We are in southern England. We have a very healthy plant in a polytunnel. I’m looking ahead and worrying about people smelling our plant when it’s nearing harvest time. We are a few metres from a country road. With neighbours not far away. I’ve attached a ground plan which explains everything.
Ideas I’ve had are : sealing the door of the polytunnel during the day and open late at night when hopefully no-one is about, creating a vent with ducting pipe taking the smell away from the road/neighbours, and some sort of charcoal filter in the door. Will it harm the plants for them to be sealed in most of the day?
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Does anyone have other ideas or comments? Thanks!
Start by creating a soft negative pressure in that polytunnel. For your exhaust, mount the fan mid-height or up in the rafters, then duct that air away from the road and down to ground level. You want to terminate it low into some dense shrubs or a mulch bed - the soil biology and vegetation will help scrub any trace odors before they can drift.
On the opposite end, cut a passive intake low to the ground with bug screen and a backdraft flap. This creates a nice front-to-back cross-flow that pulls air away from the road. Keep this running 24/7 on a low setting - odor actually spikes when the lights go off, so you need that constant flow.
For sizing, here's what works in polytunnels: They leak a bit naturally, so you're aiming for about 10-20 air changes per hour - not the crazy 1-2 per minute you'd see in tiny tents. As a rule of thumb, a 150mm (6-inch) mixed-flow fan with a matching carbon can handles most small garden tunnels. Go up to 200mm (8-inch) if you've got a larger volume or long duct runs. Keep those duct runs short with gentle bends, and add an inline silencer if noise becomes an issue.
If you're still getting some smell breakthrough, here's a trick I've used: add an internal scrubber loop. This is a second filter and fan that just recirculates air inside the tunnel without exhausting anything outside. It constantly strips odor without increasing your exhaust rate - great insurance policy for late flower when things get really pungent.
For camouflage around your outlet area, plant some strong aromatic plants like rosemary, mint, or lavender. I've even seen folks use ornamental alliums like another member suggested. This doesn't eliminate odor, but it gives you a believable source if there's a faint whiff in the area.
Now, about drying - this is usually the smelliest week of the whole process. If you can dry indoors with its own filter setup, that's your best bet. If you must dry in the tunnel, drop your fan speed but keep that filter running, and try to hold around 60-65°F with 55-60% humidity - still exhausting away from the road.
About your DIY charcoal sump idea: I appreciate the creativity, and it's not a bad concept, but to make it work properly you'd need much deeper carbon, uniform airflow distribution, and a sealed plenum system. By the time you build all that correctly, you've basically created a carbon can in disguise. A ready-made filter will be simpler, safer, and likely cheaper than buying 10kg of loose carbon once you factor in building the housing right.
