Growing high-quality cannabis is usually a warm weather endeavor. Outdoors, cannabis plants grown from seed germinate in the spring, and the seedlings grow steadily throughout the spring and summer months until they flower in late summer and fall. Mild temperatures keep the plants thriving and growing steadily.
Outdoor crops grown from clones differ in minor ways. Since clones are usually placed in a grow environment when they are at least several inches tall, their temperature needs are only slightly less specific than those of a recently sprouted seedling, which demands meticulous care and watering until it hits its stride with regard to vegetative growth. Both seedlings and young clones benefit from temperatures on the warmer side of the thermal gradient, and also benefit from higher humidity at this age than at any other time.
To properly understand the possibilities for growing cold-weather cannabis, it’s important to know what ideal cannabis cultivation temperatures are, and why deviating from them to grow a colder winter crop is a challenge, although possible.
Ideal Growing Temperatures
The ideal temperatures for robust growth and healthy young plants fall between the 70- to 85-degree (Fahrenheit) range. Seedlings and young plants in the vegetative phase of growth do not respond well to wide temperature fluctuations and appreciate consistent warmth—much like sativa plants in the equatorial regions of the world enjoy consistently warm environments. In temperate regions of the United States and other parts of the world, cannabis thrives and grows at its most rapid rate during the toasty days of summer, when temperatures remain high.
During the flowering stage of growth—which occurs outdoors when days grow shorter in late summer and fall—the optimum temperature level reduces slightly to about 65-80 degrees to coincide with shorter days and the sun sitting lower in the sky. Temperature fluctuations that do not exceed more than 10 degrees between daytime and nighttime are ideal. Although warmth this steady is difficult to maintain even in the most cannabis-friendly climes, it makes a good benchmark for what cannabis needs for perfect plant development and growth.
In the late flowering and pre-harvest phase of plant growth, which outdoors occurs in the fall, temperatures in the 60- to 75-degree range are ideal. Those who grow cannabis outdoors are limited in their ability to control their regions’ climates; their best course of action is to grow their crop in a steadily warm area—which is the reason areas like California are the most advantageous for outdoor cannabis cultivation.
When temperatures fall below 60 degrees or above 90 degrees, plants do not respond favorably. Lows and highs outside of the ideal range interrupt plant growth and development. At temperatures that are too low, optimum photosynthesis no longer occurs. Plant growth slows, and overall vigor decreases. In extreme cases of extremely low temperatures that approach freezing, cannabis plants will perish. A similar situation occurs in environments with extreme heat. Plant growth slows, and vigor decreases. Overly hot temperatures result in curled or cupped leaves. The plants will begin to turn yellow and lose the vibrant green seen in healthy cannabis plants.
In growing environments that are either too hot or cold, cannabis is also more susceptible to pests and disease—particularly if the grow area has excessively high humidity. Mold, white powdery mildew, spider mites, and root rot are more of a threat when plants are vulnerable due to improper temperatures. If the plants are flowering, trichome production reduces, and overall potency will not reach optimum levels. In cold temperatures, many cannabis plants will develop visually pleasing purple coloration. This is a different situation from strains that assume a purple hue due to their genetics. Purple due to overly cold temperatures is a sign that the plant would benefit from additional warmth.
Tricks to Extend Growing Seasons
Outdoors, a cold-weather winter grow will never truly thrive like it will during the normal growing season, which is why growers move their operations indoors for optimum year-long, uninterrupted production. Growing outdoors, however, can be done productively with some careful planning. By head-starting young plants to a decent height (2+ feet) indoors and placing them outside to flower in fall, they will immediately begin to produce buds.
Careful monitoring of outdoor temperatures during their time outdoors may prove necessary. Moving plants indoors overnight if temperatures dip too low would be wise for best results. Allowing plants to reach maturity outdoors before harvest will allow a grower to take advantage of natural sunlight during fall and early winter—if weather permits. The plants will not reach their full potential, and the harvest will be smaller, but transferring plants head-started indoors to outdoors before harvesting will make an extension of the outdoor growing season possible.
Some cannabis strains are more tolerant to cold weather than others. Pure or heavily dominant indica strains come from plants that naturally grow in a cooler environment in their countries of origin, so they make the best choices for late-season outdoor growing. Heavy-hitting indicas like Northern Lights, Granddaddy Purple, Afghani, White Widow, Afgooey, and Maple Leaf Indica would make good strain choices for grows that extend beyond what is natural for them in North America and other regions of the world.
Not only do indica and indica-dominant strains like those mentioned above come from genetics that are naturally adapted to cooler climes, but they also have much quicker finishing times than lanky, heat-loving sativa strains. As winter days grow colder and winter solstice approaches, a quick harvest is imperative. Even with some special measures taken to enjoy an unnaturally late harvest, outdoor conditions will eventually deteriorate—even in the warmest of American climates—so that a harvest proves necessary. The quick harvest time of indica makes a late outdoor harvest possible, but the long wait required for many top sativa strains would be impractical at this time of year.
Winter Greenhouse and Indoor Growing
Rather than relying on extreme measures to continue growing outdoors, the vast majority of outdoor growers move indoors to benefit from a spring harvest indoors, while the winter chill sets in outside. Greenhouses equipped with supplemental heat and lights will extend the photoperiod and provide comfortable temperatures to the winter cannabis garden. Once the sun sets, lights will trick the plants into thinking they are growing during spring or summer, and they will continue growing in their vegetative phase until the grower chooses to reduce the photoperiod for flowering.
For indoor grows, there are a wide variety of high-intensity discharge (HID) lights that work well. High-pressure sodium (HPS) are the most traditional indoor lights, but metal halide (MH) does a good job as well, although it’s not as popular as it once was. Low-intensity diode (LED) lights are quickly gaining in popularity and favor because they are more energy efficient and do a fine job of growing cannabis. They do cost a bit more than HID lights, but their monthly operating costs are lower, which lowers their overall carbon footprint.
If the indoor grow is within a heated house, HID lights emit considerable heat that will keep plants happy and toasty during the winter. If the winter grow occupies an outside building without supplemental heat, HID may keep the grow area warm enough, depending on outdoor temperatures. LED lights, however, do not produce very much heat, so supplemental heat will most likely prove necessary for optimum plant growth.
Regardless of what steps you take and your cultivation methods, there’s no reason cannabis production needs to stop once the winter chill sets in. With a little foresight and planning, you can enjoy continuous cannabis production all year long!