Humidity: The Good, The Bad & The Ugly

Everything and everyone is affected by humidity, or the lack of it. Plants are more sensitive to humidity levels and its variations than humans or animals. We manage to live through the discomfort. Plants manage humid conditions too, but the effect can be undesirable. Improper humidity levels affect plant functions and can cause irreparable damage to roots, stem, foliage, flower and fruit. The worst case scenario is pretty ugly. You could watch the death dance in horror, not knowing how to stop the process.

Moderation applies to all life forms.

As a grower you must regulate the environment and climate for your crop. Especially if you don’t want them to go on strike, get lazy and refuse to perform it is best to take your role as climate creator seriously. Understanding what relative humidity (RH) is and how it affects the plants in your indoor garden is an important part of a healthy crop and an abundant harvest. The
wrong humidity level can present you with a number of issues that vary by crop type. At early growth stages low humidity can cause your crop to go on a binge eating spree while profusely perspiring. The result is they burn to a crisp from the loss of water that maintains nutrient levels in their systems. Obviously, this is bad, at best. The plants in an attempt to improve dry conditions absorb more nutrient solution than in proper humidity. At the same time they rapidly shed water through the cell pores on the underside of the leaves known as the stomata. What
is the result of this undesirable situation? Even a weaker solution will lead to nutrient burn.

What’s the right RH level?

It would be great if it were that easy, to have one metric to aim for here. You have an average humidity level of 50-75% for best results with dark cycle RH levels being higher than light cycle. However, different plants and different stages of growth require varied humidity. So, be sure to learn what the best RH levels are for your crop at each stage, as some plants will prefer a drier climate and others need high humidity. A seed sprouts and rapidly develops a good foliage canopy and abundant roots in the cooler, shorter days of late spring and early summer when high moisture is present. As temperatures climb and day length increases, high outdoor humidity can be a bad, even ugly thing. The purpose of an indoor garden environment is to give plants optimal conditions through all stages to harvest. Coaxing them to produce that bumper crop you want includes controlling relative humidity. This is why many indoor hydroponic growers have separate vegetation and flowering to fruiting grow chambers. It is also why some plants are good indoor companions and others aren’t.


GERMINATION & PROPAGATION STAGE: Maintain rooting cuttings or germinating seedlings humidity at above 80%. A propagator makes controlling RH more manageable and allows your new crop to concentrate energy on producing the roots. Some crops can be successfully propagated in your grow tent using a humidifier with a humidistat.

YOUNG PLANTS: Between minimal foliage to breathe out water (transpiration) at night and the hot lights, many growers experience problems keeping the RH levels high enough to produce strong growth and plant vigor. In 0,6 m2 of grow space, using two 600 watt lights, you’ll generally find 35-50% RH and a temperature of 22-28 C. As described earlier, low humidity will make the plants work overtime at creating humidity. You don’t want the RH to drop below 40%. This brings issues with nutrient overload and other related problems. Your target RH level at this stage of growth should be 60-70% for faster root growth, leaf development and more compact plants.

VEGETATIVE STAGE: These larger immature plants have more shoots and leaves. They will take up increasing amounts of nutrient and emit more water vapor as they increase in size. At this stage you want to ensure the RH levels don’t climb too high. This danger increases as plants thicken.

FLOWERING & FRUITING STAGE: The ideal RH range here is 50-60% during daylight cycle and 60-70% during the dark cycle. Keep on top of RH control, as both fruit and flower are easily damaged by fungal disease with soaring humidity. High humidity related fungal problems can ruin many kinds of fruit, vegetable and flower crops.

Increasing the RH

Misters and humidifiers are used to increase RH levels. The more water vapor being emitted by your plants into the grow room environment, the less the humidifier or mister needs to be run. For plants that need tight humidity control a humidification system with HR-HRSA humidistat is best.

Decreasing the RH

g3005Ventilation drops your grow room RH level. Depending on your crop and grow space situation, at times an extraction fan with a variable speed control is all that is needed. High RH levels can also be corrected by with a dehumidifier. Great care is needed to not remove too much moisture, creating a dry environment. This can cause some plants to emit water through leaf tips, allowing it to collect on foliage and promote disease development. Ultra-sensitive plants may do best with a humidification system that delivers both humidifying and extraction functions in one with precision.

Measuring Your Humidity Levels

To stay on top of the RH level at all times, you need an instrument known as a hygrometer to give you humidity readings. Be sure to select one that is water resistant and designed for the demands of hydroponics. These will give you accurate temperature and humidity measures which are connected.

RH Changes with Temperature Variation

In your grow space, the relative air humidity is influenced by temperature. High RH disrupts a plant’s ability to get rid of excess water. Low RH makes the plant emit too much water and can cause the intake of CO2 to stagnate, leading to impaired growth. Nutrient uptake rate increases with dry air, and for about every 5,5 C temperature increase the amount up-take doubles.
Both of these fluctuations greatly affect your crop. Ideally, you want your grow room to have a lower humidity during the day cycle than the night cycle. During the darkness hours in the growing space, many assume that without the heat of grow lamps, the humidity will drop. This assumption could get ugly. In darkness, plants breathe out water, increasing the need for air flow and ventilation. Ignoring this puts your entire crop at risk for pest and disease outbreaks, among other health issues.


[alert type=white ]This article is reprinted from Garden Culture Magazine, Issue 1. [/alert]

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Tammy Clayton

Contributing Writer at Garden Culture Magazine

Tammy has been immersed in the world of plants and growing since her first job as an assistant weeder at the tender age of 8. Heavily influenced by a former life as a landscape designer and nursery owner, she swears good looking plants follow her home.