Light is the engine of plant growth. Cannabis is a photoperiod plant by nature — meaning the ratio of light hours to dark hours tells it what phase of its life cycle it is in. Understanding how to use light as a trigger, not just a source of energy, is one of the most important concepts in indoor growing.
Standard cannabis varieties — called photoperiod strains — will remain in vegetative growth for as long as they receive more than around 14 to 15 hours of light per day. When light drops below this threshold, they interpret it as the onset of autumn and begin producing flowers. Indoor growers use this biological mechanism deliberately: keep the lights on for 18 hours to grow a large plant, then flip to 12 hours on and 12 hours off to trigger flowering on command.
Autoflowering strains, developed through breeding with Cannabis ruderalis, flower based on age rather than light schedule. They will flower regardless of how many hours of light they receive, typically beginning to flower three to five weeks after germination. This makes them simpler to manage — no schedule change required — and faster to harvest, usually completing their entire life cycle in 70 to 90 days. The tradeoff is that autoflowers are generally smaller, are more difficult to train, and cannot be held in vegetative growth to build a larger frame before flowering.
During vegetative growth, the standard indoor schedule is 18 hours of light and 6 hours of darkness. Some growers run 20/4 or even 24 hours of continuous light, and plants will generally respond with faster growth under more light — up to a point. However, most experienced growers land on 18/6 as the sweet spot because it produces strong vegetative growth while giving the plant a rest period, uses less electricity, and keeps grow room temperatures more manageable.
The darkness period in veg is not strictly necessary for most cannabis strains, but there is evidence that processes important to plant health — including certain metabolic functions and root respiration — are enhanced during dark periods. Running 24-hour light indefinitely can also stress some strains, particularly those with significant indica genetics.
Switching photoperiod plants to a 12-hour light and 12-hour dark schedule triggers the flowering response. This is non-negotiable: the dark period must be completely uninterrupted. Even a brief light leak — a phone screen, a gap under a door, a timer malfunction — during the dark period can confuse the plant and cause it to revert to vegetative growth, produce hermaphroditic flowers, or develop erratic growth patterns known as re-vegging.
Most indica-dominant strains will show flowering signs within 7 to 10 days of the flip. Sativa-dominant and longer-flowering varieties may take 2 weeks or more to show the first pistils. From that point, most indica-dominant strains complete flowering in 8 to 9 weeks, while sativas can take 10 to 14 weeks or more.
The dark period during flowering must be absolute. Check your grow space for light leaks by standing inside it in complete darkness for two minutes. Your eyes will adjust and you will see any leaks. Seal everything — timer indicator lights, duct gaps, door seams — before you flip to 12/12.
Cannabis uses different wavelengths of light more efficiently at different stages of growth. During vegetative growth, blue-spectrum light (roughly 400 to 500 nanometers) promotes compact, dense vegetative development. During flowering, red-spectrum light (roughly 620 to 700 nanometers) drives flower development and is associated with larger bud sites and higher resin production.
Modern full-spectrum LED fixtures produce light across the entire spectrum plants use, often including UV and far-red wavelengths that can enhance terpene production and accelerate flowering onset. High-quality LEDs have become the dominant technology for indoor growing because they produce less heat than HPS or HID lighting while delivering comparable or better results per watt.
| Type | Efficiency | Heat | Best For |
|---|---|---|---|
| Full-spectrum LED | Very high | Low | All stages, tight spaces |
| HPS (High Pressure Sodium) | Good | High | Flowering in large spaces |
| MH (Metal Halide) | Good | High | Vegetative growth |
| CMH / LEC | High | Moderate | Full cycle, good spectrum |
| CFL (Compact Fluorescent) | Low | Very low | Seedlings and clones |
DLI measures the total amount of light energy a plant receives over a full day, expressed in moles of photons per square meter per day. It is a more complete measure of what your plant actually receives than simply knowing your light wattage. Cannabis in vegetative growth performs well at a DLI of 30 to 40. During flowering, pushing DLI to 40 to 65 (while managing heat and VPD) drives maximum resin and bud development.
You can measure DLI with an affordable PAR meter or smartphone app. If your DLI is too low, your plants are energy-limited and will underperform regardless of nutrients, training, or genetics. Light is the ceiling.
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