A police force has issued private landlords with guidance on how to spot if their rented properties are being turned into cannabis 'factories'.
Gwent Police said in the past five years the number of 'factories' it had uncovered has risen from five to 151.
The force's drugs squad head Det Insp Paul Evans said five were found this year and 38 in the past 12 months.
He said: "When drug criminals operate out of rental properties neighbourhoods suffer."
The advice to landlords includes information on how to spot the manufacture of synthetic drugs such as amphetamine sulphate or ecstasy as well as a cannabis 'factory'.
It said houses, garages, factory units and caravans have all been used as drug manufacturing or growing sites and rural locations and those with plenty of privacy are preferred.
Other giveaways include regular late-night comings and goings as well as people exchanging small packets of cash, people using drugs nearby and drug paraphernalia being left in the area.
The advice also said that wiring in the property may have been tampered with and windows might be blacked out.
Det Insp Evans added: "Landlords are responsible for their property including costs if utilities are tapped into.
"Properties should be maintained and kept in good order and we advise landlords to check their premises on a regular basis to avoid becoming a victim.
"General property values can be affected, particularly when illegal activity begins affecting the reputation of the neighbourhood.
"There is no simple solution to the problem of illegal drug use, manufacturing or trafficking. We all need to work together to make the community safer."