THE RIGHT TEMPERATURE

I presume you are taking about the temperature in the plant canopy.

Another, more accurate way to measure temperature is by using a surface temperature thermometer. This provides a reading of the temperature at the leaf surface, rather than of the ambient air.

The correct air temperature is related to the intensity of light and the carbon dioxide content of the air. Plants function in a way similar to cold-blooded animals. Their metabolism and photosynthetic rate increases with increasing temperature. When the temperature is below 15º, no matter how intense the light, virtually no photosynthesis takes place. The photosynthetic rate increases with increasing temperature.

Indoors, a well reflected 600-watt high pressure sodium (HPS) lamp area provides about 60,000 lumens after loss from light drift and reflective absorption. Placed over a M2 it is considered high intensity. Placed over 1.2 M2 it is considered moderate intensity. Whether under moderate or high intensity light when the temperature can be adjusted, the limiting factor often is CO2. With adequate CO2 levels the photosynthesis rate increases as the temperature rises. The level of CO2 and the ideal temperature is lower for plants under moderate light than plants under higher intensity.

For plants with a moderate intensity light the CO2 level required is about 900 PPM. For plants under high intensity the CO2 PPM level should run about 1300 during the lit period. Cannabis does not use CO2 during the dark period. At these levels the plants thrive at a temperature of 31º. Under moderate light they will probably do better at 26º.

If the plants are being grown in a high ventilation garden that depends on fresh air to replace the CO2, its level is a lot lower. CO2's ambient level in the atmosphere is only 380 PPM. So even with a high light intensity it cannot photosynthesize as quickly and the temperature for ideal growth is lower. Lower the heat in each situation by about 3º. Too high a temperature results in looser, lankier buds

A thermometer, even in the plant canopy provides an indirect reading of the plant temperature. It measures air temperature. A surface temperature thermometer can provide you with an exact reading of what is happening at the leaf level. Then you can adjust your thermostat accordingly.