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Weeds are a constant garden threat. Slack off on keeping them under control and they'll quickly take over. The $299 Tertill robot is a surprisingly functional alternative to constant vigilance and weeding by hand. This solar-powered, automated weed trimmer is designed to patrol and take out weeds as they emerge, so you don't have to.

The Tertill sounds like a dream come true. The minute I caught wind of the Tertill, dreamed up by startup Franklin Robotics, I was compelled to find out if it actually works. And I'm happy to say, yes it does. The Tertill is in fact a potent weed deterrent. Like any defense system, however, the Tertill isn't unstoppable.

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While it's ruggedized and weather-resistant, the robot can be defeated by the environment. Uneven terrain, mud and rocks all can cause the Tertill to fail.

Still, if you hate weeding as much as I do, it'll be tough to resist the charms of this little garden helper.

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Yup, it works

So does the $299 Tertill actually work, and is it worth buying? The answer depends greatly on how you plan to use it and where it will patrol. If you own a raised garden that gets plenty of direct sunlight, then the Tertill will do a phenomenal job.

At the beginning of my test period, I made sure the garden at the CNET Smart Home was completely devoid of weeds. I then divided the plot in half with a wooden landscape timber. One side I left untouched, while the other I reserved for the Tertill.

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Close to a month later (26 days, to be exact), the difference between the two garden plots was unmistakable. Weeds had completely overrun one side, while the side where the Tertill lived was remarkably weed-free. This was the case even with the trouble the robot had with mud, as well as losing a couple of trimmer strings to sticks and stones.

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Anatomy of a Tertill



I admit the Tertill doesn't look like a fearsome weed-killing machine. Its thick, round shape reminds me of a stack of pie plates or perhaps a fat cookie tin. The robot's bright green plastic body lends even more playfulness to the Tertill's appearance.

That goes double for the Tertill's wheels, which splay outward from the robot's center. The wheels are crowded closely together, too. This, and the highly canted angle of the wheels, give the Tertill a distinct (if awkward) cuteness.

Don't be fooled by its goofy, friendly exterior. The underside of the Tertill conceals a formidable weapon, a tiny string weed trimmer. Paired with a capacitive sensor, the trimmer string spins and flails away at any weeds the Tertill encounters.

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The Tertill's trimmer string is replaceable. Once you insert one into the trimmer hub (the moving part that spins), each string protrudes outward a distance of 1.4 inches. Over time, these strings will shorten and eventually wear out. The rate of wear depends greatly on the number of rocks, sticks and other debris you have in your garden.

In my case, I replaced the Tertill's trimmer string three times over my 26-day test period. This comes to about 9 days per string. That said, the soil in the test garden at the CNET Smart Home is particularly troublesome. It's filled lots of small rocks and sticks. And since the soil is mainly clay, it dries hard. When the soil gets wet, it becomes a sticky mess.

In fact, of the three strings I replaced, one vanished after I rescued it from mud (more on that later). The second string disappeared after I noticed it attacking rocks earlier that day. And the third string was still present, but simply too short and bent out of shape.

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There are five replacement trimmer strings bundled with the Tertill. You can also purchase $15 replacement kits. Each kit comes with 20 strings and two new trimmer hubs.

There's a speaker on the bottom of the robot too. It belts out status and warning tones based on the Tertill's present condition, and imminent actions. The robot's undercarriage also houses a watertight cap covering a microUSB port. You can use the port to charge the Terill's internal battery, helpful for when the sun isn't shining. The Tertill's main power source is a solar panel that lives on top of the robot.

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Also on the top are five LED lights arranged along a curve. They indicate the state of the Tertill's various subsystems, specifically the battery, wheels, trimmer, sensors and internal temperature. The LEDs also communicate the robot's operational mode. A sweeping pattern from side to side, with the LEDs illuminated in green, means all systems are go for weeding. Just push the big button, on the opposite side of the solar panel, and the Tertill will start patrolling.

