By Luke Sumpter

Light stress chiefly impacts indoor cannabis plants. Within this environment, growers are tasked with balancing a host of variables, including the distance of the grow lamp from the canopy, and its intensity. Position your lamp too close, and you’ll encourage light burn. Place it too far away, and your plants will underperform and stretch undesirably toward the source. Whether you’re a novice grower wondering how to fix light stress or a veteran cultivator looking to enhance your knowledge, you’ll learn everything you need to know about light burn below.

To start things off, you should know that plants technically cannot receive “too much” light; photons themselves are not the issue. Rather, an imbalance between lighting and the other resources that cannabis requires leads to light stress—the demand for water, carbon dioxide, and nutrients increases in step with light intensity.

If you hang an overly powerful light over your plant but fail to adjust these parameters, you’re going to run into trouble (in the form of light stress). Additionally, it’s the heat emitted from the light, rather than the photons themselves, that causes light burn. Position your light incorrectly, and you’ll literally cook the upper portions of the canopy.

With that cleared up, let’s take a look at the signs of light stress, how to solve it, and how to prevent light burn in the first place.

Signs of Too Much Light on Cannabis Plants

Bleached Buds

One surefire symptom of light burn is bleaching of the flowers. This phenomenon occurs when flowers are located too close to high-powered lights. You may have seen images online of pure white “albino” cannabis flowers. This might look like the intentional development of rare genetics, but the fact is, most of the time this is simply bleaching. Luckily, it’s very hard to miss your flowers turning bright white.

Although white buds may look interesting, most of the time they have been rendered useless. The heat degrades cannabinoids present in the resin, which causes buds to lose potency. The scent and taste of these buds will also be less than desirable. The terpenes responsible for these traits are highly volatile aromatic hydrocarbons, and excess heat will also cause them to degrade.

Bleached Buds

Yellow Leaves

Yellow leaves are another sign that your plant is being subject to light burn. However, yellow leaves can also signify nitrogen deficiency. The difference is that yellow leaves caused by this nutritional deficiency start from the bottom of the plant, display significant wilting, and will either fall off or are extremely easy to remove. On the other hand, yellowing caused by light burn will occur at the top of the plant, and these leaves will be much sturdier and harder to remove.

Yellow Leaves

Stunted growth

Light burn can reduce the rate of growth due to damage inflicted upon the photosynthetic apparatus. Burned leaves begin to lose their water content, shrivel up, and turn yellow. Due to a lack of water, structure, and chlorophyll, affected leaves cannot conduct photosynthesis. During the early stages of development, when plants have few leaves, this can stunt growth and impact plant health further into the growing cycle.

How to Solve Weed Light Burn

The first thing you should do is move your plants a bit further away from the lights. This can either be done by moving the plants or by moving the lights, if you have the space. Take into consideration the most affected areas of each plant when choosing a new location for them. You can also remove some of your lights. If you have these well organized, it shouldn't hurt your plants to remove a few bulbs.

If reducing the light or moving the plant isn't an option, you can always low stress train your plants. This is a technique for obtaining more yields, where you try to keep the plant's branches all at the same height. As the stem gets taller, it should be bent sideways and kept in that position with an external aid.

Try decreasing the temperature of the room too. Do this very slowly and always keep the temperature consistent. The plant will need time to adjust. Any change you'll need to make to a plant's environment or medium needs to be done carefully and patiently. It won't matter how well you water and feed your plants, if you don't take proper care of the environment, the plant will die.

How to Prevent Too Much Light for Plants

Now, let’s explore how to avoid providing too much light to begin with. The distance between your light and your plant(s) remains key here. It all boils down to the type of light in question. Typically, most indoor home growers achieve good results with a power distribution of 400–600W per m².

When using a metal halide (MH) or high-pressure sodium (HPS) lamp within this power range, aim for these distances:

  • 400W: 1 – 1.6 feet
  • 600W: 1.1 – 2.1 feet

If you prefer light-emitting diodes (LEDs), then position your lights within these ranges:

  • 400W: 1.3 – 2.5 feet
  • 600W: 2 – 2.1 feet

You can also use a lux meter to gauge where to place your lights. The metric of lux (lumens/m²) can help growers nail light positioning further, and thereby optimise plant performance. Aim for these lux ranges during the three main stages of the growing cycle:

  • Seedling stage: 5,000–7,000 lux
  • Vegetative stage: 15,000–50,000 lux
  • Flowering stage: 45,000–65,000 lux

Can Plants Get Too Much Light?

