Hemp is in flower at a Colorado test plot in Boulder County in the summer of 2014. (Elana Ashanti Jefferson, Cannabist file)

What’s a safe distance between hemp and marijuana plants?

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Hey, Cannabist!
What is the risk of pollination between hemp and marijuana? — Seedy Grower

Hey, Seedy Grower!
Unintended plant sex can be a real problem for marijuana growers when male pollen is introduced to female-only marijuana plants intended to be grown for their seedless flowers. I spoke with Anndrea Hermann, an international hemp expert, for her opinion on pollination and prevention.

Hermann, who was recently granted a DEA import/export permit for viable hemp seed into the United States, says via email: “Yes, there is a risk for marijuana growers when industrial hemp is grown with the males present.  Marijuana and industrial hemp don’t belong in cultivation together.”

A 2015 report from Oregon Cannabis Connection cited by Hermann covers pollination studies and discusses unwanted pollination.  “In three studies, pollen drift was observed across 5 to 12 kilometers (3 to 7.5 miles).  A fourth study showed pollen drift for 48 kilometers (30 miles) across the Mediterranean Sea from Morocco to Spain, summarizes Hermann.

We don’t have sea winds carrying hemp pollen into Colorado, but it can be windy nonetheless. According to Hermann, a safe starting distance between marijuana and hemp plants is 10 miles. “There is no scientific backing to guarantee that distance, but it is a safe starting point.”

In terms of the risk of pollen negatively affecting indoor marijuana grows, filtration is key. Hermann says HEPA (high efficiency particulate air) filters are the primary filter for screening pollen and she recommends consulting a filtration expert to create the necessary system to prevent pollination. Once an adequate filter system is in place, perform necessary maintenance to keep the system optimally operating.

Good communication between growers is recommended by Hermann. Hemp and marijuana growers in an area need to discuss, create and respect the isolation distance between the two kinds of crops. Hermann points out that this conversation is similar to the isolation between organic and conventional farmers growing in the same area.

It’s also a good idea to have clear and enforced employee and visitor policies to prevent pollen contamination from people entering indoor grow rooms. XO

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