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Hot in Here: Humidity

If you’ve ever experienced a New York summer, you might describe it as humid. The blankety, moisture-filled air that makes morning commutes sticky, skin glisten and wreaks havoc on our hair is essential for certain plants.

Hot in Here: Humidity
Hot in Here

Humidity is a measurement of water vapor dissolved in the air. There are two kinds of humidity, 1. Absolute and 2. Relative. Absolute Humidity is defined as the literal mass of water vapor divided by the volume of the air/water vapor mixture, i.e., how much water is literally dissolved in the air.


How Humidity Works

Air is a gas, and gasses are affected by other factors like temperature and pressure. That’s where Relative Humidity comes in on the scene, defined as a percentage that measures how much water vapor is in the air, versus how much total water vapor the air can hold. This takes into account temperature and pressure. 50% humidity means that for a given temperature and pressure, the air is half-saturated with water vapor. Most humidity readings that you will come across, like in your Weather App, are the measure of Relative Humidity.

As temperature increases, the capacity of the air to hold water vapor increases. Likewise, as temperatures fall, the water-holding capacity of the air decreases. This explains why in winter it is so dry outside. Cold air can’t hold much water, and when that air is heated in our homes, the capacity increases, but the amount of water stays the same, decreasing humidity. For example, if the humidity on a cold day outside at 32F is 10%, when that air is heated indoors to 72F, the humidity drops to ~2.7%. That’s enough to irritate your sinuses and more importantly, enough to damage sensitive plants that need humidity. This can be especially bad for plants like epiphytes (orchids, air plants, some ferns) and calatheas, for whom humidity is essential.

Humidity & Plants

Plants rely on evaporation from the sun and heat to pull water throughout their plant bodies. Water can move laterally through a plant, using potassium salts, but the pulling of water, what we call transpiration (or, interchangeably, evaporation) works like this: when the sun hits the leaves, water evaporates, cooling the leaves and pulling more water to the leaves from the stem, pulling the water to the roots which are pulling water from the ground. Water travels through the plant like a straw and is able to pull itself due to the cohesive/adhesive nature of water, or because it sticks to itself and what surrounds it.

Often, humidity-sensitive plants have lots of pores and thin leaves and parts of themselves that are only a few cells thick. Because of their thinness, these parts of the plant have poor vasculature and water cannot be efficiently transported to the cells before the water leaves the plant for the air. Thin tissues give ample opportunity for water to escape to the air, making them at high risk of crisping and dying if the humidity drops. When humidity falls, the capacity for the air to hold water increases as does transpiration and water is taken from the leaves faster than water can be replaced, causing crispy leaves, especially on the edges.

Spritz & Balances 

Now that we’ve established that humidity is the measure of water in the air, the only way to increase humidity is to put more water into the air. How? Two ways. 1. Boil water into steam, leaving a large pot of boiling water on the stove for a few hours, adding water as it gets low. 2. Supersonic vibrate water into vapor, which is just a fancy way of suggesting to invest in a humidifier. A humidifier is the best way to increase humidity levels.

Spritzing only increases humidity somewhat locally for a few minutes, but when it evaporates, the humidity goes back to what it was. Spritzing can help some plants like epiphytes and ferns. That’s because these plants have the ability to absorb water through their leaves, and directly benefit from water spritzes. Spritzing other plants that do not take it well can run them at risk for foliar infection, as bacteria and fungi activate with water on leaves and can infect them when wet.

Happy, healthy, humid environments will keep your plants happy and healthy too. For the rest of us, we’ll just have to ride out the heatwaves.