With little thought to the moral implications, a group of four engineers at a startup called Franklin Robotics are building a robot that will be programmed to kill.

Weeds.

But the four-wheel-drive bot, not much bigger than a brick, isn’t yet good at distinguishing friend from foe. On a recent test run in a small garden on the edge of a parking lot in North Billerica, where the company is based, the prototype robot had an easy time navigating uneven mulch, dirt, and a garden hose without getting stuck. But when it came to figuring out what it should slash with the tiny, spinning string-trimmer on its belly, it was confused. It weed-whacked at mulch and a periwinkle. It drove over weeds without detecting and decapitating them. It bumped into a basil plant and rolled right over a marigold.

For some people, there’s nothing more fun than getting dirty in the garden. For Joe Jones, cofounder of Franklin Robotics, “there’s nothing more fun than working on robots. You start out with something rough, but you get better and better.’’

While he confesses to being a newbie gardener, Jones has a track record in the robot biz. He was the original inventor of iRobot’s Roomba robotic vacuum cleaner, which is the best-known — and probably the best-selling — consumer robot in the United States. (IRobot says that it sold nearly 2.5 million Roombas and related bots, which clean pools, tile floors, and gutters, in 2015.)

But the great outdoors is the next frontier. Franklin Robotics hopes to begin selling its weeding bot, dubbed Tertill, in 2017, and Bedford-based iRobot may be on the verge of launching a robotic lawn mower after shelving the project several years ago.

Jones says he first had the idea for a robo-weeder in 2014, when he was working for Harvest Automation, a startup that made rugged robots for large-scale commercial nurseries. The obstacle he ran into, though, was how the robot would know the difference between a young seedling and a weed.

“I thought you might be able to put a wire stand next to the seedling, to protect it,’’ he said. “That is pretty labor-intensive if you’re a farmer with 1,000 seedlings, but it could work in a garden. So I figured we’d start in the garden, and as the technology improves, eventually it will work for 1,000 acres of corn.’’ In mid-2015, Jones left Harvest to start Franklin. (The two companies still share office space.)

The prototype Tertill has a solar panel on its back that charges an internal battery. When it has enough juice, it goes out on a seek-and-destroy mission. When it runs out of power, it sits and recharges. Anything small enough to go under its body is assumed to be the enemy, and when young plants are at that size, they must be protected with a wire loop that signals they’re noncombatants.

“The intent is to just put it in the garden and turn it on and just leave it,’’ said Rory MacKean, Franklin’s cofounder and chief executive. MacKean points out that it will need to be waterproof to survive the occasional downpour or sprinkler-drenching. Later models might be able to gather information the moisture content in the soil and relay it to a smartphone, or spot a bunny nibbling on the lettuce and move suddenly to scare it off.

This summer, the company is testing the Tertill in different gardens around Greater Boston and raising a round of seed funding (no pun intended). MacKean expects the robot to be priced between $250 and $300.

Already on the market are several robotic mowers, from companies such as Worx, Husqvarna, and John Deere. But many of them require a few hours of work laying out a perimeter, so that the mowers don’t stray onto the sidewalk or a neighbor’s turf. And they’re more popular in Europe than in the United States; John Deere doesn’t even sell its model, the Tango E5, domestically.

Dan Kara, an analyst at ABI Research, estimates that 70 percent of the sales of robotic mowers are in Europe, where lawns are smaller. In addition, Kara said, “Some people actually like mowing their lawns. My wife is an example.’’ But in a world where most people are pressed for time, outsourcing lawn maintenance to a robot seems destined to happen.

Steve Uljua, an associate product manager at Husqvarna, which makes a line of robotic mowers that start at $2,000, says American consumers seem to finally be ready for a robotic assist. Husqvarna, a Swedish company, first tried to launch its Automower here in 1997, “but it just didn’t find a foothold,’’ Uljua said. Now, though, the mowers are better at analyzing the length of the grass to avoid over-mowing, he says, and the company’s biggest machine can trim 1.25 acres of lawn.

Some challenges remain, however. Many people already own gas-powered mowers, and gas in this country is cheap relative to the price in Europe, Uljua says. Also, retailers who sell lawn and garden equipment remain focused on selling ride-on mowers and other old-school gear. (Uljua says that his company will soon start selling its least-expensive Automower through Amazon.)

A recent review of the robo-mowers on the market in the Wall Street Journal repeatedly compared them to iRobot’s Roomba vacuum cleaner — “like a Roomba for your lawn,’’ according to the headline. But iRobot won’t say anything about its plans, beyond that it is “exploring the lawn-mowing category,’’ in the words of spokesman Matt Lloyd.

That’s strange for a company that has been a pioneer in home-cleaning robots, and even stranger because iRobot sold off its defense contracting business in February so it could dedicate more attention to developing new products for consumers.

But two things have happened that suggest iRobot might have its eyes on the backyard. Last fall, the company got clearance from the Federal Communications Commission to use a specific radio frequency for robotic lawn mowers. The idea is that you’d plant stakes at the corners of your yard, and the mower would communicate with them, much like GPS satellites, to understand what was in bounds and what was out of bounds. That could simplify the setup process, which Uljua says can today involve three or four hours of burying a boundary wire in the ground.

And second is an iRobot trademark application that surfaced last month, for the brand Terra, which would cover robotic lawn mowers.

Kara, the analyst at ABI, says that lawn mowing is an obvious next market for iRobot, especially because raking leaves, shoveling snow, and cleaning windows remain technologically challenging — at least until someone figures out how to arm a drone with a spray bottle of Windex.

Scott Kirsner can be reached at kirsner@pobox.com. Follow him on Twitter @ScottKirsner.