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For thousands of years, photoperiod-sensitive was the sole option available. Even with the advancements in artificial lighting and the rise of autoflowering hybrids, growing photoperiod strains outdoors remains a top choice for farmers seeking the largest yields.
Understanding Photoperiod
When you hear terms like photoperiod, photoperiodic, or photoperiodism, think about the amount of light in a 24-hour period. Photoperiod-sensitive strains, which bloom indoors under a 12-12 light-dark cycle, flower because they are triggered by extended periods of darkness.
Outdoors, photoperiod strains generally start flowering when daylight hours fall below 15. This means that you can expect a longer flowering period. For indoor cultivation, a 12-12 light cycle is ideal, which is why most seed packs list flowering times ranging from 8 to 12 weeks.
Typically, heavy indicas like Northern Lights complete their flowering the quickest, while sativas take the longest, with indica-sativa hybrids falling somewhere in between.
Beware of Light Pollution!
Photoperiod needs uninterrupted hours of darkness to flower properly, so light pollution—like nearby street lights or any interruptions in the dark period, even briefly—can be problematic.
If photoperiod plants don't get the necessary consistent darkness, they might revert to the vegetative stage, develop hermaphroditic traits, produce lower yields, or even fail to flower entirely.
To check if your location is affected by light pollution, see if you can still read large text in a magazine at night. If so, your growing area might not be optimal.
How to Prevent Light Pollution When Growing
To avoid light pollution and ensure your photoperiod thrives, follow these tips:
Pick a Remote Location: Choose a growing site away from artificial light sources like street lamps or busy streets.
Light-Proof Greenhouses: If you're using a greenhouse, cover windows with blinds or tarps at night. You can even automate this process to maintain a consistent light-dark cycle.
Use Blackout Tarps: If growing outdoors, set up a simple structure that allows you to cover your plants with a blackout tarp during the dark period.
Avoid Light Interruptions: Even brief exposure to light can disrupt your plants. Refrain from using your phone’s light near your plants during the dark period.
Use Night Vision Lamps: If you need to inspect your plants at night, use a "dark night" lamp that emits green light, which is less likely to stress your plants.
Consider Autoflowers: Autoflowering strains are not affected by light pollution or interruptions in their light cycle, making them a good alternative if light control is challenging.
Photoperiod vs. Autoflowering Strains:
The key difference between photoperiod and autoflowering cannabis lies in how their flowering stages are triggered:
Photoperiod: This type depends on the amount of light it receives to transition between its vegetative and flowering stages. Plants require specific light-dark cycles to initiate flowering. Typically, they flower when the light cycle changes to 12 hours of light and 12 hours of darkness.
Autoflowering: Autoflowers automatically switch to the flowering stage based on age rather than light cycles. They begin to flower after a set period of growth, usually between 2 to 4 weeks from germination. This characteristic means that autoflowers can grow and flower in a wide range of light conditions.
Key Differences:
Flowering Triggers:
Photoperiod: Triggered by changes in light cycles.
Autoflowering: Triggered by age, not light cycles.
Growth Flexibility:
Photoperiod: Requires careful management of light schedules and is sensitive to light interruptions.
Autoflowering: More adaptable to varying light conditions and less affected by light pollution.
Growth Duration:
Photoperiod: Generally has a longer growth cycle with separate vegetative and flowering stages.
Autoflowering: Typically has a shorter growth cycle as they transition to flowering on their own schedule.
Yield and Size:
Photoperiod: Often yields more and grows larger, benefiting from longer vegetative periods.
Autoflowering: Generally has a smaller yield and size due to the shorter growth period.
Cultivation Environment:
Photoperiod: Best suited for controlled environments where light cycles can be managed.
Autoflowering: Suitable for both indoor and outdoor grows, and more resilient to varying conditions.
Photoperiod Strain Genetics
Typically, photoperiod strains are more potent than autoflowering varieties. This is largely due to the presence of ruderalis genetics in autoflowers, which tend to have lower cannabinoid concentrations, resulting in lower average levels.
While modern autoflowers have made significant strides in potency, they are only recently starting to narrow the gap with photoperiod strains. However, photoperiod strains still generally offer higher average cannabinoid concentrations.
