Chalk rivers and a canal
The Ver, Colne, Gade and Lea are chalk rivers, a rare kind of watercourse, and the Herts and Middlesex Wildlife Trust reckons roughly a tenth of the world's chalk streams run through Hertfordshire. Their banks, and the Grand Union Canal towpath as it climbs through Kings Langley, Hemel Hempstead and Berkhamsted, give brown rats cover, water and a corridor to travel along. Allotments, compost bays, ducks fed at park lakes and old sewers that drain towards the rivers do the rest. So a rat job here often starts at the back fence rather than under the sink: we follow the runs towards the water, lift drain covers where the signs lead underground, and seal the gaps before leaning on bait alone.