Autoflowering cannabis genetics revolutionized indoor cultivation since they flower independently, unlike traditional light cycles associated with photoperiod cannabis plants. However, different light cycles for autoflowers significantly affect a plant’s performance, including size, yield and stature.
This article will cover everything you need to know, including what makes autos unique, how different light cycles compare, and which grow light is best for auto-flowering cannabis indoors.
What Makes Autoflowers Unique in Terms of Light Needs
Autos are a stress-free and superb way to cultivate cannabis because they don’t rely on light cycles and, with consistent lighting, can achieve incredible results, making them optimal for a first-time grower or one with minimal experience.
This means autos can perform well outdoors throughout the year and thrive during summertime. A grower can create a perpetual harvest cycle outside and grow small to medium-sized plants ready to harvest within 10-11 weeks.
Why autoflowers don’t rely on light cycles to flower
Autoflowering genetics are a cross of photoperiod cannabis plants combined with ruderalis genetics. Initially found in Siberia, the ruderalis cannabis plant evolved to grow for 4 weeks before automatically inducing flower production due to the cold and harsh climate associated with Siberia. This genetic trait allows auto-flowering plants to be grown indoors without adjusting a grow timer from 18/6 to 12/12.
Importance of consistent lighting for maximizing yield and potency
Cannabis plants require light, water, and carbon dioxide, which are converted into glucose and oxygen. This means the more light a plant receives, the more energy it can convert. Cannabis plants require consistent, intense lighting to promote the maximum amount of growth, nutrient uptake, and their metabolism.
This is one reason why auto-flowering plants require intense light to maximize yield, resin, terpene production, and overall potency. If there is too little light, the yield and potency will be below par, so finding the right balance is key to optimal plant health and potential.
Best Light Cycles for Autoflowers Explained
So, you ordered your pack of autoflowering cannabis seeds and have your grow light, growing medium and nutrients ready, but you may be wondering, what is the best light cycle for autos? Below is a breakdown of the different light cycles that can be used indoors and why.
The 18/6 light cycle – the most popular option
This light cycle means the plants will receive 18 hours of light and 6 hours of darkness. Indoor growers will use an 18/6 cycle to keep photoperiod cannabis plants, clones and mother plants in a state of vegetation where they can focus on a prolific root mass, growing stems, leaves and side branches.
Once the plants mature and are ready to harvest, the 6 hours of darkness will promote autos to grow to a height of around 100-125 centimeters. This light cycle is often the most popular choice and provides the perfect ratio of light and darkness that autos need. Compared to a 20/4 light cycle, a home grower can reduce their electricity output by 14 hours per week, making 18/6 the most efficient and financially practical.
The 20/4 light cycle – more light for faster growth?
Setting your timers to 20 hours of light and 4 hours of darkness is also a great way to grow auto-flowering indoors. However, darkness plays a significant role in how much the internodes of a cannabis plant grow and stretch, affecting the final height once ready to harvest. Whilst 20/4 is well suited for autos, energy consumption will be much higher, and the temperature and humidity levels may be more challenging to maintain and keep consistent.
24/0 light cycle – does constant light help or harm?
You may be thinking that the more light cannabis plants receive, the better. How would 24 hours of light with zero hours of darkness compare? The darkness period is when cannabis plants are effectively sleeping and convert water to glucose, which is then channeled to the roots to attract bacteria and beneficial microorganisms to aid with plant growth and performance.
Autoflowering cannabis plants grown under 24 hours of lighting will grow shorter compared to 20/4 and 18/6, with minimal spacing between the internodes. When a cannabis plant has a short, stocky, and bushy characteristic, air flow and light penetration will be affected, which can cause yields to be less superior than those of autos that receive a darkness period of 4-6 hours. Besides the fact that the temperature and humidity will be tough to control, it is a worst-case scenario regarding electricity bills.
