How Different Light Affects Marijuana Plant Growth

Growing smokable weed isn’t rocket science (seriously, anyone can pop a seed into a pot and water it) but your plants can only benefit from applying different kinds of science to every stage of their life cycle. Case in point: light is key to the success of marijuana plants, and you can use various different light sources to boost the flavor, aroma, and resin production of your marijuana crop.

Why does marijuana need light?

There are several reasons that light is a critical and important factor to the growth of marijuana but let’s head back to elementary school biology for a moment and take a look at a process called photosynthesis. 

During their daylight hours, green plants like ganja convert light from the sun into energy and release oxygen (respirate). When the lights go out, even though the plant isn’t photosynthesizing it continues to respire and that solar energy that has been stored in the cells is then converted to carbohydrates that fuel the plant’s rapid growth. So, you can see that an extended light cycle during the vegetative stage would force your plant to store loads of solar energy to fuel rapid growth during the dark cycle. 

Cannabis plants are also photoperiod, meaning that their cellular structure is naturally sensitive to the changes in hours of daylight it sees. It cannot transition from its growth stage (vegetative) to flowering until it experiences at least 12 or more hours of darkness per day – this is considered the biological trigger to induce blooming. When gardeners take advantage of the great outdoors, they can expect their plants to slip into flowering late in summer as the days grow shorter and nights grow longer. 

What’s the best light to grow marijuana?

Lighting. The jury is still out. Everyone has their favorite bulb but what works for one person might not work for another. To further that, what works for one strain might not be right for another, either. It’s trial and error, but essentially, cannabis plants love lots of bright, full-spectrum light, which can be delivered by: 

• LED (Light Emitting Diode) 
• HID (High-Intensity) 
• CFL (Compact Fluorescent Lamps) 

First, we’ll look at LED lights – these kinds of bulbs are gaining in popularity for household use because they produce the same amount of light as a regular bulb but with less energy expenditure. The bad news? The physical bulbs tend to be on the expensive side, and as a single LED bulb won’t provide enough light necessary to help your plant thrive, can be prohibitive when you realize you’ll need more than one or two. 

The next options are High-Intensity Discharge bulbs, affectionately nicknamed HIDs, for short. These bulbs produce are great for producing a lot of light, but they also produce a lot of heat. Overheating your crop is bad news bears, you’ll also need to invest in a full exhaust system if you choose this route or risk wasting your time and effort. 

Just considering the above, it should go without saying that HIDs are the most expensive light choices, but they’re old school and may win you over simply for tradition’s sake. 

HID light bulbs come as: 

• Metal Halide (most often used during the vegetative stage)
• High-Pressure Sodium (HPS) (preferred during the flowering stage) 
• Light Emitting Ceramic (aka ceramic metal halide) 

Heads Up! The individual lights listed above do not, on their own, produce the full light spectrum, and it’s often recommended to stock up on several of each in order to provide your growing plants the full range they require to guarantee a plentiful, fat crop. 

Last, we’ll look at Compact Fluorescent Lights, the kind you install in the ceiling. CFLs are popular because they are energy efficient, you don’t need a bunch of different style on hand and you don’t need to worry about them overheating your grow room (they don’t produce a lot of heat). They are pretty large, however, and they aren’t the most efficient for cannabis growth unless you are cultivating strains that tend to be short and fat. 

What does the light spectrum have to do with it?

We keep talking about the importance of the full spectrum of light when it comes to cultivating marijuana – or any plant, for that matter, and now is a great time to look at just why this is key to stronger, healthier plants. 

Everybody has seen a rainbow, a phenom that occurs when light is refracted through water droplets in the sky. This isn’t just a beautiful thing, it’s the first step to understanding how light affects growth.

Light travels in wavelengths, and studies have isolated these wavelengths in order to understand just exactly how they affect the physical properties displayed by a plant – like height, color, texture, and cannabinoid content – during its life. 

The colors of the rainbow

To get even deeper, we can break down the spectrum to see just how different wavelengths affect the marijuana plant at different stages of growth. There are seven different wavelengths to consider: 

• Ultraviolet (UV) – if you know anything about UV light it’s that it can be harmful to human beings (hence all the sun protection that blocks UV rays), and the same can be said for marijuana plants. If you raise your plants without UV rays, they will exhibit enhanced growth. 
• Violet – it seems impossible, but exposing marijuana plants to visible violet light can enhance the eventual color, taste, and aroma, amazing! Violet light also appears to strengthen the performance of the plant’s antioxidants, which can prevent plant cells from becoming damaged. 
• Blue – studies have shown that visible blue light has the largest impact on the development of a cannabis plant. Research suggests that blue light can influence the formation of chlorophyll, a chemical that allows the plant to take in more energy from the sun, as well as increase the plant’s growth and maturity rates. 
• Green – studies suggest that providing visible green light will help enhance the production of chlorophyll, which facilitates photosynthesis, and also gives your plants a greener color. 
• Yellow – this wavelength doesn’t appear to have any positive effect on the growth of a marijuana plant, in fact, studies showed that growth was moderately reduced. 
• Red – exposure to red light is crucial for the optimal development of any marijuana plant. On its own, red light has very little effect, but when combined with blue light, you’ll experience a higher yield result when flowering. 
• Far-Red – here’s a wavelength you may have never heard of, but this little known color can help speed up the Phytochrome system (light detection), which is responsible for regulating growth. Far-Red lighting can help your plant enter the “night state” more quickly, which means your marijuana plants will require less time in darkness. 

Speaking of darkness… 

The Dark Cycle

Equally important to the health and success of any marijuana crop is the time the plants spend in darkness. As we know, increased periods of darkness is the biological trigger for a marijuana plant to enter its flowering phase. 

You may not know that anything short of pitch black in your grow room could stress your plant and disrupt flowering. It can even force a female plant into an intersex plant. 

• Check your grow tent or rooms for cracks/tears and repair immediately
• Cover your greenhouse at night to ensure complete darkness for the duration of the dark cycle
• If you need to tend to your indoor grow room at any time during your plant’s designated dark cycle, use only green light, as this won’t put any stress on your delicate babies. 

The Dark Cycle and Vegetative Growth

Did you know that photoperiod cannabis plants can be kept in vegetative growth indefinitly as long as they receive 14 or more hours of light per day? 

The light schedule you settle on for your vegging period is up to you – but the debate rages over which is better: 18-6 or 24-0. Anywhere between should do the trick, although you’re welcome to experiment and see what works best for you and your garden. 

Some breeders have been experimenting with a new 6-2 schedule, with three light/dark cycles occuring during a 24-hour period. The thought process behind this scheduling is the 2 hour preriods of darkness give the plant a chance to rest and process CO2 more effectively. 

The Dark Cycle and the Flowering Stage

For your plant to start budding, you need to introduce a schedule of 12 hours of light and 12 hours of darkness. This occurs naturally when plants are outdoors, as the days grow shorter. 

Master Gardener Tip: to encourage a more successful harvest, leave plants in complete darkness for 36 hours before starting the new 12-12 light schedule – this ensures the increased presence of phytochrome red, which triggers flowering. 

Next to nutrition and potting mediums, from germination to harvest, lighting is a critical tool in your belt, and can make or break your marijuana garden. Once you’ve bought your favorite marijuana seeds from Pacific Choice Seed Bank, we say… Let There Be Light!