Tacoing or bleached tops: is my light too close?

Often it's the room, not the light. A leaf cools itself by sweating water out through its pores. When the room is too hot, too dry or dead still, or the roots can't keep water moving, she can't shed the heat the light loads onto her, and the same light she handled fine yesterday is suddenly too much. Check the room before you touch the fixture.

A lot of the time, the light didn't burn her. The room let it.

What the tops are telling you

A taco, the leaf edges curling up and in, is her shrinking her own surface so less of her faces heat she can't get rid of. Read it as too much light and you pull away the one thing she might still want.

Praying, leaves angled up toward the light, is different. On its own it's often a happy plant reaching for good light. It's the tacoing and the bleaching that point at stress.

A pale, bleached top right under the light is the symptom that looks most like hunger. Usually it isn't. Bleaching is damaged tissue, not a deficiency, and feeding it only raises the salt at the roots and stacks nutrient burn on top of heat stress.

You've been reading the wrong thermometer

Your thermometer reads the air. Your plant lives at the leaf. Under an LED, a healthy leaf that's cooling itself usually runs a few degrees below the air. So point an infrared thermometer at the top leaves and compare. If the room says 80 and the leaf comes back at or above it, that isn't just a hot room. That's a plant that quit cooling.

Light or room? Run it in order

1. Look, and ask what changed. Only the tops under the fixture, while the lower growth looks fine? That points up top. Then ask what changed in the last few days: a new light, a hot spell, a fan that died, a dimmer that got bumped. And sanity check the hang height, because a strong LED a hand's width off the tops is too close by anyone's math.

2. Read the leaf against the air. Shoot the top leaves and compare to the air at canopy height. You want the leaf a few degrees below the air. Then read temperature and humidity together, never one alone, because that relationship is what tells you how hard she's being asked to sweat.

3. Check airflow and roots. You want gentle movement across the tops, leaves fluttering, not one cola getting blasted. Dead air parks heat against the leaf. Keep the root zone evenly moist, not swampy and not bone dry, because roots that can't move water shut the cooling down and throw the same cooked look.

4. Only then, the light. If the room checks out and the tops still taco or bleach, make one change: raise the fixture a few inches or dim it 10 to 20 percent. Then wait a couple of days and judge by the new growth. The leaf that already cooked isn't going to flatten back out.

What not to do

Don't back the light off before you've checked the room. If the room is what's cooking her, pulling the light back may ease the symptom, but the cause is still running, and now she's getting less light on top of it.

Don't feed a bleached top. The plant that's cooking is the one your gut says to feed. Don't.

When it's something else

• Humidity you can't get into range: see What humidity should I run, and what if I can't hit it?

• Yellow on the lower leaves, not the tops: see Yellow leaves: is it a deficiency, and is this too much?

• The whole plant drooping in the heat of the day: see How often should I water, and why is she drooping?

On our shelf in Worcester: Fixtures, hangers and timers are in Grow lights, and the fans and controllers that steady the room are in Climate, air & controls.

Keep going

Watch the episode: Light Burn or Heat Stress? Why Backing Off the Light Fails on MyRemy Live.

Fix the room first and the light usually stops being the problem. The whole system is written down in the free Stability Playbook. Get the free Stability Playbook