Marijuana grow-ops in Ontario have undergone an ominous change of late, making it more likely that a person out for a walk could be injured or killed by a booby trap, police warn.

The Ontario Provincial Police say that a few years ago, most of the marijuana growing operations they uncovered were indoors, in barns or homes.

'We had one case where this crib was mounted in the trees and it was full of rocks and lumber and had spikes in the bottom of it.'-OPP Det. Supt. Frank Elbers

These days, they are more likely to find them out in the open, according to Det. Supt. Frank Elbers of the OPP's drug enforcement unit.

"We've seen an almost total reversal," Elbers said.

"Whereas five years ago, most of our grows were indoor grows, now our outdoor grows are two to one to our indoor grows. And what I think it comes down to is economics for the bad guy."

The motivation is cheap land, he said. While the growing season is short, the overhead is low.

But outdoor operations bring with them danger in the form of booby traps intended to discourage poachers, he said. Some are crude and some sophisticated, but all are dangerous.

"We had one case where this crib was mounted in the trees and it was full of rocks and lumber and had spikes in the bottom of it," Elbers said.

"And if it fell on you, it would kill you."

Officers raiding grow ops do so very carefully now, Elbers said.

He warned that anyone who accidentally wanders into a grow-op should retrace their steps very carefully and then call police.