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is hemp photosensitive

Is Hemp Photosensitive? Learn How Light Makes Hemp Reproduce!

Researchers, Hemp farmers and those interested in Hemp wonder, “Is Hemp Photosensitive?”

Hemp is very photosensitive. Changes in light patterns induces the Hemp Plant to flower and reproduce by setting seeds. The transition from long, bright days, to shorter darker days triggers the plant hormones to send the cannabis plant into its flowering and reproductive cycle. This means Hemp flowers and sets seeds according to day length; not according to the weather or physiological maturity as with other crops. This happens because of a change in the plant proteins created by hormones that are light sensitive themselves.

What does “Photosensitive” mean?

The term “Photosensitive” is defined as “a chemical or electrical response to light.” Photosensitivity is the amount to which an object (in this case, a plant) reacts upon receiving UV Light Protons. Hemp flowers and becomes reproductive when there are specific changes in the light pattern that induces it to set seeds. We can already see Light Sensitivity is the key to cause Hemp to reproduce!

The Photosensitivty of Hemp

Cannabis, like many other crops – is photosensitive. But what exactly does this mean? How does it affect the hemp industry on both a large and small scale, and what is the scientific basis for this distinction? In this article, we’ll get into all of that and more as I explain what I found when conducting research into hemp photosensitivity.  This article will be broken up into several main components – first, a general introduction onto plant photosensitivity. This isn’t a fourth-year biology class so we won’t get in too deep there, but there will certainly be links to other resources you can check out if you want to go down that route. Second, we will talk about the practical effects of hemp photosensitivity, and what this means for hemp growers. As the industry expands across the nation, both in large scale industrial operations and small-scale personal gardens, an increase in the knowledge of the particulars about hemp production has become necessary. We will close out the article with some examples of how hemp light sensitivity can positively or negatively affect its growth cycle, and give you some suggestions about how you can use this characteristic to be more successful in your hemp production – be it on a large or small scale.

Hemp has been gaining a lot of attention in the spotlight over recent years and there’s an increase in research being done to highlight the peculiarities of the plant, be it soil sensitivity, light sensitivity, or reaction to being planted alongside other industrial plants like soybeans and wheat. Most plants have some level of light sensitivity, as we can all see every Fall and Spring when the leaves turn – but hemp is especially sensitive. We’ll talk about that a little farther below, let’s get right into it.

What is Photosensitivity in Plants?

According to research cited in a paper by D.W. Williams and Rich Mundell, of the UK Department of Plant and Soil Sciences and Kentucky Tobacco Research and Development Center states “hemp is “strongly photosensitive (flowers according to day length; not physiological maturity).” What this means, is that like other photosensitive crops, say, soybeans – the hemp plant only flowers when there are specific changes in the light pattern that induce it to do so. This happens because of a change in the plant proteins created by hormones that themselves are light sensitive (if you want to go down this rabbit hole, be sure to check out this article with some interesting new scientific discoveries made by the Carnegie Institution’s researcher Zhi-Yong Wang.) In fact, many plants go through a stage where they are hormonally not light sensitive and are driven by other factors. Let’s take a look at a small piece of that article we just mentioned:

“While under the soil’s surface, in the dark, plant seedlings grow in a special way that speeds the process of pushing the budding stem out into the air, while simultaneously protecting it from damage. This type of growth is called skotomorphogenesis. Once exposed to light, seedlings switch to a different, more regular, type of growth, called photomorphogenesis, during which the lengthening of the stem is inhibited, and the leaves expand and turn green.”

It’s an interesting branch of plant science that definitely deserves further study, but you might want to brush up on a little light plant science before you can get the full gist of what they’re talking about.

In Hemp, the most noticeable aspect of light sensitivity is the flowering process. While it takes on average 100-120 days for a hemp plant to reach maturity, the plant will simply continue to grow during this period if there is no change in light conditions. Furthermore, different varieties of plants have vastly different flowering times or times to maturation – that makes planning large scale industrial grows somewhat difficult and variable. In other words, if the plant is not given a signal by a change it light, it will perpetually remain in a vegetative state, expanding its root system, branches, and leaves, without beginning to flower or produce pollen and seeds. While this is not necessarily the case with genetically modified varieties of hemp, in its natural state this is certainly the case. No change in light, no flowering of the plant. Most growers who are growing hemp simply for fiber production use the start of flowering as a marker for when to harvest, however those looking to harvest CBD will want to go through the entire flowering process.

