Plants and Humidity
Humidity has a major impact upon the overall performance and final yield of plants grown indoors. The level of humidity will alter how your plants function and if you can successfully take control of it as a grow room environmental factor then you will see a significant improvement in the performance of your plants. This article intends to explain in straightforward terms what humidity is, how it affects plant development at various life-stages and what you can do as a grower to manage your humidity for maximum effect.
Humidity Explained
Humidity refers to the amount of water vapour that the air holds at a given time. It is usually expressed as a percentage in relation to the maximum amount of water vapour that the air will hold at current temperatures. This particular and most common form of humidity measurement is known as Relative Humidity (RH) and is usually expressed as a percentage. Taking humidity and temperature into account together makes sense since they are directly related, i.e. the warmer the air, the more water vapour it can hold. So, at 100% humidity the air cannot hold any additional water vapour - unless the temperature were to change.
Plants and Humidity: Introduction
A common misconception in indoor gardening revolves around the idea that plants do not like high levels of humidity. This quite simply isn’t true! Plants love high humidity (especially in vegetative growth), but all too often indoor growers create low humidity environments to avoid potential pathogens that are not really an issue until the flowering stage. This “play it safe” attitude is understandable as nobody wants to have a fungal or bacterial problem but through careful humidity management throughout your plants growth you can avoid any such problems whilst also significantly improving your growth rates.
Plants and Humidity: The Science
Your plants’ ability to remain suitably hydrated and capable of absorbing the necessary nutrients for growth greatly depends on how efficient water transportation is within their systems. The humidity of your growing environment can potentially disrupt this process as it is a controlling factor on how much moisture transpires from your plants and therefore how water is moving through the plant from roots to leaves.
To understand the effect of humidity on water transport there are a couple of points you need to understand. Firstly, water tends to move from high humidity areas to low humidity areas. Secondly, the humidity inside plants has been found close to 100%. The rate at which water leaves your plants (via transpiration) will increase the lower the humidity in your grow room. Since the humidity inside your plants falls in the region of 100%, the humidity of the growing environment will also need to be relatively high to avoid potentially damaging over transpiration. This is particularly relevant before fruits and flowers begin to form.
Obviously, if humidity is very low then plants will lose too much water leading to the threat of damage caused by dryness. Consequently, the development of new leaves may suffer, older leaves could curl or even shed, and flowers/fruit occasionally die before or soon after opening.
Whilst low humidity is obviously not desirable, high humidity can cause problems too. Excessively high humidity and serious lack of ventilation sometimes encourages the spread of fungal diseases including Botrytis (Bud Rot) and Mildew, as well as stem and root infections in the very worst cases. However, with a reasonable amount of care and the right equipment this scenario is very easy to avoid.
Plants and Humidity: Recommendations According to Different Life-Stages
Although the following recommended levels of humidity apply to most plants it is worth noting that exceptions do exist. For example, the majority of species respond positively to 50-80% RH during vegetative growth, but Cacti prefer much lower settings of 30-35%. Please only use the suggestions that follow as a guide.
Since humidity is clearly an important factor, we highly recommend investing in some basic monitoring equipment to allow you to accurately monitor and measure your humidity levels. Your cheapest option is a Digital Min-Max Combo Meter, which precisely reports both humidity and temperature.
Young Cutting and Seedling Stage
At the beginning of your plants life relative humidity should ideally be between 70-80% RH. Use of a Hi-Top, Aeroponic, Jumbo Aeroponic or Heated Propagator will enable you to create these conditions with minimal fuss. All of them incorporate ventilation panels which can be opened and closed to preserve, increase or decrease humidity when deemed necessary. A high humidity setting is advisable when plants are young because it encourages them to devote most of their energy to growing roots and foliage rather than attempting to correct the humidity balance by over transpiring. As a result of increasing the humidity at this crucial stage, roots form at a faster rate, plant health improves and growth is significantly more vigorous.
Vegetative Growth Stage
When your plants are ready to be moved out of a propagator and into their main growing environment the optimum relative humidity should be around 50-80% RH. Creating an enclosed grow room using reflective sheeting or by using a grow tent or BudBox will make controlling your environment much easier. The suggested humidity in vegetative growth are slightly lower than they were in propagation because at this stage your plants will start growing leaves and shoots at a rapid speed, increasing the surface area from which water is able to transpire. This puts less strain on each leaf in terms of transpiration meaning the difference between humidity inside and outside every plant doesn’t need to be so close. Your plant also needs to maintain a certain degree of transpiration to draw water and nutrients into the plant to fuel all of this new growth.
Due to the fact that your plants will now be growing in an artificial light controlled environment, you need to consider dark cycles - when humidity usually decreases. Counteract this accordingly by setting up your grow room so that humidity is about 10% higher at night compared to the day. Be warned though - if at any point humidity drops below the suggested rate for a prolonged period of time plants stand the chance of experiencing potentially troublesome issues, including over-fertilisation.
To keep your grow environment within the optimum range of humidity a humidifier should be employed.
Flowering/Fruiting Stage
High humidity can remain present in the growing environment up until the flowering/fruiting period at which point it should be lowered to significantly reduce the threat of rot. An effective solution to preventing humidity from rising too high is with adequate ventilation. By extracting hot and humid air and replacing it with cooler less humid air from outside your grow room, high humidity will automatically become less of an issue. A suitable setting at this stage would range from 35 to 60% RH.
