Who, what, why: How can you spot a cannabis farm in your street?

Montage of three pictures (clockwise), a photograph of a house taken with thermal imaging equipment (from Press Association), a cannabis farm and a gloved hand holding a cannabis leaf (from Getty)

Police say cannabis growers are moving away from commercial and industrial sites towards ordinary houses. But how can you spot a cannabis farm in your street?

Cannabis growing in the suburbs is soaring, with about 21 farms or factories being discovered every day, a new report by the Association of Chief Police Officers says.

The problem has persisted for a while, with insurer Aviva reporting a 30% year-on-year increase in cannabis damage claims last year as criminals turned to rented houses to cultivate plants.

Now the Association of Chief Police Officers (Acpo) says most of the cannabis consumed in Britain is homegrown and is urging the public to keep an eye out for suspicious behaviour in their neighbourhood.

And householders might just want to know for their own sake too.

So how can you spot an illegal drug farm on your doorstep?

Unlike many criminal enterprises, there are telltale signs of cannabis cultivation, according to commander Allan Gibson, Acpo's lead officer on the subject.

The first thing to look out for is people setting up shop, he says.

"Most of the properties are rented accommodation, and when they move in, they'll bring a lot of equipment with them.

"It will be ventilation equipment and lighting equipment for irrigation, grow bags, soil - it should stand out as unusual, which is why criminals tend to try and move in when they will be unobserved," he says.

Gibson says the next thing cannabis growing gangs tend to do is adapt the premises - often by creating venting through floors or re-wiring - but this can easily go unnoticed as plenty of new tenants renovate properties.

Another thing cannabis growers' neighbours might notice is their absence - or their coming and going at odd hours.

"They won't be sociable neighbours, they will want to keep a low profile," says Gibson.

"So if cannabis growers live in permanently, they will barely be seen - and if they are maintaining more than one growth site, they will come and go in the early hours or at night," he says.

So far, so hard to detect, it seems. But Gibson says there are a couple of obvious indicators that should be easier to pick up on.

Cannabis's distinctive strong and sickly sweet smell is one of them.

The other is covered-up windows - often constantly pulled curtains or black-out blinds - so nobody can see into the premises and the right temperature is maintained.

Rick Stephens, from West Midlands Police's cannabis disposal team, agrees windows - usually blackened or "polythened" - are a common giveaway, but he says criminals are getting increasingly sophisticated about covering their tracks.

"In some cases a bay window is created, with a overnight light or TV set up, so the premises looks normal - but actually it is just screened off, with cannabis plants lined up behind," he says.

Stephens says lighting - which can be spotted by hi-tech thermal imaging cameras which display the heat given off by the strong lights - can also alert suspicious neighbours.

"If we are talking about 500 plants, and work on the presumption that there can be six plants for a set of lights, that's 80 or 90 lights or transformers that are needed."

He says initially plants require about 12 hours of artificial light every 24 hours. Growers tend to do this at night, which means it can sometimes be spotted.

"At one house in Wolverhampton, neighbours saw bright lights coming out from a vent under floorboards, under which cannabis was being grown," he says.

Concerned neighbours can also look out for condensation on windows, or unusual levels of heat coming from a property as a result of lights, according to Bryan Dent, a drugs co-ordinator at West Yorkshire Police.

"It's going to be considerably warmer than normal room temperature - that will manifest itself in heavy condensation," he says.

Dent says criminals also often tend to bypass electricity meters, or break into the main meter, to fund the huge wattage used up by the lamps and the fans.

"This might manifest itself in strange electric cables, or digging underground to join into street furniture supplies, such as a lamp post," he says.

For those who think they are on to cannabis-growing gangs, Gibson says there is one other key tell-tale sign - the criminals do not appear to be environmentally friendly either.

"Growing material like compost and soil is often contained in plastic bags, so we often see them dumped by rubbish bins," he says.