Light Distance And Your Light Schedule

Light is the input that becomes bud. Everything else on this site is about keeping the plant healthy enough to use the light you give it.

There are only two settings to get right, and both are adjustable with a chain and a timer. How far the light sits above the canopy, and how many hours a day it runs. That is the whole chapter.

The complicated version of this topic involves PPFD meters, DLI math and photon efficiency charts. None of that is wrong, and none of it is necessary for a first grow. Your light came with a hanging chart from the manufacturer. That chart, plus the plant's own feedback, will get you a good harvest.

Distance, by stage#

Stage	Height above canopy	Dimmer setting
Seedling, weeks 1 to 3	24 to 30 inches	25 to 50%
Early veg, weeks 3 to 5	18 to 24 inches	50 to 75%
Late veg	14 to 18 inches	75 to 100%
Flower	12 to 18 inches	100%

These are ranges because lights differ. A 100 watt panel and a 300 watt panel do not deliver the same intensity from the same height. If your manufacturer publishes a chart, use their numbers and treat the table above as a sanity check.

Measure from the top of the tallest plant, not from the soil, and remeasure weekly. In flower a photoperiod plant can double in height in three weeks, and a light that was correct on Monday can be too close by Friday.

Reading light stress#

Too close, and you get light burn: bleached white or pale yellow patches on the leaves closest to the light, while lower leaves look fine. Leaf edges may cup upward into a taco shape. Unlike a nutrient problem, this is strictly top down and follows the brightest spot.

Too far, and you get stretch: long gaps between nodes, thin stems, leaves reaching upward, and a plant that grows tall instead of dense. A stretched plant is not ruined, it is just wasting energy on stem instead of flower.

The fix in both directions is the chain, not the dimmer, if you have room to move the light. Raising and lowering keeps the intensity distributed evenly across the footprint. Dimming reduces intensity everywhere at once, which is what you want for a seedling but not for a full canopy.

If you want the actual numbers#

For anyone who likes targets, the intensity figures growers work with, measured as PPFD in micromoles per square meter per second:

• Seedling: 200 to 400
• Vegetative: 400 to 600
• Flowering: 600 to 900

Going above 900 without added CO2 gives you diminishing returns and a real chance of bleaching. You do not need a meter to hit these. There are free phone apps that estimate PPFD, and they are inaccurate enough that I would not make decisions on them, but the manufacturer's chart for your specific light is reliable.

The light schedule#

A photoperiod plant reads day length to decide whether to grow leaves or make flowers. That is the lever you pull.

Vegetative growth: 18 hours on, 6 hours off. This is the standard and it is what you should run.

You will see people argue for 20/4 or even 24/0. The case for 24/0 is that cannabis has no absolute requirement for darkness and more light means more growth. The case against is that plants use the dark period for some metabolic processes, that growers report slightly better results with a rest period, and that 24/0 costs 33 percent more in electricity for a marginal gain.

Growers genuinely disagree here. The call: run 18/6. It is the most tested schedule, it saves money, and it gives your tent a few hours a day to cool down.

Flowering: 12 hours on, 12 hours off. When you want the plant to start making buds, you change the timer to 12/12 and never change it again until harvest.

Autoflowers: 18/6 the whole way through. They flower on age, not light, so the schedule never changes. Some people run autos at 20/4. Either is fine.

Buy a cheap mechanical or digital outlet timer for ten dollars. Doing this by hand will fail the first time you go out for the evening.

Flipping to flower#

For a photoperiod plant, you decide when flowering starts, and the timing matters because of stretch.

When you switch to 12/12, the plant will roughly double in height over the following two to three weeks before it settles into making buds. Some strains stretch less, some nearly triple. Plan for double.

So: flip when the plant is between one third and one half the final height you have room for. In a tent where you have four feet of usable vertical space under the light, that means flipping at roughly 18 to 24 inches tall. For most people that lands somewhere between week four and week eight of veg.

If you are not sure, flip earlier rather than later. A smaller plant that finishes properly beats a big one pressed into the light with bleached tops.

Darkness has to be actually dark#

Once you are on 12/12, the dark period needs to be dark. Not dim. Dark.

Light leaking into the tent during the dark hours can confuse a photoperiod plant. At best it delays flowering. At worst it triggers stress responses, including hermaphroditism, where the plant produces pollen sacs and seeds your own buds.

Practical checks: zip the tent fully and tape any gaps where the zipper does not close. Cover the green and red LED indicator lights on your fan controller and power strip with electrical tape. Do not open the tent during the dark cycle to look at your plants. If you need to check on something, do it during lights on, or schedule the dark period for daytime hours so you are not tempted.

Hanging and adjusting#

Use ratchet rope hangers, about ten dollars a pair. They let you raise the light two inches without unhooking anything, which means you will actually do it weekly instead of putting it off.

Keep the light level. A panel hanging at an angle delivers noticeably different intensity across the footprint, and you will end up with one bleached plant and one stretched one.

If you have multiple plants of different heights, put the short ones on upside down buckets or books to even out the canopy. Getting everything within a few inches of the same height is worth more than any other tweak at this scale.

Now the other thing that panel does besides make light: it makes heat, and that pulls you straight into the environment numbers.