The Optimal Temperature for Cannabis Growth

Most growers ask this question because they want to find the optimal temperature for each stage of a cannabis plant’s life. For a plant to develop and thrive, many different processes need to happen and each process happens at an optimal temperature. A single process (say, photosynthesis) is made up of a series of enzyme reactions (enzymes are proteins that accelerate chemical processes). Each enzyme has a temperature optimum. For example, the most abundant plant enzyme, RuBisCo, which is involved in photosynthesis, works fastest between 28 – 40°C (Vu et al. 2001). Below or above this optimal temperature will slow down the enzyme, and the overall photosynthesis process will happen slower (Vu et al. 2001). This is one reason why growing a plant at non-optimal temperatures can result in yield reductions.

This article is the first in a 3-part series exploring optimal growing temperatures for cannabis:

Since optimal temperature could vary based on growth stage, we’ll investigate the optimal temperature for each growth stage. The growth stages will be divided according to Mediavilla et al.:

  1. Germination and emergence: from dry seed to the emergence of the cotyledons
  2. Vegetative: from the first leaf pair to just before start of flowering
  3. Early flowering: half of the flowers have bracts (trichome-covered leaves)
  4. Late flowering: leaves start to dry

What Temperatures are Most Growers Using?

Germination and Emergence

There is a lot of variation in the temperatures that growers use to germinate cannabis seeds. Some go as low as 11 – 15ºC (52 – 59ºF) and as high as 31 – 33ºC (88 – 91ºF). However, it’s most common to germinate seeds between 25 – 27ºC (77 – 80ºF). What happens when you germinate in colder or warmer conditions? Many cultivators reported that very low temperatures delay germination while very high temperatures upset seed chemistry and ultimately lead to poor germination. Growers suggested using heat pads beneath the germinating tray to modify soil and air temperatures.

Vegetative

Following germination, most growers dropped the temperature down to 22 – 24ºC (72 – 75ºF) for the vegetative stage. Using 25 – 27ºC (77 – 80ºF) was another popular temperature regime. Although one cultivator suggested using temperatures as high as 31 – 33ºC (88 – 91ºF), this was very uncommon. Many growers explained that during veg, cold temperatures will slow photosynthesis and plant growth. On the other hand, growers said that too much heat can also delay growth by causing plants to stretch out, transpire more (which could lead to nutrient burn), or wilt due to oxygen deprivation at the roots. In the heat, cultivators also noticed an increase in problems such as spider mites, powdery mildew, and root rot.

Early Flowering

Many growers cautioned that extreme temperatures could cause problems during the early flowering stage. Temperatures between 22 – 24ºC (72 – 75ºF) were most common. Cultivators reasoned that low temperatures during flowering will help plants produce more potent buds compared to high temperatures.

Late Flowering

The majority of growers suggested sticking with the trusty 22 – 24ºC regime for late flowering. A few cultivators will grow in temperatures as low as 16ºC (60ºF) or as high as 30ºC (86ºF). The growers who advocated for an very cold or warm temperatures suggested that temperatures stress could increase trichome production. Cooler or warmer temperatures would affect plant processes and increase terpene content.

There is little consensus among indoor farmers on the ideal growing temperatures. For new to growers, this conflicting information can be confusing. To better narrow-in on the ideal temperature for growing cannabis, we will explore two other methods. First, we’ll look at the native growing range for cannabis and take a look at the seasonal temperatures in this area. Second, we will look at scientific studies that have also explored optimal cannabis growing temperatures.

Data available HERE and is accurate as of July 2018.