OK, this is how I understand transpiration and relative humidity. Most plants are about 90% water. Water is responsible for or helps with many things: it keeps the plant cells inflated (known as turgor); it serves as a transport medium for nutrients; it’s responsible for cooling leaves as it evaporates from leaf tissue during transpiration; and, with regards to this post, it serves as the regulator of stomatal opening and closing, thus controlling respiration and photosynthesis. For growth to occur photosynthesis must be greater than respiration.
Like everything else, I believe you try to achieve a balance between several factors to produce optimum humidity conditions for the PARTICULAR STRAIN YOU ARE GROWING.
The first factor you need to worry about is the particular plant you are growing. Different plants like to grow in different temperature ranges; if the temperature is outside of the range it likes to grow in then photosynthesis and respiration will be affected. So you need to determine the particular temperature range your particular plant likes.
The next factor you need to worry about is the thermoperiod, or the daily temperature range. Plants grow best when daytime temperature is about 10 to 15 degrees higher than nighttime temperature. Under these conditions, plants photosynthesize (build up, grow) and respire (break down, get rid of waste) during optimum daytime temperatures and then curtail respiration at night. Remember, for growth to occur, photosynthesis must be greater than respiration. The rate a plant photosynthesizes is dependent on the temperature a particular strain likes to grow in. If it’s too cold during the day for the plant photosynthesis (and therefore growth) is slowed; if it’s too hot then respiration increases (to help cool the leaves) to the point that it exceeds photosynthesis and growth is slowed.
The next factor is relative humidity,which is the ratio of water vapor in the air to the amount of water the air could hold at a particular temperature and pressure. Warm air can hold more water vapor than cold air. Basically, to compute relative humidity you take the amount of water that is ACTUALLY in the air and divide it by the amount of water the air COULD hold at a particular temperature and pressure.
Water vapor moves from an area of high relative humidity to one of low relative humidity. The greater the difference in humidity, the faster water moves. This factor is important because the rate of water movement directly affects a plant's transpiration rate, and its transpiration rate is directly related to its growth rate.
The relative humidity in the air spaces between leaf cells approaches 100 percent. When a stoma opens, water vapor inside the leaf rushes out into the surrounding air, and a bubble of high humidity forms around the stoma. By saturating this small area of air, the bubble reduces the difference in relative humidity between the air spaces within the leaf and the air adjacent to the leaf. As a result, transpiration slows down.
If wind blows the humidity bubble away, however, transpiration increases. Thus, transpiration usually is at its peak on hot, dry, windy days. On the other hand, transpiration generally is quite slow when temperatures are cool, humidity is high, and there is no wind.
So, in general, the plant will grow best with high humidity conditions…if that’s ALL we had to worry about, then I would say you would always want the highest humidity possible with no air movement, even during flowering, especially if you were growing in higher than optimal temperatures. But that’s NOT all we have to worry about. We NEED to supply air movement: it refreshes the CO2; it evens out the air temps in the grow area; it helps prevent insect and mold infestations; etc.
And since we need to supply air movement and often grow in higher than optimal temperatures, we need to increase the amount of humidity in the air to decrease the difference in the relative humidity of the leaves and the relative humidity of the air, thus slowing respiration and increasing photosynthesis and growth.
But as we increase RH we also increase the plant’s susceptibility to the likes of mold, especially in densely packed plants that are flowering. So we must achieve a balance between increasing the plants growth rate through increasing RH, while decreasing the plants susceptibility to mold with air movement at the same time. You can experiment by increasing your RH and decreasing your air movement until you get mold, OR YOU CAN GO WITH WHAT THE EXPERTS SAY ARE THE OPTIMAL RH CONDITIONS FOR GROWING WEED…it’s your choice…
