Light Schedules & Intensity: Photoperiod, PPFD, and DLI by Stage

How many hours of light does cannabis need each day?

The light schedule — the photoperiod — is the plant's calendar. Photoperiod strains read the length of the uninterrupted dark period to decide when to flower. Keep nights short and they stay in vegetative growth indefinitely; give them long nights and a hormone signal builds that flips them into bloom. Autoflowers ignore the calendar almost entirely and flower on age, which is why their schedule is yours to choose.

In veg, the three common schedules honestly compared: 18/6 is the standard — full-speed growth, a daily dark period for root work and equipment rest, and the lowest power bill of the three. 20/4 buys two extra hours of photosynthesis for two extra hours of electricity; some growers see slightly faster veg, many see none. 24/0 works — plants do not sleep the way animals do — but it costs a third more power than 18/6, removes your margin for heat, and controlled comparisons generally show little to no growth advantage. Our take: run 18/6 unless you have a specific reason not to.

Flowering photoperiods means 12/12, and the dark half matters as much as the light half. Autos typically run 18/6 or 20/4 from sprout to harvest — more total hours make up for the modest intensity of small fixtures, which the DLI section below makes precise.

How intense should the light be at each stage?

Intensity at the leaf is measured as PPFD — micromoles of usable light landing on a square meter every second. It is the number your fixture's PAR map reports, and it is what the plant experiences, unlike wattage, which only describes what the light consumes.

Two warnings belong with the table. First, intensity is not a score: light beyond what the plant can process becomes heat stress and bleaching, covered below. Second, these are targets at the canopy, averaged across it — a fixture that reads 900 in the center and 300 in the corners is a 500-ish light with a hotspot, and the corners will grow like it.

What is DLI, and how do you actually use it?

DLI — daily light integral — is the total light a plant receives in a day: intensity multiplied by hours. The math is one line: DLI = PPFD × hours of light × 0.0036, and the result is in moles. It matters because it is the plant's actual daily food ration, and it lets you trade hours against intensity honestly.

Worked example, a 2×2 tent: a small board delivering an average 500 PPFD over the canopy, run 18 hours in veg, gives 500 × 18 × 0.0036 ≈ 32 mol — right in the veg target band of roughly 25–40. Flip to flower at 12 hours and the same setting delivers only 21.6 mol, which is underfed for bloom. The fix is intensity, not hours: dim up (or lower the light) to about 800 PPFD and 800 × 12 × 0.0036 ≈ 35 mol — proper flower territory.

A 4×4 tent example shows the averaging trap: a strong fixture might map at 1,000 PPFD center, 700 mid-canopy, 450 at the edges — a true average near 700. At 12 hours that is about 30 mol: decent, not the 43 mol the center number promised. Practical targets to steer by: seedlings 10–15 mol, veg 25–40, flower 35–45 without CO2. When a grow looks light-starved or light-stressed, run this math before you touch a single nutrient.

How high should your light hang above the canopy?

Distance and dimming are two hands on the same dial, but they are not identical. Raising a fixture spreads its light wider and flattens hot spots while total intensity falls; dimming lowers intensity while keeping the coverage shape. For even canopies you usually want the light a little higher and stronger rather than low and dimmed over a single plant's head.

Starting points for modern LED boards and bars: 24–30 inches above seedlings at 25–50% power; 18–24 inches in veg; 12–18 inches in flower at full power, following your manufacturer's PAR map over any generic rule — fixtures differ too much for one distance to rule them all. Older blurple panels and HID bulbs follow their own spacing, and HID adds real radiant heat at the leaf.

Work in increments. Plants stretching toward the light get it lowered or brightened; tops curling away get the opposite, 2 to 4 inches at a time. During the flower stretch the canopy moves toward the light daily — build the headroom into your flip decision, and check canopy temperature at leaf height, not at the thermometer on the wall.

How do you measure light without expensive gear?

The honest ladder of options, best to roughest. A quantum PAR meter (the Apogee class of tools) reads true PPFD anywhere in the tent and pays for itself if you run multiple lights — it is also genuinely expensive, which is why the rest of this section exists.

Phone apps such as Photone are the budget standard now. With the paper diffuser the app asks for, iPhone readings typically land within about 10–20% of a real meter — good enough to set heights and catch the center-versus-corner problem from the DLI section. Android results vary more by model. Lux meters are cheap and consistent but measure light weighted for human eyes; the rough conversion for white LEDs is around 65–75 lux per PPFD point, so 40,000 lux ≈ 550–600 PPFD — useful, with spectrum-dependent error bars.

Manufacturer PAR maps are free and usually honest about the test height — just read the whole grid, not the biggest number in the middle. And retire one myth: the back-of-the-hand test measures radiant heat, not light. It can warn you a lamp is cooking a leaf; it cannot tell you the PPFD, and modern LEDs often feel cool while delivering stress-level intensity.

