Autoflower Light Intensity Chart: Optimize Your Grow
For autoflowers, match light intensity (measured as PPFD) to the growth stage: roughly 200-400 PPFD for seedlings, 400-600 for vegetative growth, and 600-900 for flowering. Too little light gives thin, stretchy plants and small buds; too much causes light stress and bleaching. Indoors, most growers run autoflowers on a long daily light schedule (commonly 18-20 hours of light) from seed to harvest, since autos flower by age, not light cycle. Use the chart below to set your LED’s height and dimmer for each stage, and a PAR meter if you want precision. Dial in the right PPFD and your autoflowers reward you with denser, heavier buds.
Autoflower Light Intensity Chart (PPFD by Stage)
Target these PPFD levels at canopy height:
Recommended Strains
Mamba Negra
|
THC | 18% (Medium) |
|
Type | Feminized |
|
Yield | High |
|
Phenotype | 55% Indica / 45% Sativa |
Mamba Negra Auto
|
|
THC | 8% - 15% (Medium) |
|
|
Type | Autoflowering |
|
|
Yield | Medium |
|
|
Phenotype | 80% Indica / 20% Sativa |
| Stage | Target PPFD | Note |
|---|---|---|
| Seedling | ~200-400 | Gentle; avoid burning tender seedlings |
| Vegetative | ~400-600 | Builds structure and healthy growth |
| Flowering | ~600-900 | Drives dense bud formation |
Raise or dim the light to hit these; if you add CO2 and control heat, upper-end flowering PPFD is easier to use safely.
Promos & Deals
What PPFD Is (and Why It Matters)
Light intensity for plants is best measured as PPFD (Photosynthetic Photon Flux Density), the amount of usable light actually reaching the canopy, rather than by watts or lumens, which do not reflect what the plant can photosynthesise. PPFD matters because photosynthesis, and therefore growth and yield, scales with usable light up to the plant’s limit. Give too little and autoflowers stretch and underproduce; give too much and you cause light stress, bleaching and burnt tops.
Because autoflowers run on a fixed, fast clock and cannot make up lost time, getting intensity right at each stage is especially important, there is no long veg period to compensate for a weak start. Measuring PPFD (or at least watching the plants closely) takes the guesswork out.
Best Light Cycle (Schedule) for Autoflowers
Separate from intensity is the light schedule, how many hours of light per day. Because autoflowers flower based on age rather than a change in light hours, you do not need to switch them to 12/12 as you would photoperiod plants. Most indoor growers keep autos on a long schedule throughout their life, commonly 18 hours on and 6 off (18/6), with some using 20/4 or even 24/0. An 18/6 cycle is a popular balance: plenty of light for strong growth while giving the plants (and your electricity bill) a daily rest, and it avoids the heat of running lights non-stop. Keep the schedule consistent from seedling to harvest. Pair the right schedule with stage-appropriate intensity for the best autoflower results.
Growing Autoflowers Under LED
LEDs are the go-to for autoflowers because they are efficient, run cooler than old HID lamps and offer a full spectrum, and most quality LED fixtures let you dim the output, which is perfect for matching the PPFD chart as plants move from seedling to flower. Hang the light at the manufacturer’s suggested height, then fine-tune by watching the canopy: if top leaves bleach or curl, raise or dim it; if plants stretch toward the light, lower or brighten it.
Because autos are compact, a modest LED often covers them well. Keep the light far enough away for seedlings and bring the intensity up through veg and flower, using the chart as your guide and the plants themselves as the final check.
Avoiding Light Stress
Too much intensity, or a light hung too close, causes light stress, and autoflowers are less able to shrug it off than photoperiod plants because they cannot recover lost time. Watch for the signs: bleached or yellowing top leaves nearest the light, upward ‘canoeing’ of leaf tips, and in flower, washed-out white buds.
If you see these, raise or dim the light promptly. The safest approach is to increase intensity gradually as plants mature rather than blasting seedlings, and to keep the canopy even so no single cola sits right under the lamp. For the full rundown on spotting and fixing it, see our guide to cannabis light burn. Getting intensity right protects both yield and plant health.
The Bottom Line on Autoflower Light Intensity
Set autoflower light intensity by stage: about 200-400 PPFD for seedlings, 400-600 in veg and 600-900 in flower, measured at the canopy. Run a long, consistent light schedule (commonly 18/6) since autos flower by age, not light hours, and use a dimmable LED so you can follow the chart as plants grow. Watch for light stress (bleached, curling tops) and raise or dim the light if it appears.
Right intensity plus the right schedule, dialled in with the plants as your guide, is what turns autoflower genetics into dense, heavy harvests. For bigger yields, see our high-yield autoflower seeds guide.
FAQs on Autoflower Light Intensity
What is the best PPFD for autoflowers?
Roughly 200-400 PPFD for seedlings, 400-600 for vegetative growth, and 600-900 for flowering, measured at the canopy. Too little gives thin, stretchy plants; too much causes light stress and bleaching. Increase intensity gradually as plants mature, and use the upper flowering range only if you control heat (and ideally add CO2).
What light schedule should I use for autoflowers?
Because autoflowers flower by age, not light hours, you don’t switch them to 12/12. Most growers run a long schedule throughout, commonly 18/6 (18 hours on, 6 off), with some using 20/4 or 24/0. 18/6 is a popular balance of strong growth, lower heat and a sensible electricity bill. Keep it consistent from seed to harvest.
Can autoflowers get too much light?
Yes. Excess intensity or a light hung too close causes light stress, bleached or curling top leaves and washed-out buds, and autos recover less easily since they can’t make up lost time. If you see these signs, raise or dim the light. Increase intensity gradually rather than blasting seedlings.
Are LEDs good for autoflowers?
Yes, LEDs are ideal: efficient, cooler-running and full-spectrum, and most are dimmable so you can match the PPFD chart as plants move from seedling to flower. Hang at the recommended height, then adjust by watching the canopy, raise or dim if tops bleach, lower or brighten if plants stretch. A modest LED often covers compact autos well.
