Cannabis Plant in the Vegetative Stage Part Three: Lighting, Temperature and Humidity

by Alex Robles

Well cannabis lovers, it’s part 3 of “A Cannabis Plant in Veg” and this time I’m talking about lighting, temperature and humidity.  When I started writing this series I realized I had to hit the highlights of the different parts of growing cannabis because it’s really easy to get lost in the details.  My advice to any new grower is to first learn the best, safe practices for growing quality  cannabis.  Then when you’re comfortable and ready to take your skills to the next level, learn and understand why VPD ( Vapor Pressure Deficit), LST ( Leaf Surface Temperature) or micromoles per joule are important.  Right now, let’s just get comfortable with growing a healthy plant.

Lighting

Lighting Schedule

In the wild (outdoors), summer is the main season for the cannabis plants vegetative growth cycle.  Indoors our lighting schedule is meant to mimic the long days and short nights of summer.  The cardinal rule of lighting, to keep a cannabis plant in veg is a 18-6 photoperiod.  Which means it gets 18hrs of light/daytime and 6hrs of darkness/nighttime.  Some growers go as far as giving them 24hrs of light, I don’t agree with this method because I feel every living thing needs rest in order to thrive.  Me personally, I like to keep a grow area on a 16-8 light cycle.

Indoors

I don’t use an 18-6 lighting schedule because I like to veg indoors and flower outdoors in a greenhouse and I discovered that this schedule doesn’t work for my style.  When I put plants that were under this lighting schedule outside, they would start to flower within 7-10. That’s isn’t a problem when I’m flowering in the mild California winters and expect them to flower right away and be ready to harvest in 8-10 weeks.  I ran into problems when I put plants outside in the late spring early summer when the days start to get longer (second week of May).  The plants would start to flower and then reveg and sometimes turn into a hermaphrodite because of the light stress.  I figured out it was because no matter how long the spring/summer day was, it could never compete with the artificial lighting schedule of 18-6.  So I hit the books, the internet, took a bunch of notes, experimented with different lighting schedules to find the one that works for me.  A 16-8 schedule lets me put my indoor plants outside in the spring and summer to veg but keeps the stress of a light cycle change to a minimum.

Growers Note:  Some strains will still show signs of pre-flower with a 16-8 schedule when they’re put outside.

    Light Bulbs

Choosing the right light bulb for an indoor grow it’s like choosing a religion, it’s very personal.  HPS, Metal Halide and LED all have their pros and cons.  Every grower should do their own research and choose what’s right for their situation.  There are a lot of options nowadays and I suggest you read the lighting section of Building a Grow Cabinet Part One: Lighting for more information.  I do recommend having at least 55-65 watts of light per square foot of grow space for a cannabis plant in veg.

Outdoors

When I veg outdoors, I put the plants out around Mother’s Day ( mid May).  By then the spectrum is just right for the vegetative stage in my part of the world (southern California).  In a greenhouse you could supplemental lighting to keep the plants in veg.

Watering

Indoors or out, when the temperature is high, your plants are going to need water more often.  THEY DON’T NEED MORE NUTRIENTS, THEY JUST NEED MORE WATER.  Don’t change your feeding schedule, the plants don’t need more food.  You run the risk of nutrient burn or nutrient lock out when you feed every time you water in hot weather.

Temperature

Temperature is an important thing to consider when we’re growing cannabis.  Even though it thrives in lots of different regions, it’s not happy when it gets too hot or too cold for days or weeks at a time.  If it’s uncomfortable for me indoors, in the greenhouse or outside, I know it’s uncomfortable for the plants.

I use a thermometer/hygrometer to measure the temperature and humidity.  I don’t just look at the ambient air (surrounding air) temp, I also measure the “leaf surface temperature” (LST) with a laser thermometer because this will tell me exactly how hot or cold the plant itself is.
Thermometer/Hygrometer
Laser Thermometer

Growers Note:  You DON’T need a laser thermometer, we’ve been cultivating cannabis for thousands of years without them.  It’s just a nice tool that helps me and I like playing with lasers.

A healthy, well established plant can thrive when nighttime temps are as low as 60F (15C) and daytime temps as high as 95F (35C).  Anything higher or lower than that and the plant starts to slow down and show stress in different ways.  Young plants are more vulnerable to extreme temperatures because their root and vascular systems are less developed.  I notice the most vigorous growth when I keep my night time temps at around 65-67F (18-19C) and the daytime temps no higher than 78-80F (25-26C).  I try to keep the high/low temps within 15 degrees of each other because the wider the temperature gap, the more it’ll stress them out.

