Indoor Cannabis Growing: Relative Humidity and Temperatures
The most refined techniques to grow cannabis become irrelevant when relative humidity and temperatures are not being controlled – learn more about these two major factors.
Final results of an indoor grow are greatly influenced by the way growers keep in control of parameters that influence their plants growth. There are two basic factors that can easily be forgotten when we’re busy thinking of other ways to increase yields, size, and overall health of our plants – temperature & relative humidity. This blog summarizes ways to keep both of these factors within an optimum range, and provides specific information what conditions should be maintained to achieve best results.
HOW TEMPERATURES AND HUMIDITY LEVELS INTERACT
It’s important to know that humidity levels and temperatures are closely related to one another. When we talk about humidity, we usually mean relative humidity (RH), which is the ratio of partial pressure of water vapor to the maximum vapor pressure of water at the same temperature. You get the whole idea when knowing the basic principle that warm air holds more water vapor than cold air. This is one of the reasons why it’s necessary to extract a lot of warm air from our grow room, and ideally allow cool air to enter – warm air simply holds too much water vapor in it.
HUMIDITY LEVELS AND TEMPERATURES: FROM SEEDLING TO HARVEST
We need to define what humidity and temperature control actually means when growing cannabis. It makes sense to divide the life of cannabis plants into 4 different stages in which humidity levels, and temperatures, should be adjusted to ensure healthy growth. Don’t think that humidity and temperature control is complicated and not worth it! It’s generally very easy, and more about keeping parameters within a certain range, and as constant as possible.
The first thing you need to do is to buy a hygrometer and thermometer, preferably a digital one with memory function, also showing maximum and minimum values of the past. Some hygrometers aren’t the most accurate, so don’t bother having several devices in your grow room to compare values. Now that we’re able to closely monitor our conditions, we can get to the essence of humidity and temperature control – the actual humidity levels and temperatures we aim for.
1. Seedling Stage
- Seedlings and clones like high humidity levels of 65-70%
- Reason: The root system is not established
- High humidity levels allow water intake through leaves
- Temperatures with lights on: 20-25 C° (lights off: 4-5 C° lower)
2. Vegetation Period
- Humidity levels can be lowered by 5% each week (acceptable range: 40-70%)
- Temperatures can be increased a little bit (no obligation)
- Reason: Roots absorb more water; evaporation through leaves cools plant(s)
- Temperatures with lights on: 22-28 C° (lights off: 4-5 C° lower)
3. Flowering Period
- Humidity levels need to be lowered to 40-50% (extremely important)
- You can get away with 55% (anything over 60% is real bad)
- It’s best to slightly lower temperatures in flowering
- Temperatures with lights on: 20-26 C° (avoid high temperatures)
4. Late flowering (1-2 weeks before harvest)
- The following steps are no necessity, but can improve yield, flavour and appearance
- Bring down humidity levels as much as you can: 30-40%
- Lower daytime temperatures, and also increase the temperature difference (day/night)
- Temperatures with lights on: 18-24 °C (lights off: minus 5-10 C°)
ADJUSTING HUMIDTY LEVELS AND TEMPERATURES
We’ve got a pretty good idea on humidity levels and temperatures we aim for. Now it’s time to get to the practical part, and to find ways to bring things back in balance when they’re not. Most growers will struggle to keep both relative humidity and temperatures down, which is of primary importance in the flowering period – we got that. In some colder regions, and depending on the lighting solution, the opposite scenario might be the case, and temperatures or humidity levels must be raised.
Remember the basic principle that warm air holds more water than cold air? Keep this in mind, and be aware of the fact that relative humidity and temperatures interact with one another.
Finding ways to control humidity and temperatures is crucial when growing cannabis indoors. This blog shows practical steps for best results.
Hot Tips For Growing Weed In Cold Weather
Cannabis is a plant that loves being warm, sure, but that doesn’t mean you can’t grow it in other climates. With a bit of preparation, enough knowledge, and just a pinch of technical ability, you might just be able to grow healthy cannabis plants even in cold conditions.
We all know that cannabis grows best in areas with warm summers and plenty of sunshine. But, being such a versatile and hardy crop, you might be wondering whether it’s possible to grow cannabis in cold weather. Thankfully it is, but there are some key tips and tricks you’ll need to keep in mind.
