As California considers legalising cannabis by voting for Proposition 19 in November, which will allow the state to "regulate, control and tax cannabis".

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Be careful if you are thinking of heading out on an innocent hike anywhere in California's national parks or public lands. You do not want to trip a booby trap that will trigger a shotgun aimed right at you. Fall down a bear pit lined with sharpened spikes. Or stumble upon rattlesnakes deliberately tied up along the trail.

These are the kind of hazards that are becoming all too common for hikers, riders and hunters across California. All because of the massive illegal cultivation of marijuana that is going on in public lands. Criminal gangs from Mexico have moved into the state to take advantage of the perfect growing climate and the cash strapped law enforcement agencies cannot keep up.

The better known image of cannabis in California is very different. This is where de-criminalised cannabis has become legitimate. Where you can walk into well lit dispensaries and buy a carefully weighed ounce of any strain of Sativa or Indica you like. And it is all perfectly legal - just as long as you have a note from your doctor. And everyone knows it is very, very easy to get hold of one of those notes.

It has been nearly 20 years since the Golden State effectively de-criminalised marijuana for medical use and now they are considering going much further. In November voters will get to decided on Proposition 19 - which will offer them the choice to "Regulate, Control and Tax Cannabis". Meaning it would be perfectly legal for anyone over 21 to have up to an ounce of cannabis for personal use - even if they do not have any medical reason for using it.

Sounds a bit outlandish. But polls show the proposition has a good chance of passing. Not because everyone in the state wants to be allowed to light up a spliff. But because only by legalising marijuana can you tax it. And California needs all the taxes it can get. The state is broke.

They are laying off policemen and firemen, closing schools and turning off the street lights. Any possible source of revenue has to be seriously considered. If taxing the growing, processing and sale of cannabis is a way to make money then it has got to be given serious consideration.

Cannabis entrepreneurs ready to 'cash in'

Northern California is already famous for its wine industry. And the same growing conditions that are so good for the grapes are also perfect for pot. So the region is full of cannabis entrepreneurs who are just waiting to cash in.

I visited one farm where they have already been given a permit to grow 99 huge pot plants on one plot. That grower is already making a pretty good living selling marijuana legally through his medical collective. But he cannot wait until he can expand to several thousand plants.

While in the city of Oakland, just outside San Francisco, Jeff Wilcox has already bought 43,000 square feet of warehouse space in which he plans to propagate indoors over 50 pounds of cannabis a day - just as long as Proposition 19 passes in November. Serious businessmen think they can make serious money from cannabis.

Supporters of Proposition 19 say that not only will it bring in much needed revenue, they argue the police and other law enforcement agencies will save millions of dollars if they no longer have to spend time and resources chasing after illegal marijuana.

But Sherriff Tom Allman in Mendocino County told me it is a very different story. He made it very obvious he has no problem with people growing, possessing and using small amounts of marijuana for personal use, even if they are not in strict accordance with the law. Nor does he have any moral objection to legalising it further. But as long as cannabis is still illegal in the 49 other states of the US then he fears that his county will continue to be overrun by violent and dangerous criminal drug gangs.

They come here, from Mexico mostly, to set up huge illicit cannabis gardens hidden in the wilderness of the state's massive public parks. They install complex irrigation systems that steal scarce water resources. They pollute the land with chemical pesticides and fertilisers, and they shoot at anyone who dares to come near. The cannabis is then mostly moved out of California to be sold in states where marijuana is still very much against the law.

Proposition 19 may 'attract illegal activity'

Proposition 19 would not make any of this activity legal. But law enforcement worry it would attract more illegal activity into their state. It is not just the perfect growing conditions they say that bring the gangsters here. It is the culture of acceptance of cannabis. And any further legalisation would just make it worse they fear. Instead of removing the criminal element from the pot trade it would just attract more.

Some people worry about whether Proposition 19 would not bring in much tax money at all. Comparisons are made to excise duty on alcohol and tobacco. But people do not usually grow their own tobacco or distill their own liquor so consumers have no choice but to buy them as retail products and pay the sales tax.

The same is not true of marijuana. It is cheap and easy to grow your own and any change in the law would allow individuals to grow as much as 25 square feet of marijuana for personal use. Will people really pay VAT on homegrown?

And of course the criminal gangs who cultivate most of the cannabis that is grown in California have no intention of paying any tax at all.