You can indeed expose cannabis plants to too much light, if you don’t have additional resources to help them handle this stimulus. Below, we cover optimal light cycles, more technical terminology when it comes to measuring light, and why you need to think about space before investing in a grow lamp.

18-Hour vs 24-Hour Light Cycles

How much light should you provide to your plants during different stages of growth? Photoperiod plants need 18–24 hours of light during veg, and 12 hours of light during bloom to remain in the flowering stage. Autoflowering plants can receive 18–24 hours of light during the entire growing cycle, as they don’t require a certain light schedule to initiate flowering. But which cycle should you opt for during the photoperiod veg stage and the entire autoflowering life cycle?

A debate rages around this topic. Some growers note excellent results when maxing out the light cycle, whereas others claim it makes little to no difference. However, because light drives photosynthesis, and photosynthesis drives growth, you can expect to see a faster rate of growth when keeping your lamp on for 24 hours each day.

So, why don’t all growers follow this strategy? There are two main reasons. First and foremost, it costs more to run a lamp for this amount of time. If you grow cannabis indoors all year long, you’re looking at a substantial increase in your electricity costs. Second, increased photosynthesis and growth require higher quantities of water and nutrients. The longer you keep your lamp running, the more you have to tend to your plants’ needs across different phases of growth.

PPFD and Lux Meters Explained

We’ve briefly touched on lux meters—devices that measure the amount of lumens across a particular surface area. While some growers value this metric, it doesn’t tell us the whole picture. Why? Because lumens are a measure of light visible to the human eye. However, our eyes are only sensitive to a portion of the spectrum that plants use for photosynthesis.

Therefore, many cultivators prefer to measure photosynthetically active radiation (PAR)—the range of photons that plants convert into energy. This is done by measuring photosynthetic photon flux density (PPFD) using a PAR meter. These devices inform growers of the amount of photosynthetically useful light hitting an area of 1 square metre per second (µmols/m²/s). Aim for a PAR level of between 300–400 µmols/m²/sec when positioning your lamp.

More Light Means More Resources

If you want faster growth over a shorter period, it makes sense to use a stronger light. However, this benefit comes at a cost. An increased metabolism through higher rates of photosynthesis means you’ll have to match PPFD with other plant demands. These include:

  • More nutrients: Your plant will require higher levels of key nutrients, particularly nitrogen, when under more powerful lights. Increase your weekly dose if you’re using synthetic nutes. If you’re an organic grower, liquid seaweed and fish emulsion are good sources.
  • More water: You’ll need to keep a keen eye on plant hydration. Observe frequently for a loss of turgidity, and water every time the top few centimetres of the growing medium dries out.
  • Dial in temperature and humidity: Stronger lights give off more heat. Use fans to help reduce excessive temperatures and a humidifier to stop foliage from getting too dry.
  • Consider carbon dioxide: Using stronger lights will give you the opportunity to leverage supplemental CO₂. This combination can increase plant resistance, speed up growth, and result in better yields.
18:6 Ratio
24:0 Ratio
12:12 Ratio

Match Lighting With Space

Select a light that’s compatible with your growing space. If you’re micro growing, you can get away with a small 200W LED. If you’re growing in a 2 × 2 × 2m tent, you’ll need a larger and stronger light to make the most of your environment. It might be tempting to place a powerful 600W light in your tiny grow tent for the best yield possible, but things can easily backfire. You’ll find your plant desperate for more nutrients and water, and (depending on pot size) it’ll likely start to grow too close to the lamp and suffer from inevitable light burn. Dial in PPFD, feed your plants well, keep them hydrated, and you’ll watch your plant hit its genetic potential—without frying it in the process!

How Much Light Does a Cannabis Plant Need?

Cannabis plants need enough photons to survive, but they can handle plenty more than this. Technically, cannabis plants cannot receive too much light. Autoflowers and vegging photoperiods will thrive under a powerful light source for 24 hours each day, provided they receive enough nutrients, airflow, water, and humidity to deal with the sharp rise in metabolic processes. However, most home growers strike a balance between optimal performance, energy costs, and plant health. To achieve this, just follow the recommendations above for light distance, lux, and PAR. Happy growing!

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