For large scale farmers and small-scale growers, this light sensitivity can in fact have a downside. The plant’s growth cycle is completely dictated by light, so if you’re growing outdoors, planting too early may lead to a plant that is too large and unwieldy to be harvested easily – be it for flowers, seeds, or the hemp fiber itself. According to an article that cites David Williams, an agronomist, “We like to manage the plant size with the grain varieties by delaying the planting date to not allow them to get too tall.” This can also be used to the grower’s benefit – by controlling when the plant is first sown, the farmer can control the overall height of the plant to an extent. For indoor growers, once the plant reaches maturity, it can be forced to flower at any point, giving the grower a bit more leeway in how large they want their plants to get.

Photosensitivity refers to the hormonal responses a plant has to different light conditions, and we know some cannabis varieties can even change colors based on different light and temperature conditions. But what it doesn’t mean are things like light burn or slow growth due to lack of light. That kind of reaction by the plant is more a matter of just pure energy input, rather than a sensitivity to light. It’s a subtle distinction, but it’s there!

Light Sensitivity and CBD

The Photosensitivity of hemp is intertwined deeply with CBD production – because the whole process is initiated by the flowering process of the hemp plant.

There are different reasons for considering light sensitivity when growing cannabis. For growers who are harvesting hemp seed, using light sensitivity to ensure mature seed production is critical! This means that those farmers need to let male and female plants mature together, to ensure maximum seed production. The plant only truly starts to show it’s sex organs when it’s begun its transition to a flowering state, which is induced by light sensitivity, so you can see how that’s a critical factor for seed farmers and they wouldn’t want to obstruct it.

For other farmers it can also be an important point. Those looking to harvest CBD from cannabis will be aware that it is namely in the flower of the plant, along with some other chemicals that we all know and love. The CBD is located in the trichomes for the most part, although it can be found in other parts of the plant, even the root. That is why most people looking to harvest CBD are going to focus heavily on the flower of the plant. Because of this, the photosensitivity of the plant is critically important. The production of those chemicals is reduced when a plant needs to spend energy creating seeds – a process that is initiated by the pollination of the flower by the male plants. Because male plants only show their true colors after photosensitivity takes its toll on them, it’s a necessary step for cannabis farmers to identify and be able to separate out the wheat from the chaff as it were, or rather, the male from the female plants. In this way, light sensitivity is an important tool for a cannabis grower no matter what their final product is going to be.

According to research, the only difference between hemp plants made for CBD production and those made for fiber production is the actual CBD content, which needs to be 0.03% or lower for it to be considered purely a hemp production plant. According to that paper, this is analogous to the difference between sweet corn and field corn. While they are the same species, and have very little discernible difference, there is in fact a difference – one is sweet and the other is not! The excess sugar in sweet corn is analogous to the excess CBD in some hemp varieties. This difference, however, does not change the photosensitivity of a hemp plant. According to the same article, many industrial growers have pointed to the fact that hemp has been touted as a low input crop, however they say that is not necessarily true. While hemp may perform well in non-ideal conditions, if you’re looking for maximum output, the article says, you should certainly still give it the same conditions you would give to other industrial crops such as soy or wheat. This makes sense – while hemp is a robust plant that can grow well in imperfect environments, the most favorable conditions will obviously be more productive. With hemp, the same is generally true of light conditions. While it is difficult to give southern varieties too much sun, this is possible with northern varieties of hemp, so it’s important to know what kind of plant you’re working with.

So, is Hemp photosensitive? The answer is yes! As you can see light sensitivity is absolutely critical to the development of a productive cannabis plant – and is definitely an important thing to consider (if not the most important.)

Indoor Vs. Outdoor Photosensitivity

The source of light is an important consideration when it comes to the effects of Photosensitivity on the plant. Indoor lighting has different advantages to using the natural sun, so it’s important to consider your needs and do your research before deciding how you will pursue your grow, if that’s what you’re looking to do.