Comparing light cycles: yield, efficiency, and energy consumption
| Light Cycle | Yield | Efficiency and energy consumption |
| 18/6 | The yield will be excellent, and the buds produced will be top quality, with the plants being the tallest they can be once matured. | 18/6 is the most popular choice and allows the plants 6 hours of darkness to convert water into sugar for the roots. Temperature and humidity will be far easier to control and maintain and 18/6 will also be the most efficient as far as energy consumption and electricity bills are concerned. |
| 20/4 | The yield will also be excellent and may be slightly more than the 18/6 light cycle, however plants may be shorter than the 18/6 cycle, | A beginner level grower may find it harder to maintain the temperature and humidity. 20/4 light cycle will increase energy bills as it will use 14 hours per week more light than 18/6 and produce slightly shorter plants with a more compact shape and structure compared to 18/6 or 24/0. |
| 24/0 | The yield may be less than the 18/6 and 20/4 light cycle, as plants will not have the opportunity to convert water to glucose to help feed the beneficial microorganisms and bacteria in the root zone. | A grow light and ballast left on for 24 hours a day will become extremely hot and present a challenge for growers as far as temperature and humidity control is concerned. Plants will also transpire water at a much faster rate than the 18/6 or 20/4 cycles. The electric bills will also be as expensive as can be, as this light cycle will use 28 hours of electricity consumption more than the 20/4 cycle, and 56 hours more than the 18/6 cycle. |
Choosing the Right Grow Lights for Your Autoflowers
When deciding which indoor grow light is best suited for autoflowering cannabis plants from seed to harvest, you will typically choose either LED or high-pressure sodium. Below is an explanation of the difference between the two for indoor horticultural use.
LED vs HPS: Which works best with long light schedules?
Both LED and HPS work very well with long light schedules. However, you must consider which lights are most efficient regarding wasted heat energy, the number of watts used, the photoactive radiation, and how that converts into electricity costs.
LEDs
These grow lights are extremely popular among growers due to the low levels of heat they produce, allowing growers to keep their lights much closer to the canopy. LEDs are also an excellent choice for indoor growers as they provide a far more efficient light spectrum and photoactive radiation (P.A.R) combined with their low energy consumption compared to HPS setups.
HPS
High pressure sodium lights require a ballast, reflector and bulb that can produce much wasted heat energy over long light cycles. As a result, the light cannot be as low to the top of the plant canopy compared to an LED, and the growing spectrum and P.A.R levels are not as efficient as LED models. Sometimes, a 600w or 1000w HPS light can use two to three times more watts than a highly efficient LED-manufactured grow light, resulting in a more significant energy bill.
Understanding PPFD and spectrum for each stage
PPFD stands for Photosynthetic Photon Flux Density, and it measures how much P.A.R. (photoactive radiation) hits the plant's canopy. PPFD is expressed as μmol/m²/s, which means micromoles per square meter per second. It is measured using a digital light meter, and the sensor is placed at the exact point where you want to test the intensity of the P.A.R levels. Below is a list of the best PPFD levels for autoflowering plants.
The seedling stage: Autoflowering cannabis plants do not require a lot of intense light during this stage. Using a PPFD of 200-300 μmol/m²/s will be perfect.
The vegetative stage: PPFD can be increased as the plants use more nutrients and grow much faster. Using a PPFD of 400-600 μmol/m²/s is ideal.
The flowering stage: At this stage, plants require as much intense light as possible to produce terpenes, trichomes, and dense buds. Aim for a PPFD range between 600 and 900 μmol/m²/s.
How DLI (Daily Light Integral) influences autoflower growth
Daily light integral is a way to measure the amount of light received over 24 hours in a square meter calculated as per square meter per day (mol/m²/d) using the following formula:
DLI = PPFD (µmol/m²/s) × Daily Light Hours (hours) × (3600/1,000,000)
As technical as it may sound, DLI influences auto flower growth significantly and will determine if the plants are receiving too little or too much light based on the recommended PPFD values for each stage of plant growth provided above.
