What Happens if Your Plants Get Too Cold and How to Prevent It

What Happens if Your Plants Get Too Cold and How to Prevent It

As a seasoned cannabis grower, you’re probably aware of the damage heat can cause to cannabis plants. Heat is often the result of inefficient lighting, a lack of proper airflow or faulty environmental controls – and can cause irreversible damage, such as fried, crispy buds.

But what about the other end of the spectrum: what happens if your plants get too cold? This is an important consideration that doesn’t get enough attention. But with energy-efficient LEDs (with a comparably low heat profile) going mainstream, low temperatures may represent a mounting concern for growers across the map.

Ambient Air Temperature vs. Leaf Surface Temperature

Temperature matters. It plays a critical role in plants’ survival, because it influences transpiration. Transpiration is the process by which plants regulate internal temperature through absorbing water and expiring vapor through small pores called stomata. At an indoor grow facility, growers will want to monitor and maintain optimal temperatures for plant growth – the ambient air temperature, as well as the leaf surface temperature (LST).

  • Ambient Air Temperature: This is the temperature that we typically think of when we say, “it’s 76 degrees in this room, and 68 degrees in that room.”
  • LST: This measurement provides a more accurate representation of how your plants might be feeling internally. Leaf temperature is important, as this is where many metabolic processes occur.

Generally, cannabis plants’ LST should fall between 72 and 86 degrees. Reaching this optimal temperature requires a slightly warmer ambient air temperature between 75 and 89 degrees.

Vapor pressure deficit – referring to the relative humidity in the ambient air compared to the relative humidity within the leaf – can influence how well a plant transpires, and therefore its LST. That being said, growers will want to regulate both humidity and temperature to ensure their plants are transpiring and growing at their highest potential.

What Happens if Your Plants Get Too Cold?

The signs of heat stress in cannabis plants are obvious. They’ll frequently react with curled, browning leaves and produce dry, unhealthy buds.

On the other hand, cold stress can be harder to recognize. Cannabis plants exposed to low temperatures for an extended period of time might bring once-vibrant growth to a screeching halt. They may become more susceptible to pathogens or pests.

Of course, genetics determine just how well various types of cannabis respond to different temperatures. Some have simply evolved in cooler climates, making them more resistant to low temperatures, while other strains might quickly shrivel and die.

Temperature Considerations for the LED Cultivator

With outdated lighting technologies, such as HID or HPS bulbs, heat was a primary concern, placing additional pressure on year-round cooling systems to regulate indoor temperatures. LEDs, however, usually only emit around 25% of their energy as heat – they’re highly efficient. While this is a benefit in most cases, it can result in cooler indoor temperatures than one might expect. During the winter season or particularly chilly nights, cultivators will want to ensure their grow facility stays warm enough. In turn, lower temperatures may lead to a high relative humidity, requiring a dehumidifier to encourage proper transpiration.

A few tips for the winter growers out there:

  • During the winter, look for strains that are naturally more resilient when it comes to the cold.
  • Some expert growers recommend giving cannabis a little cold snap toward the end of the flower phase to encourage additional cannabinoid production (some cannabinoids are believed to offer a defense mechanism).
  • Integrate your environmental controls, including HVAC, temperature and lighting, to help you maintain the ideal conditions through each stage of your crop’s growth.

Overall, keeping your grow room warm enough takes less energy than perpetually cooling it down. Installing LEDs provides an energy-efficient option to support healthy plant growth, regardless of season.

Keeping an eye on that thermometer, and occasionally measuring LST, is key to a successful, year-round harvest.