The team cleared a large area of the cliff side at Moulin Huet of the weed
In an attempt to save the native plant life at Moulin Huet, St Martin's parishioners and the Guernsey Probation Service cut back Japanese Knotweed.
If left unattended it can smother other plants and cause erosion of the soil.
Senior constable Gerry Tattersall said: "Until a few years ago the area was covered in hydrangeas... now we have half a dozen and the rest is knotweed."
He also said that if not dealt with the knotweed could threaten the stability of the cliff face.
The area being cleared used to be covered by hydrangea bushes
Gerry explained: "It grows to about 8ft (2.4m) high and the roots go down the same distance.
"As well as overtaking and suffocating the hydrangeas it's also eroding the cliff face... I believe in another generation these cliffs will have fallen into the sea if we don't do anything."
The weeds could be removed using herbicide but Gerry said would be too costly and that removing all the earth containing the plants would be "stupid and ridiculous".
Instead the parish decided to simply cut it back and burn the remains several times a year. Gerry said: "This won't kill it but it will weaken it."
Parishioner Bill said: "When it first arrived and was first realised as a problem it should have been put on the list and it should have been got rid of then."
St Martin's Douzenier Colin Pickard said: "It's a very important job... I just hope we can find a way to control it in the long term."