Understanding Light Cycles for Growing Weed
Created on Dec 19, 2022
Last updated on Oct 7, 2024
Article written by
Homegrown Cannabis Co.American Seed Bank and Cultivation Experts
Content reviewed by
Dr. Lewis Jassey
Cannabis requires unique lighting conditions throughout its life stages, and the amount of light and its timing largely determines yield quantity and quality.
The light cycle for growing weed is 18/6 in the seedling and vegging stages and 12/12 in flowering. It lets plants photosynthesize, stretch, bloom, and yield an abundance of sticky buds.
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Why Is Monitoring Light Cycles Important?
The light cycle refers to the rotation of light and darkness a marijuana plant receives daily. In most cases, it’s relevant for the progression between growth stages.
Most strains (cultivars) are photoperiodic, so they respond to seasonal changes in light. The plant’s life cycle nears its end as the days get shorter, and it responds by blooming and reproducing. You can mimic this change in grow rooms to trigger flowering.
Autoflowering strains contain non-photoperiod ruderalis genes and don’t depend on light schedule changes. Instead, blooming occurs after a certain age (usually 5-6 weeks).
We’ll spend most of this guide discussing photoperiod weed and dedicate a separate section to autoflowers. In the former case, monitoring the cycle is essential for the following four reasons:
- Launching the seedling stage: Seeds don’t need light to pop, but the resulting plant only stretches if there’s a lamp above it.
- Maintaining the vegetative stage: Weed keeps stretching while it believes it’s summer outside. You keep it vegging until it’s big enough to carry an abundant harvest, but it can bloom sooner if the days suddenly shorten.
- Triggering the flowering stage: Reduce the daylight duration to get your plants to bloom.
- Maintaining the flowering period: Cannabis needs 12 hours of uninterrupted darkness to remain in bloom. Fluctuations revert it to vegging and might cause hermaphroditism.
Types of Grow Lights
When growing cannabis outdoors, you rely on the sun to keep your weed plants developing. Indoor growers must manually manage the light schedule with lamps. Which one should you get?
Gardening stores sell three types of grow lights. They differ in spectrums, intensities, and suitability for different stages of plant development. Let’s see how they measure up.
Note: Vegging and flowering plants need stronger lamps than seedlings. Blue light is better for vegging, while red kickstarts bud production.
CFL Grow Lights
CFLs (compact fluorescent lights) are the most common option. They’re available in home improvement and grocery stores and come in two varieties:
- Daylight CFL bulbs emit a blue-leaning spectrum, suitable for vegetative growth.
- Warm white CFL bulbs emanate a reddish light, better for the flowering stage.
Daylight CFLs are perfect for the seedling stage. Since they don’t emit much heat, you can place them 2-4 inches above the young plant. These lamps are also appropriate for small spaces with 2-3 mature plants.
HID Grow Lights
HIDs (high-intensity discharge) lamps have been the gold standard in the cultivation industry for decades. Growers may choose between two types:
- Metal halide (MH) lamps produce a cool blueish light suitable for vegging.
- High-pressure sodium (HPS) lamps are reddish and better for flowering.
If using one option for vegging and flowering, HPS is the better fit. These lamps lean red but emit the whole light spectrum, ensuring large plants and high yields.
One thing to remember is that HID grow lights emit a lot of heat. They should stay 12-24 inches above the canopy to avoid plant stress, especially while your marijuana is young.
LED Grow Lights
LED (light-emitting diode) lamps are cooler, longer-lasting, and more sustainable than HIDs. At the same time, they provide higher intensity and penetration rates than CFLs.
Most quality LEDs emit the entire spectrum, making them suitable for veg and flowering. Some come with switches, letting you provide blue or red lights depending on the phase.
The initial cost is the main downside: LEDs are a considerable investment. They’re also pretty intense, so you should keep them around 24 inches above the canopy. You may also supplement your grow room with additional carbon dioxide (CO2) to help photosynthesis keep up and push them closer for faster development.
When to Start Monitoring Light Cycles
Growing weed consists of four stages: germination, seedling, vegetative, and flowering. The latter three require you to monitor the light cycle.
The germination stage awakens the growth hormones in a dormant cannabis seed. It revolves around warmth, air, moisture, and darkness. Light exposure becomes a factor as soon as the taproot emerges, and you place the young plant in a medium.
Let’s explore the best light cycles for each subsequent section of your cultivation journey.
Seedling Light Cycle
- Lamp type: CFL or HID
- Light cycle: 16/8, 18/6, 20/4, or 24/0
The optimal schedule for seedlings is 18 hours of light and six hours of darkness. Young cannabis plants aren’t as sensitive to changes, though, so you can play around with different light cycles.
For instance, 16 hours of light make growth slower and leaves time to resolve health issues. So, this option is better for total beginners. Those who want to speed-run this stage might keep the lamps active for 20 or even 24 hours.
