Each country has its own invasive species and rampant plants with a tendency to grow out of control. In most, the techniques for dealing with them are similar - a mixture of powerful chemicals and diggers. But in the US a new weapon has joined the armoury in recent years - the goat.
In a field just outside Washington, Andy, a tall goat with long, floppy ears, nuzzles up to his owner, Brian Knox.
Standing with Andy are another 70 or so goats, some basking in the low winter sun, and others huddled together around bales of hay.
This is holiday time - a chance for the goats to rest and give birth before they start work again in the spring.
Originally bought to be butchered - goat meat is increasingly popular in the US - these animals had a lucky escape when Knox and his business partner discovered they had hidden skills.
"We got to know the goats well and thought, we can't sell them for meat," he says. "So we started using them around this property on some invasive species. It worked really well, and things grew organically from there."
They are now known as the Eco Goats - a herd much in demand for their ability to clear land of invasive species and other nuisance plants up and down America's East Coast.
Poison ivy, multiflora rose and bittersweet - the goats eat them all with gusto, so Knox now markets their pest-munching services one week at a time from May to November.
Over the past seven years, they have become a huge success story, consuming tons of invasive species.
"I joke that I drive the bus, but they're the real rock stars," says Knox, who also works as a sustainability consultant.
Typically, chemicals and/or machinery are used to clear away fast-growing invasive plants, but both methods have their drawbacks. Chemicals can contaminate soil and are not effective in stopping new seeds from sprouting. Pulling plants out by machine can disturb the soil and cause erosion.
Goats, says Knox, are a simple, biological solution to the problem.
"This is old technology. I'd love to say I invented it, but it's been around since time began," he says. "We just kind of rediscovered it."
One of the reasons goats are so effective is that plant seeds rarely survive the grinding motion of their mouths and their multi-chambered stomachs - this is not always the case with other techniques which leave seeds in the soil to spring back.
Unlike machinery, they can access steep and wooded areas. And tall goats, like Andy, can reach plants more than eight feet high. A herd of 35 goats can go through half an acre of dense vegetation in about four days, which, says Knox, is the same amount of time it takes them to become bored with eating the same thing.
"When they move on to a new site, you can see the excitement in the way they eat," he says.
"They like the magic of getting on the trailer when all the food has gone and then they ride around for a bit and the next thing, the door opens and there's a whole new smorgasbord to eat."
Even more plant species could be added to the goat's diet, judging from some new research.
At Duke University in North Carolina, marine biologist Brian Silliman has spent 20 years working on understanding and eradicating the invasive species phragmites.
This reed, which thrives in salt marshes, can grow up to 10 feet tall, pushing out native species and blocking bay and sea views for coastal residents.
Silliman says at first he tried insects and other forms of "bio control" to tackle the plant, but nothing worked.
"Then I took a holiday to the Netherlands, where the plant comes from, and saw it wasn't a problem there because it was constantly being grazed by animals," he says.
In studies, Silliman found that goats were very effective - in one trial, 90% of the test area was left phragmites-free.
"I think all wetland managers should take up this method," he says. "It's cheaper, less polluting, better for the environment and goat farmers get paid."
One plant goats are increasingly being used to clear is kudzu. This fast-growing vine, native to east Asia, was first introduced into the US in 1876, as a ornamental plant that could shade porches and prevent soil erosion.
But it is now often described as "the vine that ate the south" because of its ability to grow up to a foot a day in the warm environment of south-eastern states like Mississippi, Alabama and Georgia.
Over the last 10 years, however, many landowners have successfully removed it using goats who repeatedly graze the plant until it loses the will to grow back.
Brian Cash runs one of three animal grazing businesses in Georgia where kudzu is a huge problem, not just because of the ground it covers but of the "kudzu bug" - a small beetle which thrives on the plant and which causes a burning sensation when squashed by bare skin.
He learned about keeping a grazing herd on the US West Coast, where there are several dozen well-established goat grazing companies, but decided to adapt the formula.
"In the end we used herds of mostly sheep with some goats mixed in as we found the goats were harder to control," he says of his company Ewe-niversally Green. "We found that the goats led all the mutinies."
Brian Knox, in Maryland, agrees that some goats can be troublesome and even admits to donating his grumpiest animal to a local butchery class.
But overall, he says he has a happy relationship with the animals.
"They certainly earn their keep," he says.
One of the more high profile jobs they have worked on was cleaning up the Congressional cemetery in Washington two years ago.
Large crowds came to watch as the animals spent a week chomping the overgrowth of Honeysuckle, Ivy and Poison Ivy. The goats even featured in newspaper and news programmes around the country.
This is one of the things he likes about taking goats into urban areas - the response of the city-dwellers, who are "fascinated", he says, to see how efficiently the goats gobble up the vegetation.
"It's still quite novel," says Knox.
Goats aren't a silver bullet. Knox often combines the goat clearance with some manual root cutting and even with a chemical treatment if needed.
But his goats have started to make an impact on the weeds choking America and, he says, they are having a lot of fun doing it.
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