

If your property has long fencelines of thick weeds, or you frequently cut down overgrowth, you’ll outgrow this trimmer. For routine lawn care, it hits a sweet spot of performance, comfort, and simplicity.

The bottom line and my recommendation

The LST320C is a genuinely user-friendly trimmer/edger that makes weekly yard maintenance easier. It’s light, well-balanced, and quick to adjust; the automatic feed and no-bump operation reduce the usual fiddling; and the included two-battery setup plus a one-hour charger mean you can cover most suburban tasks without waiting.

I recommend this trimmer for homeowners who want a lightweight, low-maintenance tool for regular trimming and clean edging on small to medium lawns. It’s not a brush-clearing powerhouse, and the runtime per 1.5 Ah battery is modest, but the overall package—comfortable ergonomics, solid cutting performance for grass, quick edging conversion, and straightforward charging—adds up to a practical, pleasant tool to live with. If your needs match its strengths, it’s an easy yes. If you routinely tackle heavy overgrowth, look for a higher-voltage or gas alternative.