Thieves have been stealing plants related to cannabis in the mistaken belief the crop has a psychoactive effect, police have said.
Surrey Police said hemp plants growing in fields along the A25 in Guildford, Dorking and Reigate have been targeted.
Supt Matt Bristow said the plants were being grown for use in the production of fibre and oils.
Hemp is related to cannabis, but contains only trace amounts of THC, the substance found in marijuana.
Mr Bristow said: "We believe thieves are targeting the hemp plants for use as cannabis but these hemp plants have been specifically cultivated for industrial purposes.
The level of THC was "extremely low and unlikely to have any effect", he said.