Growers tend to focus on certain critical factors contributing to the health and vigor of plants such as growing media, seeds, lighting conditions, temperatures and fertilizer compositions. These are all important considerations, but we may be overlooking one major variable: relative humidity and the role it plays on stomatal opening and closing.

In a controlled environment like a house or greenhouse, we sometimes fail to monitor relative humidity and, in the process, may be producing inferior plants. In this article, the mechanism of stomatal opening and closing, relative humidity and the interaction of both will be discussed. A little understanding will go a long way in getting better results.

Plants, like people, respond to stimuli. People respond to stimuli such as hunger, noise and fatigue. For plants, stimuli can encompass numerous factors such as temperature, light and water. Growers often gauge success or failure based on visual cues like growth patterns, form, yellowing leaves, wilted appearance, days to maturity and flowering.

Factors that are sometimes not externally or overtly visible can go unnoticed or, worse yet, attributed to other factors. Simply put, monitoring stomata conductance or the influx of carbon dioxide into the stomata with an accompanying efflux of water vapor may be an overlooked factor for any plant.

Without being bogged down by details, the stomata are tiny gates or pores covering a plant. While they are often on the undersides of leaves, they can be found on the upper sides and stems as well. As diverse as the plant kingdom is, it should be no surprise that there are a myriad of adaptations to this particular component.

Stomata are the gas exchange sites that allow carbon dioxide in for photosynthesis and release oxygen and water out. The opening and closing of the stomata is controlled by a series of complex steps, but put simply, water movement into the guard cells of the stomata inflates the cell, causing the stomata to open.

A loss of water from the guard cells causes a decrease in turgor pressure, which quickly closes the stomata. Since water loss from your plant is attributed to the stomata, there is one factor we must pay close attention to: relative humidity.

Relative humidity or RH, is a measure of the amount of water (vapor) in air relative to the saturation point, expressed as a percentage. When air is saturated with water, the value at a given temperature would be 100%. Two integral components of RH are temperature and the total amount of moisture present.

Air is able to hold more moisture as the temperature decreases. Likewise as temperature increases, the carrying capacity of moisture in air decreases. This concept will have important ramifications that will be discussed later.

So why does relative humidity matter when we are talking about stomatal opening and closing? Life exists in a constant battle to reach equilibrium. Chemicals at a high concentration will want to move to areas of lower concentration to maintain this equilibrium. Water is a chemical compound and as such will want to diffuse or move accordingly. This is where the plant stomata come into play.

For example, if the relative humidity surrounding the leaf is low, then this gradient will want to pull water from the plant. This is why plants, especially grown in containers, can dry out quickly and wilt when the wind is blowing and the relative humidity is low.

This stimulus (along with darkness) can trigger the closure of the stomata. This may sound great, especially when you think about plants under drought or water stress, but this also significantly reduces uptake of carbon dioxide, which reduces photosynthesis.

A reduction in the rate of photosynthesis is obvious as there will be a stunting of growth and an overall lack of vigor. If this scenario played out continuously, it would result in energy starvation and plant death.

On the other hand, exposing plants to relative high humidity will have the opposite effect as stomata will remain open. However, while you might think that growing plants in a humid environment would cut back on watering needs because that pull to lose water is eliminated or significantly reduced, there are three main problems associated with growing plants in high humidity.

Plants, like people, can use evaporative cooling as a means of regulating internal temperatures during adversely hot days. While some plants possess coping mechanisms, in general, hot temperatures coupled with high humidity will lead to reduced growth. Another reason high humidity is not ideal for plant growth is directly tied to pathology. Fungal spores, like seeds, require moisture for germination.

Since high humidity is equal to high moisture, fungal diseases are more likely to develop. Lastly and most importantly is the effect of stomata opening with high humidity. One research study has shown the vase life of cut roses is significantly reduced when the plants are grown in greenhouses under high (>85%) humidity.

The plants responded to their environment and became acclimated to these humid conditions, resulting in a lack of functionality of the stomata. The cut flowers became insensitive to the cues that usually helped reduce water loss.

I use this point to illustrate the impetus of this article: your growing conditions can and will dictate the type of plants you produce. Make them lazy in their environment and they will be ill-prepared for the real-world.

At the very least, growers should invest in a cheap hygrometer and try to maintain RH at 50 to 80%, avoiding situations where RH exceeds 90%. The lack of water uptake by growing plants in a humid environment would directly reduce nutrient uptake, leading to reduced growth and nutrient deficiencies over time.

To correct high RH, simple ventilation may be effective at decreasing the moisture content. Humidifiers placed accordingly can be used to increase RH during times of low RH such as the winter months when heaters are used in the house.

You can also increase the temperature as a means to lower RH, but for most growers, the added heating expense prohibits this action. It is important to note that not all plants respond the same to certain stimuli.

Some plants favor lower RH for optimal growth while others push the envelope toward saturation. Use the guidelines (moderate relative humidity, measures to control relative humidity) as general recommendations and seek additional information on a plant-by-plant basis.

While growers often pay close attention to many details, make sure relative humidity does not fall into the overlooked category. Remembering this will truly pump up your plants, promoting faster growth and increasing overall health.