Highs

One of the first signs of heat stress that I notice is that the serrated edge of the leaves will start to curl up and “cup.”  If I don’t do something to help the plant with the heat, the leaf will start to look burnt, get crispy, dry up and die.  Sometimes the leaves will just yellow or get brown burnt spots on them.  Eventually the plant will choose survival over growth and use all its resources to help keep itself alive.  That’s why plants get stunted, those times it gots too hot, it just stopped growing, we don’t want that.

Indoors

The first thing I do indoors is cool the room, then I figure out why the room is getting hot and fix it.  So i’ll ask myself a few questions.

  • Is the indoor location too hot?  An attic grow is going to get hotter in the summer than a basement grow because heat rises.
  • Do I need a bigger intake/exhaust fan?  If you have heat building up in the room, the answer is more than likely “yes”.
  • Are my lights too intense for the space?  The biggest producers of heat indoors are lights.  I can’t cram an industrial light into a 6ft x 3ft space it wasn’t designed for and not have a lot of heat build up.

Outdoors

The best thing I can do for plants that outside is watch the weather report and put shade cloth over them, especially if they’re small/young (4ft or smaller).  I also make sure not to put the plants next to building or structure that could block the prevailing wind.  Ventilation is important to help a plant deal with high temperatures.

Lows

Prolonged cold/low temperatures (below 60F) are going to slow down a cannabis plants metabolism, which is going to slow down or stop growth.  The active, healthy growth that we see with warmer temps won’t be there.  During these cold periods, the root system struggles to absorb some nutrients that are vital for growth, so the plant will focus on staying alive instead of growing.  This is going to leave your plants small and weak.

Growers Note:  Don’t forget to check your soil temperature if your plants are outdoors in containers, especially if you’re in black nursery pots.  You could wrap the pots with white plastic bags to help reflect sunlight off the pots.

Indoors

When you’re vegging indoors and cold starts creeping in you’re going to have to increase the heat  somehow.  Space heater are good for small grow areas.  You could also think about running your lights at night to help heat the room.

 Outdoors

If you’re in a greenhouse, space heater might work, it depends on how it’s built.  If the greenhouse has a concrete floor, make sure you get the pots off of that floor.  The concrete acts as a heat sink, it will suck the heat from the pot.  Put the pot on a piece of wood, the points is to get it off the cold ground.  If you’re completely outdoors and winter has moved in, you’ve run out of time.  Pack it up and use the winter to plan for next spring or move indoors.  Unless you live in place where winters are mild, you’re going to be fighting “Mother Nature” the whole time.  I have no advice for heating the outdoors.

Growers Note:  Be smart and safe when using a space heater. MAKE SURE NOTHING IS GOING TO FALL OR DRAPE OVER THE HEATER AND CATCH ON FIRE.  That includes the grow tent.  Please be safe, this plant is not worth you burning down your house.

Humidity

Humidity is an important thing to understand, but not an easy thing to explain.  You see temperature and humidity are two heads of the same body, we can’t think about one without thinking about the other.  When experienced growers talk about humidity they’re talking about “relative humidity” (RH), which is the amount of water vapor that the air is holding.  Think of it this way, 60% relative humidity in a room means 60% of that air is holding water.  The warmer air the more water vapor it can hold.  As the air cools, the vapor turn back into water falls as dew.  The bigger the swing between daytime highs and nighttime lows, the more moisture is going to fall on the plants.  I don’t want a thick extra layer of water sitting on the plants with cooler temperatures in the room, that added moisture could invite mold, fungus and disease.  I like my veg area to have around 50%-70% humidity.

Lows

If the humidity gets too low it could make the plants more likely to struggle with heat. The leaf edges could start to cup up with low humidity even if it isn’t that hot.  I can increase the humidity in an indoor room by running a humidifier which will put moisture in the air.  With small grow cabinet I’ve used wet rockwool cubes in a party cup with mixed results.  In a greenhouse I’ve use solar fountain pumps in 7gal buckets with water to help put moisture in the air also.

High

When the humidity gets too high, it makes it more likely for molds and fungus to grow on cannabis plants.  In order to avoid this indoors I make sure all parts of my ventilation is working the way it should.  Exhaust fans should be taking hot moist air out, while intake fans are pulling in fresh cool air and stand alone fans are circulating all the air.  If this still doesn’t solve the humidity issue, I’ll throw a dehumidifier in there.  Outdoors the best thing I can do is choose a well ventilated area that the wind blows threw to put my plants.  In a greenhouse, circulating fans and exhaust fans are great and solar exhaust fans are the best.

Growers Note:  Different things are going to cause humidity in different growing situations.  Work towards figuring out what is causing it to build up and fix it.

Well my fellow cannabis enthusiast, there it is, as plainly as I could put it.  Leave me a question or comment, I’d love to hear from you.  Remember to hit the subscribe button before you leave and as always.

Grow Learn Teach

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