Growing cannabis - Is light leak a myth?

Published Oct 19, 2022 09:00 a.m. ET
iStock / Jeremy Poland

Now that we’ve made it easy, even for the least experienced grower to utilize the best and most innovative technology to produce a top-shelf product, more than ever before are doing their best to perfect their approach. Going the extra mile with some grow lights, and high-quality nutrients to benefit long-term where yield is concerned makes absolute sense, but not all commonly implemented precautions are necessary to take.

What is light leak?

Light leak is one of the most highly debated topics among amateur growers, and it’s a term used to describe when light makes its way into a grow room uninvited.

UV from the sun is often the most concerning, as is light leak in situations where cultivators are turning out more than one crop at a time, with both requiring different lighting schedules. There is less disagreement over exposure to stray rays of light from regular household bulbs, which aren’t as strong, but with so many UV lights being utilized indoors these days some say it’s worth noting the potential for light leak.

Why is light leak a problem?

The main reason growers do all they can to combat light leak is that too much exposure to sunlight can interrupt the plant's ability to know how and when to grow based on the way it perceives the season in its immediate environment, something that is controlled by strictly timed grow lights to optimize yields, bud size, and potency.

There is some truth to this idea, as photoperiodism depends on maintaining certain day and night durations. However, the requirements of growing cannabis plants do leave some room for error. A dim glow from the outer edges of a curtain likely isn't going to confuse your crop, but a wide-open window with a bright stream of powerful sunshine could be enough to cause a big problem.


Most cannabis cultivars today are what is referred to as short-day plants, which means that they flower during the shortest days of the year, an evolved feature to allow for vegetative growth in the hottest months, and seed production before the harshness of winter takes over. This response is mimicked in controlled environments with grow lights, and times nights, but an extra hour of sunlight isn’t going to be enough to throw a whole crop off course.

Is it something all growers should worry about?

Yes, growers requiring the most control over a crop for commercial purposes are absolutely going to want to pay attention to light leaks, and even small-time, personal cultivators may want to hang a blackout curtain or keep the door closed (for sanitation reasons) but small leaks aren’t going to be too much of a problem, so there’s no need to go taping up every single crack or crevice in a room, which could reduce air quality and contribute to mould growth.

In conclusion

Having control over your grow lights to maintain a clear day/night regimen is an excellent idea, but there’s no reason to panic if there is a little stray light that makes its way into your grow room or greenhouse at night.

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