A Secret Ingredient of Trees

Sam Westreich, PhD
Aug 31, 2018 · 6 min read

The trees in our lobby are sad and weak. I know why.

A strong, healthy tree, unlike those in my lobby. Photo by Faye Cornish

I’ve always been fascinated by trees. When I was a child, my parents took me on a trip from our native Minnesota out to San Francisco. We spent half a day hiking in the Muir Woods redwood grove, and I remember returning to our hotel with a crick in my neck from staring up at the massive trees.

Perhaps memories of that childhood trip helped drive my determination to pursue graduate school on the west coast. My graduate school in Davis had its own (miniature) redwood grove, and even though the trees didn’t compare in size to those in the Muir Woods National Park, I still loved looking up at them, wondering how a trunk less than a foot in diameter could support more than a hundred feet of leaves and branches.

My lobby. Those are not amazing looking trees.

Redwoods are majestic — but the trees in the lobby of our office building are perhaps their direct opposite. While redwoods stand tall and straight, the small trees growing in our lobby are crooked and stunted. Their branches kink at unusual points, they don’t seem to have nearly enough leaves for trees of their size, and they lean at odd angles, like the drunkest man at a college party.

This isn’t something unique to these particular trees, either. California likes to plant indoor trees in its shopping malls, and I’ve been struck many times by how these indoor growths fail to compare to the majesty of their outside counterparts.

Why? What makes indoor trees so weak and sickly compared to their outdoor fellows?

Ingredients to build a tree

Trees, like almost all plants, obtain their primary nutrient, carbon, from the air. Their leaves absorb carbon dioxide from the air, which they break apart to separate the carbon atom from the two atoms of oxygen. The oxygen is returned to the atmosphere, while the carbon is used to build the structure of the tree. The tree needs to receive sunlight, which it uses as an energy source to break apart the carbon and oxygen in carbon dioxide.

Trees also need water, nitrogen, phosphorous, potassium, and other trace elements. Since the nitrogen found in the air as a gas is inert and very difficult to break apart and use, these other nutrients are instead pulled in from the earth where they grow.

This is where most biology textbooks stop. Trees need light, carbon dioxide, water, and some trace nutrients. Pretty simple — and all these things can be supplied indoors. But this isn’t a complete list.

First thing missing: wind.

Consider the element of wind. Whoosh, air moving around. Ask most people, and they wouldn’t consider wind to be necessary for tree growth, although it does make the leaves move about in a lovely manner.

As it turns out, however, wind is vital for growing larger trees indoors. Wind constantly presses on trees, pushing them in different directions and adding stress to their structure. To counteract the negative effects of being pushed around, trees grow more durable reaction wood, a component of their trunk that provides more sturdiness and strength. Without reaction wood, trees tend to become very weak and prone to collapse before they reach full maturity.

Wind also combats another danger of indoor trees — dust. Dust can settle on the leaves of a tree, acting like a million tiny shields to block the leaves from receiving the sunlight that they need. Outdoors, wind clears the dust from tree leaves. Indoors, however, there’s nothing to dust the tree leaves, and dust and other particles can starve the tree of needed sunlight.

Second thing missing: humidity.

Ever notice how most offices, malls, or commercial buildings tend to have fairly dry air? Humidity is generally discouraged inside a building; it can increase the risks of mold or rot in building construction materials, and can interfere and cause damage to electronics. Many buildings have dehumidifiers or other facilities to strip excess water out of the air. The average humidity inside a building is usually below 10%.

Trees, however, prefer more humidity — much more. Many trees prefer close to 50% humidity on average, while some tree varieties, such as citrus trees, prefer over 50% humidity. Imagine if you were constantly parched with a dry throat, and couldn’t get any water to alleviate that suffering. That’s how trees feel indoors.

Third thing missing: water balance.

Yes, I just said that trees were too dry because of a lack of humidity in the air — but paradoxically, they can also be too wet, if they’re overwatered.

Too much water can be especially dangerous for indoor trees if they are growing in pots, or in some contained amount of dirt. Normally, excess water poured onto the base of an outdoor tree will drain away through the soil, diffusing away from the tree. Indoors, however, there may not be enough dirt for the water to fully drain, and it instead lingers in soil near the tree roots. This can cause fungus and rot to spring up, eating away at the tree beneath the surface of the dirt.

On the other hand, not enough water for a tree will starve it of nutrients. All those trace minerals and nutrients that a tree needs — nitrogen, phosphorous, potassium — must be dissolved in water for the tree to absorb them. This is why we put liquid fertilizers on plants, instead of just dumping powder onto them! If a tree isn’t watered, its leaves will curl over as it loses water that it can’t replace, and it will wither away as it starves from lack of absorbable nutrients.

Growing better indoor trees

Given what I’ve written so far, it may seem like growing indoor trees is impossible, and commercial spaces (like my building’s lobby) are the equivalent of medieval torture dungeons for trees. It is, however, possible to grow trees indoors — with the proper caveats and considerations.

First, pick the right type of tree. Some trees, like a ficus, are better suited to indoor growth because they don’t require deep taproots and are comfortable not receiving full sunlight. Other types, like the redwoods I mentioned at the beginning of this article, will not thrive as well indoors.

Second, make sure that the tree receives as much sunlight — and wind — as is possible indoors. Remember, glass blocks some sunlight, so even if a tree is beneath a skylight, it receives less sunlight than its outdoor fellows. Placing a tree near a commonly open window where a breeze blows can help provide some stress — or, barring that, give a tree a good shake whenever you walk past. Think of it as a deep tissue massage.

Finally, keep in mind that a tree needs water — both in the air and in the ground. Most mature indoor trees require water about once a week, especially if they’re planted in a pot. If the soil isn’t totally dry at the surface, the tree doesn’t need water. And while we prefer our environments to be dry, remember that trees prefer more humidity. Consider giving a wilting tree some time with a humidifier, or at least some open water nearby to help humidify the air.

And next time you’re outdoors and feel the wind brushing your hair, hear it rustling the leaves of the trees, you can know that the trees appreciate the breeze just as much as you do.


Sam Westreich is a microbiome scientist working in Silicon Valley, and has spent years immersing himself in science and the nerdiest of pursuits. He blogs about science, biology, microbes and microbiomes, and his thoughts on graduate school and finding success.

Sam Westreich, PhD

Written by

PhD in genetics, bioinformatician, scientist at a Silicon Valley startup. Microbiome is the secret of biology that we’ve overlooked.

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