Seedlings don’t need high-intensity light to grow well, but they may overstretch and tip over if the lamp sits too far away. Place lower-intensity or dimmable light 2-4 inches away from plants.
Outdoor cultivators should keep seedlings indoors for 1-2 weeks. If you take them outside earlier, ensure they get at least six hours of daily direct sunlight exposure.
Vegetative Light Cycle
- Lamp type: HID or LED
- Light cycle: 18/6 or 20/4
The vegetative stage is the period of the most intense cannabis growth. Your focus is ensuring your garden doesn’t go into flowering before it’s big enough.
Let’s see what the vegetative light cycle should look like indoors and out.
Indoor Growing
Indoor vegetative plants require a minimum of 16 hours of light. You can provide 20 hours to speed up development, but 18/6 is the gold standard for fast yet sustainable vegging.
If nights are longer than six hours for a prolonged period, your garden may flower early. Small plants can’t carry big buds, so it’s best to re-introduce an 18/6 schedule for a few more weeks.
Intensity is the name of the game here, which you get from HID or LED lamps. Use blue-leaning or full-spectrum light and avoid yellow and orange bulbs.
Outdoor Growing
Outdoor cultivators should ensure their plants get at least six hours of sunlight during the vegetative phase. Choose a sunny spot for your garden and move the pots under indoor lamps if the weather is overcast.
In general, you can’t affect the duration of vegging outdoors. It ends in late July or early August when the days shorten. There’s one trick that gives you control of this process: light deprivation.
Deprivation pushes cannabis to start the flowering process. It’s the go-to technique for growers in long-summer areas looking to produce multiple harvests per season.
To accomplish early outdoor flowering, cover your plants with dark cloth or opaque buckets for 12 hours. That way, you mimic the amount of sun they’d naturally get in flowering. Continue covering them until mid-August, and be consistent to avoid stressing your cannabis.
Flowering Light Cycle
- Lamp type: HID or LED
- Light cycle: 12/12
Except for ruderalis, marijuana is a short-day plant, requiring at least 12 hours of uninterrupted darkness to flower.
Plants produce buds in this section of the grow cycle. It occurs naturally when the days shorten in late summer. Indoors, you trick it into thinking winter is coming by shortening the light cycle.
Let’s see how to maintain and optimize indoor and outdoor flowering light cycles.
Indoor Growing
Once your plant reaches around three-quarters of its final desired size, set up a schedule of 12 hours of light followed by 12 hours of darkness indoors. You might use a 10/14 ratio, but buds are smaller and slower to ripen under these conditions.
This cycle should be constant and precise. Receiving over 12 hours of light makes the plant revert to vegging (re-veg) or turn hermaphrodite and self-pollinate.
Use HPS or LED for this stage and rely on full-spectrum or red light. Distance lamps appropriately, as excessive heat and light dry out the trichomes.
Outdoor Growing
Choose a spot with full sun for a prolific flowering cycle. You can’t do much to support bud production for your outdoor garden, but you can protect it from re-vegging.
Most gardens are near billboards, cars, and street lamps, and these sources cause trouble if they fall directly on the canopy. If that’s the case in your set-up, cover plants with black buckets at nighttime to protect them from light leaks.
Light Cycles for Autoflowering Plants
Autoflowers bloom based on age rather than the amount of light and darkness. You can maintain the same light cycle throughout the seedling, vegetative, and flowering stages.
Indoor Growing
Most cultivators maintain an 18/6 light schedule for autoflowers when growing indoors. The theory behind this approach is that plants remain healthier when they get a recovery period. If you’re a newbie, go with this option.
Long periods of light only make auto cannabis plants more prolific, and growers might keep their lamps active for 20 or 24 hours. If choosing this alternative, be extra careful about other growing conditions to keep your garden healthy.
Can you use a 12/12 schedule during flowering for autos? That’s only useful if you keep photoperiods and autoflowers in the same grow room. Otherwise, you get smaller plants and buds with this approach.
Outdoor Growing
You don’t have to consider the seasons when planting autoflowers outdoors. You can take seedlings outside in April, collect a harvest in July, and do another batch before the growing period ends.
If you’re not planning to get two harvests per season, sow in May and harvest in August. That way, your plants get as much sunlight as possible and produce bigger yields.
The Bottom Line
Managing the light cycle is among the vital aspects of the growing process. Seedlings need gentle light; long days supercharge vegging; uninterrupted darkness maintains flowering. Stay mindful in each stage for the best results.
Once you master the management, you can start experimenting with spectrums and tweaking the daytime and nighttime hours. These tricks make your marijuana healthier, faster-flowering, and higher-yielding.
Why not put this theory into practice? Visit our partner Homegrown Cannabis Co. to buy the highest-quality seeds and start your cultivation journey.
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