In this article, we’ll walk you through everything you need to know about growing cannabis during those chilly winter months.
Getting It Right From The Start – Picking The Perfect Genetics
Every cannabis grow is dictated by genetics. In turn, if you’re planning a winter or cold weather grow, your best bet at achieving a respectable harvest is investing in the right genetics from the start.
Ideally, you’ll want to look for strains with genetics linked to cooler areas of the globe. Indica-dominant strains and landrace varieties from mountainous areas of Asia, for example, tend to be much better suited to cooler temperatures than sativas.
Autoflowering strains are another great option for growers living or growing in cold regions. Cannabis Ruderalis, the variety of cannabis bred with indicas and sativas that gives descendants their autoflowering properties, is native to Central/Eastern Europe and Russia, and is renowned for being especially hardy. Plus, the fact that autos can go from seed to bud in as little as 8 weeks makes them optimal for sneaking in an extra early Spring or late Autumn harvest.
How Cold Weather Affects Cannabis Plants
There are 2 main concerns when it comes to growing cannabis in cold weather or winter. The first is shock to the plant’s root systems.
Healthy roots are the heart and soul of a healthy cannabis plant, and cold weather can directly impact the health of a plant’s roots.
Once soil temperatures drop below 12°C, a cannabis plant’s metabolism starts to slow down considerably. As it does, the plant will struggle to take up water, nutrients, and oxygen from its soil, and many of the enzymatic processes needed to fuel its growth will come to a standstill. Over time, the plant will likely suffer from stunted growth or may even start to wilt.
In some strains, cold weather may also trigger hermaphroditism, meaning that the plant grows both pollen sacs and buds, leading to self-pollination and dramatically decreased yields in turn.
Mold is the second major concern when growing cannabis in cold weather, especially if there’s moisture in the air.
Moisture from rainfall, snow, or frosts can build up and become trapped on a plant’s leaves or flowers, triggering mold issues that can be extremely hard to get rid of. That, in turn, can cause the flowers of your plants to rot well before they’re ready for harvest.
How Cold Can Cannabis Get?
This leads to another question: How cold can cannabis plants get before problems arise? Well, to answer that question, we first have to consider the perfect temperature for cannabis plants.
For the most part, they’re happiest at temperatures between 20-25°C, depending on which stage of their life cycle they are in. Vegetating plants, for example, like it warmer, as they’re usually in that stage during the peak summer months. In that same way, flowering plants tend to like slightly cooler temperatures, as they’re usually in this stage around the beginning of autumn. Meanwhile, the ideal soil temperature for healthy, stable root growth is between 15-20°C.
Straying a little outside of these ideal temperatures, though, won’t kill your plants. After all, cannabis growing in the wild isn’t always exposed to that ideal range. However, if you’re growing indoors and have the opportunity to meticulously control every aspect of your grow, staying within these temperatures will ensure your plants grow at their best.
Getting That Extra Late (Or Early)-Season Crop
One of the reasons you might be considering growing weed during the coldest months of the year is to get in an extra harvest, either before or after the growing season. If that’s the case, we recommend planting autos and germinating them either late in the fall or, preferably, later in the summer.
Depending on the flowering time of the particular strain you’re growing, your plants could be harvest-ready either by mid-winter (if you germinated your seeds in fall) or mid-fall (if you germinated in summer).
Note that this technique only works in regions with a mild winter or a warm spring. Don’t try this technique if you’re living in an area with harsh, cold winters and unstable spring temperatures. In these areas, you’ll need to build a greenhouse to protect your plants from the elements.
What Effect Do Cold Temperatures Have On Flowering Cannabis Plants?
Many indoor growers opt to bring down the temperatures in their grow room during the bloom phase. This helps mimic the natural temperature drop plants would be exposed to when growing outdoors, and can also help promote bud development as the plant prepares for winter and focuses on reproducing. Many growers also find that gradually dropping the temperatures in their grow room can help encourage their plants to produce buds with purple hues.
Can You Grow Cannabis Outdoors In Winter?
No. Unless you live in an area with very mild winters and low rainfall, we don’t recommend growing cannabis outdoors during winter, as you’ll usually end up with sickly plants and/or poor harvests.
