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.
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.