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The Cannabis Light Cycle

Feb 10, 2020
Lighting Problems

A Guide to the Cannabis Light Cycle

When cultivating cannabis, the most crucial part is attaining a great yield that brings profit (and enjoyment) when it’s harvest time. However, the cultivation process is often tricky for beginners, with success depending on an important factor: proper lighting.

Good lighting is a vital part of cannabis cultivation, whether you’re growing indoors or outdoors. The light cycles used during flowering are directly correlated to the overall yield and quality of the crop. You’ll need light for successful growth, and in this guide, you’ll learn the basics of the cannabis plant’s light cycle.

Light Cycle During the Vegetative Stage

The vegetative cycle occurs when your plants are growing, and it’s essential to the health and success of your crop. During this time, the leaves and stems of your plants will grow taller and larger. However, the plants won’t produce buds, which means you’ll have to monitor their shape and size. Here’s where the light cycle comes in: you can manipulate your plants’ lighting for better growth. The more light your plants get, the better they’ll grow and the bigger your yield will be.

When cannabis plants have entered the vegetative cycle/stage, allow them to receive at least 18 hours of light per day (a cycle sometimes referred to as 18/6). If you’d prefer plants that are as large as possible, keep them under lighting for 24 hours per day.

Marijuana plants don’t start to flower until they begin to receive 12 consecutive hours of darkness; otherwise, they’ll stay in the vegetative stage. Even if your crop is exposed to 13 hours of lighting each day, it will stay in the vegetative stage.

The Inside Vegetative Stage

Light isn’t the only way to yield large plants during the vegetative stage. If you’re cultivating cannabis indoors, you’ll need a proper garden room with an elevated ceiling. It’s important to consider that some strains, such as Jack Herer and Northern Lights, typically produce higher yields even when these techniques aren’t used.

The Outside Vegetative Stage

In most cases, growers start their plants indoors under proper lighting before bringing them outside for some natural sunlight. Most cultivators start cutting clones or planting seeds during March and April, keeping the seedlings under 18-24 hours of continual light before taking them outside in May or June.

If you’re planning an outdoor cannabis garden, it may help to keep your plants inside until dangers such as frost have passed for the season. A surprise drop in temperature or a late-spring snowfall can have disastrous results for your crop. However, once these risks have gone away, your plants will remain in the outdoor vegetative stage from late spring into late summer.

The Light Cycle During the Flowering Stage

For your cannabis plants to shift from the vegetative stage into the flowering stage, they’ll need at least 12 hours of daily darkness. Most gardeners that grow plants indoors start on the 12/12 light cycle once their crop has reached the right shape and size during the vegetative stage. Many growers allow an indoor vegetative stage of 3 to 16 weeks under 24/0 or 18/6 light periods.

To successfully cultivate cannabis indoors, you’ll need to closely imitate its natural growth patterns. When plants are grown outdoors, they begin to flower as the days gradually get shorter, receiving at least 12 hours of darkness. To mimic this, simply switch from an 18/6 or 24/0 hours light/darkness cycle to a 12/12 hours light/darkness cycle.

Flowering Outdoor-Grown Cannabis

If you want to grow an outdoor crop, just let nature take over. By doing so, you’ll allow your plants to bud on their own, which typically happens after June 21 as the days get shorter. This doesn’t mean the plants will stop growing or flowering; rather, they’ll likely double in shape and height after entering the budding stage. It’s important, though to ensure that your plants aren’t exposed to any light during the 12-hour darkness period. Even a misplaced streetlight or flood light can disrupt a crop’s flowering period.

Light Cycle Scheduling Provides a High Yield

Have you decided whether to grow cannabis indoors or outdoors? It’s crucial to understand cannabis seedlings’ light cycles in order to choose the right lighting schedule for your crop. By creating a lighting plan, you can ensure a maximum yield come harvest time.

