Light: How Close, How Long

What You Need to Know

Light is the energy your plant runs on. The leaves capture it and use it to build sugars out of CO2 and water — those sugars are the actual food and material the plant grows with. So light isn’t the food itself; it’s the energy that powers the building. That distinction matters, because it explains why light sets the ceiling on everything else. You can have perfect water, perfect air, perfect feeding — but the plant can only grow as fast as the light lets it photosynthesise. Weak light and the whole operation runs on a camping stove.

For Level 1 you need to get two things roughly right: how close the light sits, and how long it stays on. Get those in range and the plant does the rest.

Your Eyes Are Liars

When a cheap “blurple” LED looks blindingly bright, that’s your eye talking, not the plant’s. Your eye is most sensitive to green-yellow light, so anything in that range looks intense to you even at low power. Plants actually absorb most of the light that hits them — including a good share of green. They just reflect and pass through a touch more green than red or blue, which is why leaves look green in the first place. Red and blue are absorbed most strongly, but green still gets used, and it penetrates deeper into the canopy. The takeaway for now: how bright a light looks to your eye tells you almost nothing about how well it feeds the plant. The number that matters is how many photons actually land on the leaves.

Seb’s Corner — PPFD and the Inverse-Square Rule

[SEB] PPFD — Photosynthetic Photon Flux Density. The number of usable-light photons landing on one square metre of canopy each second, measured in micromoles per square metre per second (µmol/m²/s). Why it matters: it’s the honest measure of how much light the plant is actually getting, regardless of how bright it looks. A seedling wants it gentle — roughly 300–400 µmol/m²/s. Crank a tiny plant up to 700+ and the tops can bleach and stall.

Now distance. There’s an inverse-square rule: for a point source, doubling the distance drops the intensity to a quarter. That quarter is the worst case, though. A broad LED panel sitting close above the canopy falls off much more gently than that, because it isn’t a single point of light. So the rule to keep is the direction, not the exact maths: every centimetre you raise the light costs you intensity, and every centimetre you lower it adds intensity fast. Set the height, then check the plant.

The Hand Test

You don’t need a meter for a first grow. Put the back of your hand at canopy height, palm to the light, and hold it there. If the heat is comfortable — warm but you could leave your hand there a good while — the light is roughly in range. If it’s uncomfortably hot in a few seconds, the light is too close; raise it. It’s crude, but it keeps you out of trouble while you learn to read the plant.

18/6 — The Schedule

During vegetative growth, the plant reads long days as summer: time to grow. Run 18 hours on, 6 hours off. That gives enough daily light to drive strong growth while leaving six hours of dark for the processes that happen at night. Some growers run 20/4; the gains over 18/6 are thin and you’re paying for electricity the plant can’t use efficiently. Autoflowers run on 18/6 (or 20/4) the whole way through — they flower on age, not on the light schedule, so you never change it. Photoperiod flowering (the switch to 12/12) is a later level. For Level 1, 18/6 is your number.

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How To Apply This

1. Set the height with the hand test. Hand at canopy, palm up. Comfortable warmth = roughly right. Hot fast = raise it.
2. Start gentle for a seedling. If your light dims, run it lower for a small plant — think gentle, not full power. A seedling doesn’t want the same intensity a big veg plant does.
3. Run 18/6 and leave it. Put the light on a timer so the schedule never depends on you remembering. Consistency matters more than the odd hour either way.
4. Read the plant after a day or two. New growth tight and compact = light’s about right. Tops bleaching or curling away from the light = too close, raise it 10–15cm and check again tomorrow. Plant stretching tall and spindly = not enough light reaching her (see below).

One change at a time. Adjust the height, then wait a day before you touch anything else.

Stretch as a Signal

Stretch — long gaps between branches, a tall, leggy, reaching plant — is the plant telling you it isn’t getting enough light. It’s reaching for more. The fix is usually to lower the light (or raise the plant) so the canopy gets more intensity. The stretch that already happened won’t reverse, but new growth should tighten up once she’s getting enough. Stretch isn’t a disease. It’s a message. Read it and respond.

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Watch Out For

• Buying a light by the headline wattage. A “1000W” LED that draws 130W from the wall is a glorified desk lamp with a rave filter. The plant stretches under it for months while you blame the genetics. Wall draw is the only wattage number worth trusting.
• Hanging it too close because “more light, more growth.” Past the sweet spot, the tops closest to the light can bleach, curl, and go crispy. If only the very top leaves are affected and the rest looks fine, you were just a touch too close — raise it.
• Hanging it at the very top for “maximum coverage.” Even coverage at low intensity is still starvation. You can spread weak light over the whole canopy and feed nothing properly.
• Fiddling with the schedule. Changing the hours, leaving the timer off, “just topping up” with an extra few hours. Pick 18/6, put it on a timer, leave it alone.
• Mistaking heat-curl for a nutrient problem. Leaves curling up like a taco near the light is usually heat or light proximity, not hunger. Raise the light before you reach for a bottle.

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Quiz

1. (True / False) Plants barely use green light, which is why leaves look green.

2. (Multiple choice) PPFD is measured in:

• a) Watts
• b) Lumens
• c) Micromoles per square metre per second (µmol/m²/s)
• d) Degrees

3. (Multiple choice) What light schedule should you run for a Level 1 grow (veg / autoflower)?

• a) 12 hours on, 12 off
• b) 18 hours on, 6 off
• c) 6 hours on, 18 off
• d) 24 hours on

4. (Scenario) Your plant is tall, thin and leggy with big gaps between the branches. What is she telling you, and what do you do?

5. (True / False) Hanging the light as high as it goes for “even coverage” guarantees the plant gets enough light.

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Answer Key

1. False. Plants absorb most of the light that hits them, including a large share of green. They reflect and pass through a touch more green than red or blue — that’s the colour you see — but green is still used and penetrates the canopy.
2. c) µmol/m²/s. It counts usable-light photons landing on a square metre of canopy each second.
3. b) 18 hours on, 6 off. Drives strong growth with a dark period; autos run this the whole way through and never flip.
4. She’s not getting enough light and is stretching to reach it. Lower the light (or raise the plant) so the canopy gets more intensity. Past stretch won’t reverse, but new growth tightens up.
5. False. Even coverage at low intensity is still too little light. You can spread weak light thin and starve the whole plant.