Marijuana grown indoors functions better at moderate temperatures between 70 and 75*F (21-23*C) during the light period and a drop of no more than 15*F (9*C) to 60*F (15*C) during the dark period. CO2-enriched plants will produce more at a marginally higher temperature of just under 80*F (27*C).
If the temperature drops below 60*F (15*C) during the dark period, plants will grow more slowly and yields will not be as abundant. This won't be readily apparent if you aren't particularly familiar with the garden's normal output. A few nights of cool temperatures won't significantly damage your crop, but if it continues to occur throughout the flowering period, it can definitely be cause for concern. A CO2 generator or electric heater can heat the room adequately.
If the floor can have a steady temperature at around 80*F (27*C), the roots will be warmed and the stems and leaves will withstand influxes of colder air. A heating mat is ideal if you've only got a few plants to worry about. Larger gardens might require the use of a recirculating hot water heater to ensure optimal temperatures.
Most outdoor varieties can endure temperatures as low as 50*F (10*C) without any problem. That being said, fifty degrees is still not an ideal temperature because it tends to slow down tissue growth and photosynthesis later in the day. Anything below 40*F (4*C) can result in damage to the tissue. Gas patio heaters can keep gardens warm on frigid nights. Maintaining a temperature of 60 degrees will promote plant growth substantially.
These outdoor plants can also benefit from a polyethylene plastic covering that keeps things nice and toasty while also protecting the garden from the elements. Heaters can increase protection even more. Download my free marijuana grow bible for more information about growing marijuana.
Most plants can withstand high temperatures if they have a large root system that can bring up an adequate amount of water to keep the plant comfortable during transpiration. During vegetative growth, temperatures that extend into the 80's and 90's (27-37*C) can produce elongated stems. During flowering, it can produce airy buds. This is the same for both indoor and outdoor marijuana plants.
For indoor plants, you can lower the temperature with ventilation, air conditioning, or by using air- or water-cooled lights that eliminate heat. Just to be clear, it isn't the temperature in the aisles of a room that you need to worry about; it's the temperature directly under the lights at the plant canopy level on which you need to focus your attention. Another great way to keep the temperature lower in an indoor grow room is with a LED grow light. To find out more about LED grow lights, click on the banner below:
For outdoor plants, you can cool the air with micro-sprayers that lower the temperature by an average of 20 to 30 degrees during the day. The spray emitted is full of droplets that are 5 microns or less in diameter. The droplets evaporate, providing a cooling sensation in the surround air.
Outdoor plants that are kept in small pots or small planting holes that are surrounded by poor soil will have a difficult time growing even if the soil is moistened regularly. This is largely because their roots are often smaller and less abundant and cannot draw water to the leaves. The roots can also be overheated if they are in containers that absorb light and produce heat.
First, check to see if the plant happens to be root-bound. If it is root-bound, then you should transplant it to a heftier container. To avoid overheating, paint over dark containers with light colors that will reflect instead of absorb most of the light.
If you see tip burn or if the buds closest to the lights are stretching and appear bleached, the area directly by the lights is too hot. Tip burn is occasionally misdiagnosed as too bright of a light. The intense light is fine; the heat is what gets to the buds.
For a quick fix, just keep the lights farther away from the plants. If you want something more substantial, try installing air-cooled lights with reflectors that will reduce the heat near the light. Water-cooled lights are actually more effective at diminishing light-generated heat.
For the most part, loon-watt lights should maintain a distance of around 3 feet (1 meter) from the tops of the plants. Air-cooled lights make the acceptable distance range between 18 and 24 inches (45-60 cm). Water-cooled lights make the acceptable distance about 12 inches (30 cm) or even less from the plant tops. With light movers, you can move the lights closer or farther away depending on your preference.
Check out my marijuana seedshop for a wide selection of top quality marijuana seeds. We ship to the United States, Canada, and number of other countries. If the order doesn't get to you or the seeds don't germinate, then you can get new seeds free of charge. And don't forget to download my free grow guide.