Cold weather marijuana growing is popular because grow room heat is an asset in the winter rather than a liability.
But sometimes winter cold weather marijuana growing can be a grow room disaster.
For example, I read on BigBudsMag.com about a marijuana grower in Michigan who struggled through an ice storm that knocked out electricity for several days.
He found out why serious cannabis growers own generators!
His cannabis plants sat in the dark, when his grow room temperatures went below 40 degrees Fahrenheit for extended periods of time.
He reported that only because he’d used marijuana-specific hydroponics nutrients and supplements that “armored” his cannabis plants against stress, his crop survived.
Here are other things to consider about making the most of cold weather marijuana growing:
Add More High-Intensity Discharge Light During Cold Weather Marijuana Growing
The first thing I do when cold weather arrives is add more HID lights to my marijuana grow room.
I add metal halide and high pressure sodium. It means I can grow more plants because I increase my light coverage. I get more pounds per crop in cold weather marijuana growing.
HID bulb heat is a problem for indoor marijuana grow rooms during warm months.
You spend a lot of money to get rid of HID heat.
But in winter, HID heat helps warm my house. I’m not spending money on air conditioning to remove grow room heat.
If your lights push your indoor marijuana grow room past 74F (or 84F if you’re adding C02), the extra grow room heat can be sent back into a central heating return intakes or otherwise transferred to help reduce home heating costs in winter.
Just be careful not to remove grow room C02 when you remove extra heat.
Another tactic for keeping temps in range is to transfer colder air from other parts of the dwelling into your marijuana grow room.
In some grow houses I’ve used, just having the central house fan on circulating air has been all I needed to keep the grow room temps where I want them.
If you’re increasing ambient grow room C02 using C02 generators that burn propane or natural gas, they generate heat that helps keep your grow room warm.
But they also generate humidity, and you don’t want humidity higher than 59% in your grow room, especially during bloom phase.
Cold Weather Marijuana Growing Water & Air Temperatures/Humidity
When I’m growing marijuana using water culture or other pure hydroponics methods, cold weather marijuana growing means I turn off the chiller and maybe use a water warmer in my res or master bucket.
Depending on your marijuana strains and grow room conditions (especially your ambient grow room temperature), you could have your water as low as 66° and not hurt your marijuana roots or plants.
If water temperatures get too low, however, root function slows, oxygen levels in the water drop, and roots can be damaged.
In general, you don’t want your marijuana root zone to go below 66° or above 74°.
You want your marijuana nutrients solution at a constant 68°.
And if winter outdoor temperatures are making your hydroponics marijuana grow room ambient air temperature colder than 70F when your lights are on, compensate for that by keeping your nutrients water around 71F, or heat your room.
The goal is to not stress your cannabis plants with cold air temperatures and/or cold root zone temperatures.
Monitor your hydroponics res temps and ambient grow room temps, along with your marijuana plants’ growth rate, leaves, and overall health.
If you see cold weather marijuana growing plant problems, they could be caused by wrong temperatures instead of pH, nutrients, bad genetics, mites, etc.
Keep your humidity between 55-64%.
If you have big fat buds that are prone to gray mold, keep humidity at 52-56%.
A dehumidifier generates heat in the grow room.
Cold Weather Root Health
Your marijuana roots can function well even if temperatures in the root zone are 66°.
The problem for some marijuana growers, especially if they’re growing in a basement or on the first floor of a house with a poor foundation, is that the floors can be very cold.
You want to put insulation between roots and your cold concrete, basement floor, or other floors if the root zone is going below 66°.
You can use those big sheets of R-24 or higher foam insulation that looks like drywall except it’s about 2 inches thick.
Or you can put your plants on heat mats.
Put a thermometer on the same surface your plants are sitting on, but not so your hydroponics lighting is shining on the thermometer.
That’s how you’ll know the temperature transferring into your roots.
Whatever you do, don’t let your roots get too cold.
Difference Between Night & Day Marijuana Growing Temperatures
Experiment with lights-off temperature drops. I have a couple of 1250-watt space heaters in my hydroponics grow room that I set to keep the ambient air temperature at 65-67F during night cycle.
During bloom phase in cold weather, my heaters are running quite a bit during the 12 hours when my hydroponics lights aren’t on.
My lights-on temperature is 74F.
Many marijuana growers believe that having the lights-off temperatures at least 7-10 degrees cooler than lights-on temperatures is more natural for marijuana plants, and could induce purpling in cannabis strains that have a tendency to go purple.
Allegedly, this day and night temperature differential could create earlier harvests, more purpling, higher THC and other cannabinoid percentages.
However, I’ve seen no absolute proof that those beliefs are accurate.
Please read this very important article on how night and day temperature differentials affect plant stretch during bloom phase.
Energy Efficiency in Cold Weather Marijuana Growing
Winter is a time when energy consumption for home heating is very high, and very costly.
To make my home as energy-efficient as possible, I installed the highest-level of quality double-paned windows at my grow house, and they’re tightly sealed and caulked, with white poly plastic over many of them on the inside.
I walk around inside my marijuana grow room and the rest of the house when gusty frigid wind is blowing outside, looking for drafts or other vectors for cold air.
I injected special insulation into my walls. I removed the existing old insulation in my attic and replaced it with 48 inches of new insulation.
Along with that, I reconfigured my ductwork and furnace. Energy efficiency is your friend, so get the best heating and heat distribution equipment possible.
Some marijuana growers who use a basement or ground floor need to put insulation and/or heating mats on the floor so the cold doesn’t come up through the ground and into your roots.
Anything you can do to make your home and marijuana grow room energy-efficient is going to pay off in reduced electricity costs, and in the health of your cannabis plants.
In my view, winter is a better time to grow marijuana than summer.
In general, you have lower humidity during winter.
Cold weather kills molds, fungi and pests.
You don’t have to spend money to remove HID heat from your indoor marijuana grow room.
Cooler temperatures in your bud zone (as long as they’re not too cold), make for thicker, heavier, more resinous buds.
Cooler temperatures during drying and curing marijuana mean that your THC and other cannabinoids aren’t degraded by heat.
Now that you’ve read our cold weather marijuana growing tips, I know you’ll have high-THC, high-yielding plants every winter!
