Tips for Growing Cannabis Indoors in Winter

When growing indoors during cold-weather months, lights-off grow-room temps drop several degrees, mimicking natural conditions that are favorable to plant morphology and bud quality when growing indica and Kush strains.

Cooler lights-off, or nighttime, temperatures promote bud development and resin production, while limiting the stretch in plant height that occurs in the first two to three weeks of bloom phase. Night temperatures are optimum when they're three to five degrees less than lights-on temps. This day/night differential is most helpful when you're growing Kush strains whose genetic lineage comes from the mountainous regions of Central Asia.

Cooler night temperatures in a sealed indoor grow room often result in high relative humidity. In grow phase and in pre-flowering bloom phase, a humidity range of 55 to 75 percent is acceptable. However, in peak and late bloom phase, when thick indica or Kush buds are present, relative humidity above 54 percent creates conditions favorable to the dreaded gray mold that can destroy a crop overnight. Many growers install dehumidifiers in their grow rooms so the moist air is kept under control. Dehumidifiers add heat to the grow room, which could be of benefit if the grow room tends to get too cold.

Grow Room Temperatures

Growing with Hydroponics

Now, it doesn't take a genius to figure out that you will probably have to provide your indoor cannabis plants with additional heating during the winter. As a rule of thumb, the temperature you should be providing for cannabis plants during the daytime is 24-30°C (75-86°F), while temperatures overnight should be somewhere between 18-22°C (64-72°F). What's crucially important to remember is that the rate of growth and the quality of your plants in general will be impacted severely if the temperature during the night falls any lower than 16°C (61°F).

It's also important to be aware of the fact that you will need to keep a close eye on the difference between the temperature during the daytime and the temperature at night, particularly during the initial three weeks of the flowering period. If the difference in temperature from day to night is too great, you may find that your plants end up noticeably stretched with excessively wide spaces between the internodes. By contrast, if you ensure that the discrepancy between night-time and daytime temperatures is as small as realistically possible, space between nodes will be reduced. Ideally, you should be looking at differences in temperature of between 2-4°C (3.6-7.2°F) for optimum results during the first three weeks of the flowering period. For the rest of the time, the discrepancy should be kept below 10°C (18°F) from night to day.

Taking all of this into account, the most important thing to be aware of is whether or not you are using the kind of lighting setup that generates a lot of heat. The reason being that if you are, the primary advantage is that this kind of rig will usually provide your plants with plenty of heat to keep them warm during the day. However, this can also lead to a significant and potentially dangerous drop in temperatures overnight, after the lights have been turned off. If this is an issue, one extremely simple yet effective way of getting around it is to switch your day-night cycles. Roughly translated, try keeping the lights on overnight to compensate for the cooler temperatures, while turning them off during the daytime when ambient temperatures will remain at a more appropriate and consistent level.

Of course, you may not find that the ambient temperature during the daytime is enough to replicate the 'overnight' temperature you are looking to create of 18-22°C (64-72°F). Which in turn means that you may need to think about installing some kind of heating device, in order to ensure that the plants are kept at the optimum temperature throughout the day (or in this case night). Be sure to invest in quality thermostats to keep an eye on temperatures at all times, given the way in which they are often subject to a fast and significant changes during the winter months.

Controlling Humidity

Often overlooked though of equal importance, keeping an eye on ambient humidity during the winter months is something that needs to be stepped up significantly. The reason being that as the temperature of the air decreases, the amount of water that sticks around in the form of vapour also falls. Air that is 25° C for example can hold considerably more water than air at a temperature of 15° C. But it isn't just the fact that the amount of moisture in the air itself will continually have an effect on your cannabis plants. Instead, there is the way in which the temperature conditions during the winter significantly increase the likelihood of condensation build up - particularly as temperatures fall during the night.

Chances are, you probably already know that cooler temperatures and damp conditions are exactly the kinds of conditions favoured by powdery mildew and other harmful moulds in general. If condensation and moisture are allowed to build upon your plants and remain over time, it is considerably more likely that you will find yourself facing these kinds of potentially devastating problems. In terms of combating the problem, keeping the temperature slightly higher than you normally would be a great way of preventing condensation. In addition, it may be worth investing in a decent dehumidifier for the room, which can make a big difference throughout the winter months.

Interestingly, winter has a tendency to be the driest time of the whole year in many regions, which in its own right can present a number of problems when it comes to growing cannabis. Just as it is possible that the air in the grow room or grow tent may become overly moist, air that is brought in directly from the outdoors may be excessively dry and perhaps even call for a humidifier. Suffice to say, it is important to keep a close and continual eye on humidity levels at all times, in order to strike the best possible balance.

Winter Lighting for the Indoor Grow

Lighting for indoor grows can remain the same year-round, for obvious reasons. Other than adding supplemental heat, the plants remain unaware of changing seasons outside, so indoor growing practices remain the same regardless of the date on the calendar. When growing indoors, there are many lighting options with a variety of fixtures and hood style to maximize light intensity and manipulate direction.

Indoor commercial growers rely on clones for consistent, high-quality production. Clones will be several inches tall when placed into the growing environment, so HPS grow light is perfect right from the start. Light-emitting diode lamps LED grow lights are gaining rapid popularity in the cannabis industry and provide another option for indoor growers. While LED lights cost more than typical HPS lights initially, they consume less electricity, so the cost of operating them is lower. Eventually, anyone who invests in LED will recoup the extra money spent to purchase them. LED lights also runs cooler than HPS, so they won't increase the temperature as much in the grow area, and they are safer overall.

Many growers who plan a winter grow indoors grow from seed. Growing with seed is quite different from growing with clones, so metal halide (MH) lighting is a great option for very young cannabis sprouts. Metal halide does a particularly good job of growing very young plants and helping them develop a root system. For initial fast growth, MH will provide a small advantage over HPS. The MH lights cause rapid growth initial development while HPS prove more effective in the later stages of plant growth. After the first two or three weeks, the cannabis plants will appreciate a lighting change. Switching from MH to HPS will boost plant branching and secondary root development. The use of HPS lighting is particularly beneficial during the flowering stage of plant growth, when plants really fill out and add a great deal of size with the development of fat, sticky buds.

Separate lighting fixtures, hoods, and ballasts exist for both MH and HPS lights. Because these types of lights cost quite a bit of money, investing in both of them might not be practical unless you have a good budget for your grow. Fortunately, there are ways around this to avoid spending too much, while still enjoying the benefits of using both MH and HPS.

Conversion bulbs are effective in changing lighting from MH to HPS. The size of the lights is identical, but the rays they emit are noticeably different. The MH light emits a very bright white light compared to the HPS conversion bulb, which has reddish-orange rays. The HPS conversion bulb effectively replicates the HPS rays of a true HPS bulb, while using a MH setup with MH ballast and hood.

Better yet, there are conversion ballasts that will change from MH to HPS with the flip of a switch. Although the conversion bulbs work quite well, the conversion ballast works even better and completely converts to using true HPS bulbs. When the time comes to switch from MH to HPS, merely flip the ballast switch from MH to HPS and change the light bulb from an HPS bulb to a true HPS bulb. The HPS light from this setup is true HPS instead of a close replication from a conversion bulb.

Both of these options allow growers to benefit from the different MH and HPS grow light rays and enjoy the best of both worlds. Whether using a conversion bulb or a conversion ballast, there is no need to overspend and buy completely different lighting systems specifically for both MH and HPS.