If you do try your hand at keeping a bunch of outdoor plants during winter, make sure to follow these tips to keep them healthy and get the best possible results:
- Keep your plants’ roots warm. Using heat mats to keep your soil temperatures above 12°C can help minimize the stress to your plant’s root zone and promote healthy vegetative growth and flowering.
- Protect your plants from rain or snow. Remember, any humidity trapped on your plant’s leaves or buds can cause serious mold issues.
- Know your dates. If you have your plants growing outside at all, remember to bring your plants indoors before the temperatures outdoors get too cold.
Avoid Light Stress Or Revegetating (When Moving Your Plants Indoors)
If you need to bring your cannabis plants indoors to shelter them from the harsh winter, make sure you do so carefully. Pay close attention to the daytime hours and temperatures, and do your best to match those conditions indoors.
If your plants have started to flower, you’ll need to be even more careful. If you give them more light indoors than they’ve been getting outside, they may revert back into their vegetative cycle. This process is known as revegetation, and causes extreme stress for the plant.
Force Flowering Outdoors
If you can’t move your plants indoors in time to save them from the winter, plan ahead and consider force-flowering your plants outdoors. The easiest way to do this is to cover the plants with a light-proof structure or fabric (like a tarp) during their new “nighttime” hours. This will give them the impression that the seasons are starting to change, and they’ll jump into the flowering phase accordingly.
Move Plants To A Greenhouse
Greenhouses essentially give you the freedom to grow weed all year round, given you have adequate control over the lighting, temperature, and humidity inside your greenhouse. If you’re considering using a greenhouse to grow your own weed, here are a few things to consider:
- The initial cost of building and setting up a greenhouse can quickly add up and usually top the costs of setting up an indoor grow room/tent.
- Greenhouses allow you to harness the power of the sun all year round (given your region gets plenty of sunlight).
- Growing in a greenhouse helps keep your plants away from prying eyes.
Why The Old School Indoor Grower Welcomes Cold Weather
Old school HID systems emit a great deal of heat, as well as light. Keeping cannabis plants in vegetative growth with an MH bulb under an 18+ hour light cycle is actually easier in cold weather and a simple method to preserve mother plants and/or cuttings during the winter months.
Flowering with HPS lighting and vegging with MH bulbs usually requires serious air cooling. The old school grower can turn cold weather to his/her advantage and save some cash on the air conditioning bill at the same time. Some even add extra lights to boost yields when the weather gets cooler.
Why The Next-generation Grower Needs To Turn Up The Heat In Winter
LED lighting systems are rapidly overtaking HID bulbs as the cultivation lamps of choice for Millennial weed growers, but the technology is still to be perfected. LED lights run cooler, because they emit almost 75% of their energy as light. This is fantastic most of the time and usually makes for easy environmental control in the grow tent.
However, LEDs can run too cold during the winter months. The simple solution is to turn up the heat. Unfortunately, this might negate the electric bill savings made by running the low wattage drawing LEDs.
High RH may also be an issue with LED systems in cold weather, a dehumidifier may also need to be added to the grow show and definitely during the bloom phase.
CFLs are more or less redundant during prolonged periods of cold weather, even the highest wattage bulbs available, with 300W, cannot generate sufficient heat to curb a frosty winter.
Bud rot is a real threat for growers cropping in cold weather with next generation lighting like LEDs and CFLs. Hydroponics and LEDs can be a recipe for disaster in cold weather, as both root rot and bud mold can be real threats if feed water is below room temp and RH exceeds 60% during flowering.
Maintaining environmental control year round is the only long-term solution; keep an eye on those thermometers and make adjustments when necessary. It’s a whole lot easier to heat a grow room in winter, than it is to cool a sweltering HID grow show in high summer.
Old school HID growers may be able to avoid turning on the heating during the dark cycle if they run their lights during the cold winter nights. Daytime temperatures are usually a little higher, so it might be possible.
LED aficionados face the prospect of running the heating virtually nonstop. Perhaps a combo of HID and LED is the smartest compromise in freezing conditions for the modern indoor cannabis grower.
We almost forgot to mention cropping cannabis in cold weather has the huge advantage of increasing the chances of certain strains displaying beautiful lilac and deep purple shades every weed grower covets most.
Wondering how you'll be able to brave a winter cannabis grow? Click here to learn all there is to know about growing weed in cold weather.