Your attention is still required

In theory, setting up and using the Tertill is dead simple. The first step is to fully charge its battery, either by putting it in direct sunlight or via USB for at least an hour. Next, place the Tertill on the ground at your chosen location. It's ideally suited for an enclosed garden, or a flower bed. Finally, you press the robot's power button to kick the machine into patrol mode. In reality, I found the process is a little more complicated.

Tertill navigation

The Tertill works best on level surfaces. Uneven ground with bumps and divots can cause the robot to get stuck or flip over. To help it avoid that outcome, Tertill won't climb inclines steeper than 22 degrees.

That's a problem for gardens like my home garden, which sit on a slope. Unless you decrease these grades, the Tertill will become trapped in one section. The manual also recommends that you give the Tertill at least 12 inches of clearance space between plants. Any less than that and the robot will avoid navigating around them.

The small test garden at the CNET Smart Home contained six plants spaced as advised (at least 1 foot apart). The Tertill didn't have any problems navigating this area. For instance, it never got stuck on plants. That said, its pattern of navigation seems quite random, with the machine changing course haphazardly when it encounters obstacles.

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Mud is the worst

By far, I found that the biggest threat to the Tertill is mud. The natural clay soil at my house is particularly problematic. Dense and made of fine particles, this type of dirt becomes a sticky quagmire when it gets wet. After a heavy rain, my Tertill test unit usually became hopelessly mired.

Freeing and cleaning the machine isn't fun, but it's not an awful chore if you do it properly. As suggested by the manual, a quick wash with water from a low-pressure garden hose does the trick in about 10 minutes.

That said, my first few attempts to clean mud from the Tertill didn't go well. Without hose water, wet mud stuck to the wheels and inside the wheel wells like thick glue. Dry mud became a hard plaster-like shell that clung with more tenacity. When I washed the robot inside (in a utility sink), it made a big, muddy mess.

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The worst aspect to becoming bogged down by mud isn't the clean up. It's the downtime. Weeds never take a break, and all they need to resurface is a few days of inattention. And since the Tertill's trimmer only targets sprouting weeds (plants under 1 inch tall), the window for the robot's effectiveness is tight.

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Sunlight is a must

Another lesson I learned is just how critical it is for the Tertill to have access to strong sunlight. My mulch bed at home is situated in between several trees. Their branches overhang enough to shade the area much of the day. So even when the sun is shining, some parts of the bed remain in shadow. When I placed the Tertill there, it ran less often. Unfortunately the decrease in the Tertill's activity was the opening weeds needed to re-emerge.

Watch out for drops

Tertill relies on a pair of obstacle sensors (on its left- and right-front side) and an accelerometer to keep it on course. Unlike a Roomba, the Tertill lacks cliff sensors. That means it will tumble out of any areas with accessible drops. In fact, on many occasions I had to rescue the Tertill I'd placed in a mulched garden bed.

One section of this bed sits on a steep hill. Its borders are guarded by a landscaping timber that's partially buried. In spots, only about 1.5 inch of the timber is above ground. The robot always managed to climb over this low hurdle, tumble down a retaining wall, then land on the hill below. To prevent that, I had to wall the area off with barriers at least 3 inches tall. You'll have to use the same tactic to block the Tertill from escaping its patrol zone.

To Tertill or not to Tertill?

Spending this much money on the $299 Tertill is a little extravagant. It costs nothing, save some sweat and physical effort, to remove weeds by hand. Still, weeds never give up. It only takes a short lapse on your part for them to return with a vengeance. If you look after it properly, and if it can operate in the right conditions, the Tertill is just as relentless.

Yes, the machine isn't perfect. I wish you didn't have to box it in with barriers to keep it from wandering away. I also feel that if the price dropped to, say, $99, it would be very tough to resist. Then I would be tempted to drop multiple Tertills outside the house. Think flower beds in the front and back yards, plus the vegetable garden.

I also can't express how joyful it is to watch the Tertill in action. It patrols and attacks garden invaders with ruthless determination, and that's just what they deserve.