What does light stress look like — and how is it different from heat stress?

Bleaching is the signature of too much light: the parts of the buds and leaves closest to the fixture turn pale yellow to white, starting with the single highest cola while identical buds inches lower stay green. Bleached bud looks striking and smokes weak — that growth has damaged chlorophyll and stalled. It can happen at perfectly comfortable air temperatures, which is exactly how it differs from heat stress.

Tacoing — leaf edges curling upward into a V or canoe — is the shared symptom: light intensity, heat, and dry air all produce it, because the leaf is reducing its exposed surface. Foxtailing, new spires of fresh pistils stacking on finished buds late in flower, points at heat or intense light near the tops, though some genetics foxtail on their own schedule. The diagnostic that separates the causes: light stress strikes by position (highest, closest, center of the beam) while heat stress follows hot spots and poor airflow and usually lifts when canopy temperature drops.

Fixes, in order: raise the light 2 to 4 inches or dim it 10–20%, confirm leaf-surface temperature sits roughly 75–82°F, and give the plant several days before judging — existing bleach does not green back up, but new growth tells you within the week whether you solved it.

Do sunrise dimming, UV, and far-red actually help?

Three popular extras, graded by evidence rather than marketing. Sunrise/sunset dimming — grade: comfort feature. Ramping light up and down over 20–30 minutes may ease the shock of instant full power, and it is pleasant to work under. Controlled evidence that it changes yield or quality is thin. Enjoy it if your controller includes it; do not pay a premium for it.

UV supplementation — grade: promising, inconsistent. Some studies and many grower trials report increased trichome density and resin response from UVA/UVB exposure in the last weeks of flower — the plant appears to treat UV as stress worth defending against. Results vary by strain and dose, the lamps add cost and a real eye-and-skin hazard, and overdosing burns leaves. A reasonable experiment for an experienced grower, not a first purchase.

Far-red and the Emerson effect — grade: real physics, modest returns. Mixing deep-red and far-red light genuinely boosts photosynthesis beyond either alone, and end-of-day far-red can make plants read the night as longer — which can encourage stretch if mistimed. Bars are cheap and the science is sound; the gains are percentages, not transformations. Bottom line for all three: light quantity and coverage first, spectrum toys after — nobody's harvest was saved by a sunrise ramp over an underpowered panel.

Can a little light leak in flower really cause hermies?

The flowering signal is built during darkness, and meaningful light during the night period interrupts it. A plant that keeps getting its nights broken can stall, reveg, or — the feared outcome — produce male flowers on a female plant (a hermie) and seed your crop. The trigger is real; the panic around it is often out of proportion.

The leaks that matter are stronger and longer than most people fear: tent pinholes along seams and zippers, a window or room light shining in during lights-off, glowing power strips and equipment LEDs inside the tent, and opening the tent under full room light mid-night. The check is simple and free: during the dark period, zip yourself inside the closed tent with the room lights outside switched ON — that is the condition you are testing against — wait four or five minutes for your eyes to adjust, and look for glowing seams and indicator lights. Black tape fixes most of what you find.

Genetics share the blame with leaks. Stable lines tolerate a small indicator LED for a whole run without a single male flower; unstable genetics hermie under mild stress with no leak at all. If you do find a few pollen sacs: remove them carefully with wet fingers, increase your inspection rhythm, and keep growing — a handful of sacs caught early is a chore, not a catastrophe. Chronic, repeated light exposure is the thing to fix properly.

Frequently asked questions

Do cannabis plants need a dark period during veg?
Not strictly — photoperiod plants will veg under 24 hours of light. But 18/6 grows them essentially as fast in most real tents, costs a third less electricity, and gives equipment and the room a daily cool-down. The dark period becomes non-negotiable only in flower, where 12 uninterrupted hours trigger and sustain blooming.

Can I make up for a weak light by just running it longer?
In veg and with autoflowers, largely yes — that trade is exactly what DLI measures, so 300 PPFD for 20 hours feeds a plant about like 450 for 13. In flower with photoperiod plants, no: the 12-hour dark requirement caps your hours, so intensity has to do the work.

What is the best light schedule for autoflowers?
18/6 or 20/4 from sprout to harvest. Autos flower on age, so every light hour counts toward their short life — but 24/0 shows the same diminishing returns it does elsewhere, and the modest savings of 18/6 usually win.

The tops of my buds are turning white. Is that a good thing?
No — that is bleaching from too much light, covered in the light-stress section. The white growth has damaged chlorophyll and reduced potency. Raise the light or dim it 10–20%; the bleached parts will not recover, but healthy new growth confirms the fix.

Will one interrupted night ruin a flowering crop?
Almost never. A single brief exposure is a blip the plant absorbs. What causes trouble is a leak that repeats every night — a glowing power strip, a pinhole aimed at a lamp — so fix patterns, not incidents, and do not panic-chop over one mistake.