Heat or Light? How to Tell if Your Plants Are Getting Too Much of a Good Thing

When it comes to growing weed at home, more light usually means more buds… right? Well, not always. While cannabis loves light, there's a fine line between ideal conditions and a grow room that's a little too intense. Sometimes the problem isn’t the light at all—it’s the heat that tags along for the ride. So how do you tell which one is stressing your plants? Let’s break it down.

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Heat Stress vs. Light Stress: What’s the Difference?

These two issues often show up in similar ways—curled leaves, slowed growth, crispy edges—but the causes and solutions are totally different.

Heat Stress

• Caused by high temperatures, usually above 85°F (29°C).

• Can come from grow lights, poor airflow, closed tents, or hot ambient temps.

• Typically affects upper leaves and may spread to the entire plant if unchecked.

Light Stress (Photobleaching)

• Caused by too much light intensity, especially from strong LEDs placed too close.

• Commonly appears as bleached or faded spots on the tops of colas.

• Doesn’t always coincide with high room temps.

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Signs of Each Problem

Signs of Heat Stress

• Leaf edges curling upward (taco-ing)

• Dry, crispy leaf tips even though the soil is moist

• Slow bud development in flowering stage

• Wilting during lights-on but recovering during dark hours

• Thermometer reading above 85°F consistently

Signs of Light Stress

• Bleached, white or yellow top leaves directly under the light

• Burn spots that are irregular and not from nutrient burn

• Stretching despite intense light (plant trying to escape the beam)

• Lower leaves remain healthy while top canopy suffers

• No signs of excessive heat on thermometer

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The Ruling Out Method (A Simple Grower’s Test)

Here’s a step-by-step way to determine the culprit:

1. Check Your Temps
   Use a digital thermometer with a probe placed canopy-level. Temps should be:

   • Veg: 70–85°F (21–29°C)

   • Flower: 65–80°F (18–27°C)
     If you're above this range, heat may be the issue.

2. Measure Distance from Light
   Refer to your light manufacturer’s PAR chart or recommended hanging height. Most LEDs need:

   • Veg: 18–24 inches

   • Flower: 12–18 inches
     If your plant is too close, even if the room’s cool, light stress could be sneaking in.

3. Run the Hand Test
   Hold your hand at canopy level for 30 seconds. If it’s uncomfortably warm, it’s likely heat stress.

4. Observe Leaf Color & Texture

   • Pale, bleached spots? Probably light stress.

   • Crispy, curling tips? More likely heat.

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How to Fix It Fast

If It’s Heat Stress:

• Increase airflow with oscillating fans and better exhaust.

• Add an AC unit or inline duct fan to cool your space.

• Use a fan speed controller to balance out temp swings.

• Raise your lights a few inches to reduce radiant heat.

• Run lights during cooler parts of the day/night.

If It’s Light Stress:

• Raise the light or lower its intensity (many LEDs are dimmable).

• Use a PAR meter or phone app to track intensity levels—aim for:

  • Veg: 300–600 PPFD

  • Flower: 600–1000 PPFD

• Spread the light out using side lighting or light movers.

• Train your plants to create a more even canopy (topping, LST).

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Pro Tips from the Grow Tent

• Light & heat often go hand-in-hand, especially with high-powered LEDs. Don’t assume one without checking the other.

• Invest in a thermometer/hygrometer combo—you need both temperature and humidity data to make informed choices.

• Leaf surface temps can be hotter than room temps. Use an infrared thermometer if you want the full picture.

• Watch your plants during lights-on, not just when it’s convenient. Problems often show up in the hottest part of the cycle.

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It's All About Balance

Cannabis is a sun-loving plant, but even it has its limits. Too much heat can cook your yields, and too much light can bleach your dreams. By learning to recognize the difference—and making small, smart adjustments—you can keep your grow thriving and your buds big, sticky, and stress-free.

If your plants are telling you something’s off, don’t panic—diagnose, adjust, and watch the green bounce back.

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