How Much Light Do My Cannabis Plants Need?

As a first-time grower, it can be confusing knowing exactly how much light to provide cannabis plants with. You do not want to give too little and do not want to do it over and cause light burn. Should you have many grow lights or keep it simple and only use one or two?

In this article, I explain why providing the right amount of light is essential, the ideal PPFD for indoor photoperiod plants, the right amount of PPFD for auto-flowering plants, and how much light outdoor plants need. I also break down cannabis light burn and what causes it.

Why Providing the Right Amount of Light Is Important

When growing indoors, providing your plants with light, water and carbon dioxide is crucial for photosynthesis. The more light energy a cannabis plant receives, the greater the amount it can convert into growth energy for the plant.

There is a delicate balance between providing the right amount of light and burning and bleaching the tops of the plants, which can cause tremendous stress and potentially ruin your indoor crop.

The Optimal Levels of PPFD for Autoflowering Plants

Auto-flowering, unlike photoperiods, has a much shorter vegetative phase and lifecycle, and will begin to stretch and enter the flowering phase by day 28 of growth. To be safe, an indoor grower should provide their autos for the first four weeks with 400-600 μmol/m²/s.

At the start of week 5, Autos will enter flowering mode. A good PPFD range here is 600-900 μmol/m²/s. Any higher, and you could risk light burn, your auto-flowering cannabis plants recommended by Auto-Seeds and stunting plant growth.

The Right Amount of PPFD for Photoperiod Plants

The amount of light your indoor cannabis plants receive every second in a square meter of grow space is called PPFD (Photosynthetic Photon Flux Density). Grow light manufacturers display this value on the packaging or product description.

When photoperiod cannabis plants are vegetative, they focus on growing roots, leaves, a thick and strong stem, and side branches. A PPFD level between 300 and 600 µmol/m²/s will be ideal, allowing plants to grow healthy without causing any stress.

Once you have changed your grow timer from 18 hours of light to 6 hours of dark, the plants will begin flowering. This is when they will stretch and grow as tall as possible. Then, by day 21 of 12/12, they will start to show pre-flowers, which turn into small-sized buds.

Your PPFD levels should be between 600 and 1050 µmol/m²/s to achieve robust plants capable of producing large, resinous, and terpene-rich dense buds without getting too hot and stressed.

How Much Light Do Outdoor Cannabis Plants Need?

Even outdoor-grown cannabis plants can receive too much light and UV. Cannabis can be overloaded with sunlight, particularly in equatorial or high-altitude regions with the strongest solar irradiance (light intensity). According to Dutch Passion, optimal photosynthesis typically occurs around 1000–1500 µmol/m²/s of photosynthetically active radiation (or PAR).

If you are growing in a high-altitude region, especially during June and July, a polytunnel or shading net will help reduce light intensity outdoors.

What Is Light Burn and How to Prevent It

Have you ever looked at your flowering cannabis plants and noticed they looked dry, crispy, malnourished, and some of the buds had bleached white? These are signs of heat stress and light burn, which occur when the light is too close to the buds or the intensity is out of the plant’s comfort zone.

Royal Queen Seeds advises that light burn can reduce the growth rate due to damage inflicted upon the photosynthetic apparatus. Burned leaves lose their water content, shrivel up, and turn yellow.

Signs of Light Burn and Heat Stress

Cannabis plants can become burned and suffer from heat stress caused by indoor grow lights. The plants may have stretched too close to the light when flowering, or the grow lights may be too close to the top of the canopy.

It is an easy mistake to make. However, the damage inflicted is irreversible and will significantly affect the quality of the buds and overall yields. Below is a short list of the various signs to watch for.

Curled Dry Leaves

The leaf tips will have a crisp, curly appearance, usually combined with yellowing. Plants suffering from symptoms of light burn will turn yellow from the top of the plants downwards, so it is not the same as a nitrogen deficiency.

Stunted Growth

One of the worst things about light burn is that it dramatically affects the growth rate and flower production. Stunted growth is one tell-tale sign that your plants are unhappy and will not grow at the same rate and speed as you may be used to.

Bleached buds

The first time you see a bleached cannabis bud, you may have to look twice! The entire bud and affected area turn white, ruining the colour and aromas present before the bleaching. You may notice that only the tops of the highest colas in your grow room turn albino white, and the affected buds will not be viable to harvest and are lifeless.

My Final Thoughts

Maintaining the optimal temperature and humidity levels indoors can be challenging when the grow light is too close to your canopy. Keeping the distance between the top of the canopy and your grow light between 60 and 90 centimetres will allow you to maintain a consistent PPFD range without the concern of inflicting heat stress or bleaching.

You can always place the back of your hand underneath the grow light and see if it feels too hot. Use fans to generate a fresh air current at the top of the canopy.

Feature photo: A flowering indoor cannabis grow tent full of plants using LED. Photo credit: Stoney Tark.