What Really Makes Plants Flower and Veg? PR and PFR Explained
We all know you turn your grow lights on for 18 hours on/6 hours off during the vegging cycle and 12 hours on/12 hours off during the flowering cycle (and if you didn't, you know now). But have you ever wondered why you have to give plants that specific amount of light? And what about people who let their plants sleep for longer, or don't let them sleep at all? In this article, we'll tell you why plants love the light cycles they do, and some drawbacks of over/under lighting your plants.

Phytochrome Red and Phytochrome Far Red: What's Triggering Your Plants to Flower?

Before we get into specifics let's get a little scientific, shall we? Leaves have light receptors that bring in certain wavelengths of light to help them grow. Two receptors that regulate vegging and flowering growth are Phytochrome Red (PR) and Phytochrome Far Red (PFR). They bring in red wavelengths from the sun (or your grow lights) to either prevent or allow flowering. Now hold on because here’s where things get tricky:
Spectrum and Defuse Colors
PFR tells a plant to not flower, and it's triggered when you expose your plants to light containing far red (760/800 nm) light. You manipulate the amount of it your plant’s getting by regulating the amount of darkness you allow your plants to be in. PR takes in red (660/760 nm) light and remains neutral throughout the plant’s life, so when your lights are on PR and PFR are in balance. However, once you turn your lights off that PFR starts to turn in to PR, and once they come back on that new PR starts to turn back into PFR. From Vegging to Flowering Early on when you’re vegging, PFR told your plants to stop flowering. The more darkness you give your plant the more PFR will turn to PR. At 0-6 hours of darkness, lots of that PR reverted to PFR. At 12 hours of darkness over time there won't be enough PRF to tell your plants to stop flowering. Hence, they start to flower. Any time your plants are exposed to light PFR will try telling your plants to stop flowering. That’s why it’s so important to not have light leaks in your grow tent, grow room, or garden during flowering: it takes almost no effort for that new PR to go right back to PFR and tell your plants to keep vegging, which extends your flowering time. Light cycles tend to mimic a plant’s natural growth outdoors, but we don’t run them simply to mimic outdoor conditions. Rather, we do so to make sure plants get all the light they need to veg and flower the proper amount of time to give you the yield you want.

PFR: the Hormone That Keeps on Vegging

When it comes to vegging your plants there are two main lighting cycles: leaving your lights on for 18 and shutting them off for 6 hours or running your lights 24 hours a day throughout the vegging cycle. Your plants need lots of light to develop a strong root system so it can grow foliage to capture more light for the flowering and fruiting stages.
Those who use 18/6 light cycles do so because it’s closer to the natural light plants get from the sun. It’s also not a bad idea to let your plants rest for a little while, especially when they’re young (you don’t want to burn young plants). But while plants naturally get to a rest period, it’s not necessary for the vegging stage. Remember that PFR tells your plant to avoid flowering, and it won’t convert to the neutral PR as long as it receives lots of light. You’ll need around 14 hours of light to make sure a lot of your plants’ PFR doesn’t convert to PR. Less than that and your plant may start flowering earlier than expected, but any more light than that won’t harm the plant. We all know that during the vegging stage your plants are stretching out to get a light to keep growing roots and foliage for flowering sites. Well, the longer they grow without 10+ hours of rest the bigger they can become, which means potentially more flowering sites and fruit. It makes sense, then, why some grower opts to run 24 hours of continuous light during the vegging stage. You can also extend the vegging time to get them where you want them to be before the big switch. But remember: never let your plants get less than 14 hours of light during the vegging stage unless you want to start flowering early.

Photoperiodism: Turning Your Vegging Plants into Flower Machines

When it comes to the flowering stage of growth one word you should know is photoperiodism. That's the way some plants, animals, and organisms physically change in response to the amount of light it receives. In plants, photoperiodism is what determines the threshold of light you need before it triggers hormones to start flowering. Once you go from a 24hr or 18/6hr to a 12/12hr lighting cycle your plants will slowly convert the flower-restricting PFR into neutral PR. Usually after about 2 weeks of this cycle to completely neutralize PFR, but your plants are very sensitive to light during this time.
Illustration of short vs long day plants growing stages
Any disruption in darkness can have a strong effect on plants. It takes a little while for a plant to switch from vegging to flowering, but it doesn’t take more than an hour of light to set your plants back. Will they completely stop flowering? Of course not, but depending on how much light your plants are accidentally exposed to and the length you could be confusing your plant. It’ll have a hard time going from veg to flower and back, which results in poor fruits and flowers come harvest time. Short vs Long Day Plants Plants are known as short-day plants while others are long-day plants. All that means is that your plant will experience photoperiodism when either a certain amount of darkness is allowed to the plant or a certain amount of light is given to a plant (usually never more than the threshold for your plant’s PFR trigger). Will moonlight affect your plants? If you’re growing outdoors moonlight will not send your plants back into vegging. The moon’s glow is a reflection of the light from the sun, and the light it casts on to the earth (and your garden) is only around 5-10% of the sun’s strength at the most. This isn’t nearly enough to trigger any significant change in your plants.

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