Response of Sativa Strains to Photoperiod
Sativa strains, originally from tropical regions, thrive in abundant sunshine and extended growing seasons. Due to their native environment, they are less sensitive to fluctuations in daylight hours, as equatorial regions experience nearly equal day and night lengths throughout the year.
Many pure sativa strains, such as Hazes, can start flowering under a consistent 12/12 light cycle from the outset. After completing their vegetative growth phase (which typically takes several weeks), they will continue to flower effectively under the same light schedule. However, a downside is that sativas generally have a slower growth rate and require a longer flowering period to mature.
Response of Indica Strains to Photoperiod
Indica cannabis strains, which originated in more northern latitudes, are well-adapted to respond quickly to changes in light cycles. They are designed to flower faster than sativas, having evolved in climates where daylight hours shorten rapidly as winter approaches.
Indicas generally flower effectively under a 12-hour photoperiod, but some strains can also flower under a 14/10 or 13/11 light-dark cycle, depending on the specific variety. However, providing less than 12 hours of darkness is not advantageous for photoperiod indicas. Reducing the dark period can delay bud maturation and negatively impact yields.
Critical Factors of Outdoor Cultivation:
The Life Cycle Annual plant and, as a short-day plant, it is typically ready for harvest in the fall. Generally, if planted in the spring after the equinox, will transition from vegetative growth to flowering after the summer solstice, with harvest occurring in the fall. The amount of sunlight each plant receives will determine the growth pace.
For greenhouse growers, tarps and light-proof plastic sheeting can be used to reduce sunlight and speed up flowering. However, this requires constant vigilance to manage light exposure and remove shade materials before sunrise.
Response of Sativa Strains to Photoperiod Sativa strains, which originate from tropical regions, are adapted to long growing seasons and stable light conditions. They can begin flowering under a 12/12 light cycle, but they often require more time and patience due to their longer flowering periods. Understanding their response to light changes is crucial for successful cultivation.
Location, Location, Location In the Northern Hemisphere, the outdoor growing season spans from April to November, with shorter summers and colder winters as you move north. Conversely, in the Southern Hemisphere, the growing season is from August to April. Research is essential unless you are near the equator, where daylight hours remain relatively constant year-round.
Sunrise and Sunset Data Accurate sunrise and sunset data is crucial for planning your grow. Use online calculators to analyze monthly daylight hours in your area. Determine when you will have 18+ hours of light for vegetative growth and when daylight drops below 12 hours to estimate your harvest window. Keep in mind that outdoor photoperiod cannabis will flower gradually as nights lengthen.
Weather Forecast Consistent sunshine is vital, but so is monitoring the weather. Review historical weather data and forecasts to ensure optimal growing conditions. Thrives at temperatures between 20–28°C. If you live too far north, the growing season may be limited, making hardy autoflowering hybrids like Blue Cheese Automatic more suitable. In more favorable regions, exotic strains like Kali Dog and Chocolate Haze can be grown.
Site Inspection Once you've conducted your research, visit your grow site to ensure it's free from obstacles that cast shadows and is secure from prying eyes. A private, unobstructed space is ideal. Use chicken wire or fencing to protect your plants from herbivores and rodents.
Genetics Choose seeds that are suited to your local climate. One advantage of photoperiod strains over autoflowers is the ability to take cuttings from successful plants. If you find a strain that thrives in your garden, clone it to replicate your success.
Climate Change Climate change poses a growing threat to outdoor cannabis cultivation. Extreme weather events, like the 2017 wildfires in Northern California, can devastate crops and affect quality. Stay informed about local weather patterns and be prepared for potential challenges to safeguard your cultivation efforts.
Pro Tips
Starting your cultivation indoors can be a smart move. A sunny windowsill is ideal for delicate seedlings if the weather is favorable. Alternatively, if it's still too cold and rainy outside, use a 200W cool white CFL to support vegetative growth. Combining indoor and outdoor growing isn't a compromise—it's leveraging the best of both environments.
For outdoor cultivation, use cannabis-specific soils in white plastic pots or Smart Pots. Purchase high-quality, cannabis-specific soil from a local grow store. When growing in the ground, your primary task will be to stake plants with bamboo for support. Containers offer the flexibility to move your plants as needed, especially as seasons change. Since the Earth orbits the sun, you may need to reposition plants on a balcony or terrace throughout the day to ensure they receive ample direct sunlight.
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