If you’re growing a crop indoors and there are no issues with room area or height, just give your plants an 18/6 or 24/0 light cycle during their vegetative stage. This should go on for about 60 days.

Conversely, if you’re planning an outdoor garden, the light cycle for optimal flowering should also be set at 24/0 or 18/6. However, you’ll need to keep the plants indoors until dangerous conditions such as overnight freezes and snowfalls have passed. After that time, it’s okay to bring the plants out to grow and thrive in natural sunlight and open air.

From the time immediately after the vegetative stage, it will take approximately three to six months to achieve a bountiful, enjoyable harvest.

Is It Possible to Provide Your Plants with Excessive Light?

Though it is theoretically possible to give cannabis plants too much light, it’s quite rare when lamps are positioned at the correct height. Bleaching or light saturation is indicated by oversize white spots on the leaves. Narrow-beam angle LED lights without the correct TIR lenses are more likely to cause this. Heat damage is a much greater concern. Though the ambient indoor temperature may be within the appropriate range, the temperature at plant level is much more important. If your grow room isn’t equipped with a thermometer, put a hand near the top of the plant canopy. If you’re uncomfortably hot, your plants will be equally stressed.

Autoflowering Plants

With auto-flowering marijuana plants, things are done a little differently. These plants will bud under any of the above light cycles, which is a significant advantage. You’ll be able to cultivate auto-flowering plants alongside those in the vegetative stage, or you can have several auto-flowering plants at varying growth stages.

Unfortunately, stray light beams may still cause issues with these plants. It’s important to ensure that dark periods are fully dark with no irregular light exposure. Most cultivators grow auto-flowering plants under an 18/6 light cycle for their entire lifespan. Some go with a 20/4 light cycle, but it’s quite uncommon to keep auto-flowering plants at 20/4 once they’ve grown out of the seedling phase.

Light Disruption and Power Outages

Despite the prevalence of automated grow rooms and light timers, things sometimes don’t go according to plan. If a bulb burns out or there’s a power outage, make light cycles a top priority. When your garden is running on a 12/12 cycle, a short-term outage isn’t a big problem.

A few additional dark hours won’t jeopardize your plants. Though they won’t grow in the absence of light, they’ll be safe for at least a day or two. After that, they may become stressed.

When growing on a vegetative 18/6 light cycle, an extended blackout may cause the plants to flower early. You’ll need an alternate light or power source for your garden; a gas or electric lantern should keep PFR levels from going into decline.

If you must alter the garden’s lighting schedule and disrupt the scheduled dark period, simply leave the lights on until the dark period rolls around again. An extra few hours of light won’t cause any harm.

Switching Spectra: Is It Really Necessary?

The short answer is no, as long as you’re starting with full spectrum grow lighting. However, if you’re using metal halide lights, things are different. Full spectrum pulse-start or ceramic metal halides, most fluorescent lamps, and bi-spectrum induction lights can be used in both indoor cultivation phases.

The sun doesn’t change, even at the end of summer. Despite a popular misconception, the sun’s light spectrum doesn’t shift toward red wavelengths in autumn. The spectrum shifts far red during the pre-dawn hours, becoming lighter as noon approaches. After that time, the cycle is completed inversely. Today’s growers are getting healthy, large plants even without mimicry of these daily spectra shifts.

In Closing

There are numerous reasons to cultivate cannabis at home. However, you’ll only get the highest possible yield and the greatest benefit from your homegrown crop by knowing the light cycles used during the planting and growth phases.

It may take some experimentation to familiarize yourself with the light cycles we’ve discussed in this guide. If you’re new to the world of cannabis cultivation, simply stick to the guidelines we’ve provided and you’ll eventually find success!

Proper lighting is crucial to the success of a cannabis crop. When plants don’t have the right types and amounts of light, they grow slowly or fail to grow. A lack of light plays a substantial role, as cannabis plants time their flowering around times of darkness. Once you’ve grasped these concepts, you’ll become a better grower with higher yields.

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