Anyone who has been paying attention to the recent public change in sensitivities towards hemp has noticed a huge increase in both industrial and personal hemp production. There’s a vast amount of resources online ranging from how to set up indoor grows (down to the smallest and most minute detail) to how to get certification for an industrial scale growing operation. When thinking about your own practices with regards to hemp production, it’s important to think about a critical difference between two methods of growing – indoor or outdoor?

As an outdoor grower, the elements are in charge. You simply can’t accurately predict when the last frost will be, when it will rain, snow, or when exactly your crop will be ready for harvest. You can, however, predict when the solstice will be, and when you can expect the photosensitivity of the hemp plant to start making its presence known. By doing this, you’re giving yourself a leg up on the competition, which is principally mother nature herself. As previously stated, it’s important to consider when to first plant your hemp so that you get just the right plant you’re looking for – and the answer to this question will be different if you’re growing for fiber versus if you’re growing for flowers.

While indoor growers give up the power of the sun, (something your power bills will certainly make you aware of if you haven’t noticed already) they gain complete control over the light environment of their grow, and therefore the can harness the photosensitivity of their plants to their considerable advantage. Let’s take an example scenario – you are planting hemp outdoors to produce CBD, but due to circumstances beyond your control, end up planting a little bit late in the season. By the time the days begin getting shorter, your plants haven’t quite reached maturity, or are in the earliest stages of it and still have a lot of room to grow. As such, you’re going to lose out on the potential of your crop. This would never happen with an indoor grow! First of all, the indoor grower can extend the vegetative period of the plant for as long as they want. Although the plant will certainly reach a natural limit on its size, and there is definitely diminishing returns in terms of whether or not it’s worth it to continue to  let the plant “veg,” the point here is that you as the grower are in control. I can’t tell you how helpful it is to be able to decide “No, I think these plants need a few more weeks” and at the flip of a switch (or lack thereof) continue to let your plants grow. The indoor grower can use this light sensitivity to give their plants more or less of the light the plant expects during a particular grow cycle (blue during vegetation, and red during flowering.) Furthermore, at the end of the plant cycle, the grower can choose to fully remove light from the plant if they want, which is some believe increases the production of the CBD in the flower. Another area of the plant that is photosensitive is the root structure. Hydroponic growers will know that shielding the root system from light is critical to maintaining their health.

Light Causes Hemp to Flower

Plant photosensitivity is a huge discipline and there’s a lot of information on it online if you’re looking to get more information on the general topic. When it comes to hemp light sensitivity, the bottom line is that yes, hemp is very photosensitive and this light sensitivity is what drives a hemp flowering cycle. The transition from long, bright days, to shorter darker days triggers the plant hormones to send the cannabis plant into its flowering or reproductive cycle. It makes sense when you think about it, the plant realizes that winter is coming, and it will soon die off. In order to make sure it passes on its genes; it starts its reproductive cycle – for the females this means producing flowers (and then seeds) and for the males it means producing pollen. For the grower of hemp that’s aiming for CBD, they’ll want to focus on large flower production as opposed to allowing the male flowers to pollinate the females – which would allow the females to produce seeds and take away from CBD production. Sorry males, this means you’re probably going to die.

Is Hemp Photosensitive? Yes, Photosensitivity is central to hemp production. If you’re looking into industrial growth, there are other considerations to be aware of like regulations and health concerns like soil quality, pesticides, and water pollution. If you’re interested in photosensitivity with regards to indoor growing operations, whether small or large, there’s a wealth of knowledge to be found on the topic so you should be able to easily find the resources you need to be successful.

For those of you who are simply interested in this topic for scientific interest, rest assured that because of new laws and regulations, there are more and more strict scientific studies being done on hemp and there will soon be a wide range of studies to dig into and satisfy your knowledge.


Is Hemp Photosensitive? The answer is yes, Hemp is strongly photosensitive. We learned Hemp only flowers and becomes reproductive according to day length; not according to the weather or physiological maturity. Click here to discover our collection of organic Hemp Extracts!

Discussion question: Did you know Hemp was photosensitive?

Thank you for taking the time to read Is Hemp Photosensitive? My mission is to make you an expert in all things Hemp.

To your wellness,

Jess Etchemendy

Have questions? Email me at