What to Know When Buying an Indoor Grow Light

Growing cannabis indoors is a way to be self-reliant, grow out your favorite strains, and learn as much about plants as possible. All you need to get started is a grow room, medium, nutrients, pots and a grow light.

Suppose you are a first-time grower who is unsure which is the best grow light to invest in. In that case, this article covers the different lights, what light intensity photoperiod plants need, light cycles for autos and common mistakes to avoid.

What Is the Best Thing to Check for When Buying a Grow Light?

When it comes to buying a grow light, the price of the light and how much electricity it uses may be one way to make a decision. However, light intensity is what counts! At one time, grow bulbs were measured in lumens, and the higher the lumen count, the better the bulb. In the last decade, grow lights have evolved massively, along with an understanding of what really matters before spending your hard-earned cash.

What Is the Difference Between Photosynthetically Active Radiation and PPFD?

PAR means photosynthetically active radiation, and the light waves that plants use for photosynthesis. When growing any plant, how well it can photosynthesise plays a massive role in plant health, the rate of growth, and how it converts sugar for food.

PPFD is related to PAR and stands for photosynthetic photon flux density. The PPFD measurement that a lighting manufacturer states means how much PAR the plants receive in a specific area of the plant canopy every second. Even though it may sound technical, all you need to know is that PPFD is measured in micromoles per square meter per second and is represented as μmol/m²/s.

The Different Grow Lights Available to Choose From

Not all grow lights are the same; some are best suited for clones, vegetative stage, and flowering. Below is a list of the different lights available for indoor cultivation.

Compact Fluorescent (CFL)

CFL lights can be bought as strip lights or bulb lights. They are ideal for cloning cannabis plants and the early stages of plant growth; however, they do not possess the intensity cannabis plants need for vegetation or flowering.

Metal Halide (MH)

These are the same lights you would see above you in a car park or sports stadium. They are classed as high-intensity discharge lights and are best suited for the vegetation stage.

High Pressure Sodium (HPS)

HPS grow lights are another high-intensity discharge light, and the spectrum they produce is optimal for flower production. HPS and MH lights combined are an excellent way to create the optimal spectrum for growing cannabis.

LED

LED lights have the most superior spectrum for indoor cultivation. Royal Queen Seeds advises that LEDS have a fixed light spectrum that works for both the vegetative and flowering phases of cannabis. This way, you can use the same light throughout your entire grow from seed until harvest.

How Much Light Intensity Do Photoperiod Cannabis Plants Need?

As a cannabis plant grows from seed to harvest, it will require different PAR and light intensity levels. Below is an explanation of what you should provide your indoor plants at each stage of a cannabis plant’s life cycle.

The Seedling Stage

The seedling stage is when your germinated seed grows for the first 14-21 days. At this stage, they only require low light intensity, ranging between 100-300 µmol/m2/S.

The Vegetative Stage

When a cannabis plant passes the seedling stage, it will demand greater light intensity and an increase in nutrients and carbon dioxide. Aim for a light intensity between 400 and 800 µmol/m2/s.

The Flowering Stage

When flowering cannabis plants indoors, Light intensities of 800-1000 µmol/m2/s PPFD are used, more if CO2 supplementation is available. According to Dutch Passion, during cannabis bloom, the red wavelengths power the photosynthesis that creates the buds, resin cannabinoids, and terpenes.

The Different Lighting Schedules for Autos

Autoflowering cannabis plants are different from photoperiods and do not depend on specific light cycles. Below is a breakdown of an indoor grower’s options when growing from seed to harvest.

12 Hours of Light and 12 Hours of Dark

Autos grown under a 12/12 light cycle will remain small in size and vigour. The yield will be low, and the size of buds will also be small and lack density. 12/12 is an inefficient way to grow autos.

18 Hours of Light and 6 Hours of Dark

Auto-flowering plants grown under 18/6 will be good in size and structure. The yields will be far greater than those of the 12/12 cycle, with excellent yield and bud quality. Mature plants will grow around 90 – 120 cm tall.

20 Hours of Light and 4 Hours of Dark

With 20 hours of light and 4 hours of darkness, the 20/4 schedule maximises photosynthesis while allowing for a brief rest period. This schedule can lead to higher yields due to the extended light exposure as recommended by Auto-Seeds.

24 Hours of Light and No Darkness

Autos grown consistently for 24 hours will be much more bushy with shorter internodes. Cannabis plants require darkness to allow the sugars converted during photosynthesis to be transferred to the roots, so it is best to use an 18/6 or 20/4 light cycle.

My Final Thoughts

Always check the PPFD rating provided by the lighting manufacturer before committing to a purchase. Combining metal halide and high-pressure sodium is optimal for indoor cultivation. When growing autos, avoid a 24-hour light cycle and opt for either 18 or 20 hours of light to reduce electric bills and lower the light’s heat output.

Feature photo: A large grow space that uses HPS grow lights for flower production. Photo credit: Bill Sutherland