by Nebula Haze
You’ve got a grow space that gets cold at night (or during the day). You don’t want to pay to heat a grow space when you’re already paying to run the grow lights. What’s the best grow light for your situation? There is a clear answer.
Anything but small LEDs! Note: LEDs can be a good choice if they are big enough to keep the grow room warm.
LEDs are the best cannabis grow light for most situations, but in a cold grow room they lose to big-bulb grow lights.
Best cannabis grow light for a cold grow room: Big bulb lights (especially HPS and LEC/CMH)
Grow lights with big bulbs give off plenty of heat, which helps warm up a cool room. But more importantly, cannabis plants under these grow lights tend to be more resistant to the cold in general. That’s because big bulb grow lights give off significant amounts of infrared light, which helps keep leaves warm from the inside and overall increases photosynthesis in cooler temperatures.
Cannabis plants grow better in cooler temperatures under big warm bulbs.
Example: You can grow a ton of weed (over 1 lb if done right) with a 600W HPS grow light even if it’s getting cold. These plants thrived despite the fact that the grow room got close to 60°F/16°C at night. With LEDs, those cold temperatures would have dramatically reduced the yields but with a 600W HPS, the marijuana buds still got enormous.
Pros of HPS, LEC, or CMH in a cannabis grow room (vs LEDs)
- Typically faster vegetative growth
- Buds fatten up well in cool temperatures
- Enormous yields (often better than LEDs as long as it doesn’t get hot)
- Excellent bud density and potency
Cons
- High electricity use
- Big grow light bulbs get hot and bright like mini suns, putting off a ton of heat and light. You can’t safely look at or touch these bulbs when they’re on.
- All HID bulbs (including MH, HPS, LEC, CMH) need a 5-minute break to cool down after being turned off before the bulb is ready to re-spark and turn on again. That means you can’t quickly flip HID grow lights on and off like with LED panels. Not a huge deal but kind of annoying.
Companies used to offer more sizes of HIDs, like the mid-sized 250W HPS, but they stopped getting produced once LEDs hit the market. You can still find 150W (harvest up to 5 oz) but if you want something a little bigger I recommend getting a dimmable 400W HPS (here’s an example of a complete 400W HPS cannabis setup) and adjusting it to your desired brightness/heat level. 400W produces a lot of heat at 100% brightness, but a much more manageable amount of heat when turned down to 50% or 75%. A 315W LEC is also a good mid-size choice.
A dimmable 400W HPS puts off a lot of heat (but not an insane amount) and grows amazing weed with excellent yields
LECs are another kind of HID (big bulb) grow light. They tend to grow weed with a lot of trichomes and above-average terpene production (buds are SMELLY).
HID Ideal Air Temperature: 70-85F° (20-30°C)
HPS grow lights produce fat, dense buds as long as you control the temperature and make sure the bulb is the right distance away.
This manifolded cannabis plant was grown in a 2’x4’x5′ grow tent with a 250W HPS kept about 12″ (30 cm) away.
If you get any big bulb grow light, I recommend getting a “dimmable” model, which means you can adjust the brightness levels as needed. Make sure to only adjust the brightness while the light is off (no dimming on the fly like with LEDs).
Don’t forget the hand test. Light distance is also crucial with HIDs. In order to test the distance, put your hand at plant level when the light is on. If the light feels too hot for you after 30 seconds that means it’s too hot for your plants and the light should be moved further away. HIDs give off infrared light that raises the temperature of leaves (and hands). This means you should test the temperature of both the air and how the light feels on your hands at plant level.
A huge bonus tip for cold grow rooms
- Grow autoflowering strains – These allow you to keep grow lights on 18-24 hours a day from seed to harvest (without ever needing to give your cannabis plants long cold dark periods). Plus they tend to be more cold-resistant in general.
Check out this list of 10 Cold-Resistant cannabis strains
Autoflowering strains are a good choice for cold grow room, but not the only choice. Check out a list of 10 recommended cold-resistant strains.
What about LED grow lights in a cold grow space?
LEDs are not the best choice for growing cannabis if your grow room is always cold. And it’s not just because LEDs don’t put off much heat. Unlike grow lights made out of a big bulb, LED grow lights don’t produce significant infrared light to help warm plants from the inside. Leaves under an LED grow light stay about the same temperature as the air, whereas under a bulb (like an HPS, LEC, or CMH grow light) the surface temperature of a leaf under the light is significantly higher than the air. I tested this when I first learned about the importance of leaf temperature, and if you happen to have both types of grow light you can buy a temperature gun to measure the difference yourself. It’s pretty remarkable to see how all the leaves in an LED tent match the air temperature (even the leaves directly under the light), while in an HPS or LEC tent the leaves under the light are significantly warmer than the air. Even if you don’t have a temperature gun, you can feel the difference on your skin. Your hand under a big bulb light feels much hotter than your hand under an equivalent wattage LED in the same temperature.
All cannabis plants grow poorly without warmth, but LED-grown plants are particularly weak to the cold.
Summary: LEDs don’t warm up leaves from the inside so leaves stay the same temperature as the air. That’s an issue in the cold because warm leaves are able to do more photosynthesis than cool leaves for the same amount of light. As a result, cold plants under LEDs grow poorly.
Plants under LEDs in a cold grow room tend to grow slowly and may show weird symptoms
In general, LED-grown cannabis plants grow slower and produce smaller buds so with LEDs you want to keep a nice warm environment for the fastest growth and biggest buds. I’ve found cannabis plants under LEDs seem to perform best when the air temperature is about 80°F / 27°C or a little warmer when the lights are on. No cold nights.
LEDs Ideal Air Temperature: In the range of 77-85°F (or 25-30°C)
If it gets too cold, cannabis plants under LEDs grow slow and buds don’t thicken up properly.
Ready to start growing? Learn how to grow cannabis in 10 steps.
