The Royal Horticultural Society has some useful advice on controlling Japanese knotweed, Himalayan balsam, giant hogweed and ragwort. 

We endeavour to control Japanese knotweed, Himalayan balsam and giant hogweed on council land where possible.

Ragwort is poisonous to cattle and horses but is a very important food source for over 70 insect species, around 30 of which use ragwort as their only food source. As a result, we do not remove ragwort from council land unless it is grazed by horses or cattle or near to grazed land.