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Gardening myths: plants that release oxygen at night

I was recently asked, "what's your favorite part of working at a garden center?" Without hesitation I responded, "answering customer questions." Who doesn't enjoy sharing their expertise with others on the topics that they're the most passionate about? I'm particularly energized by discussing the scientific explanations of how plants work. Of course, human beings have been gardening for longer than we've had the explanatory tools of science, and so many gardening myths persist, passed down as traditional knowledge.

Gardening myths are especially pervasive on the internet. We all want quick fixes to everyday problems, which makes clickbait-y listicles so tempting to share. As an example, Reader's Digest recently published 11 Pest-Control Plants You Need in Your Yard. The problem is, they left out all of the science. It's true that plants and bugs communicate through smell in all sorts of fascinating ways. And as I wrote in my garden center FAQ post, many strong-smelling herbs contain essential oils that can repel bugs. In the plant world, being smelly makes you less likely to become a bug's dinner. But for humans to benefit from this effect, we have to extract the fragrant oils from the plant materials and slather them on our bodies as bug spray.

I got a good chuckle out of the last plant on the Reader's Digest list, Venus flytraps. Of course, Venus flytraps don't repel bugs at all; they attract bugs in order to eat them! (In my experience, Venus flytraps are difficult to care for. If you are interested in carnivorous plants, I recommend American pitcher plants for outdoors and tropical sundews or butterworts for sunny windowsills indoors.)

More recently, customers have been asking for plants that release oxygen at night. This one really stumped and confused me. You see, photosynthesis happens in the presence of light and plants receive natural sunlight during the day, not at night. So under normal photosynthesis, carbon dioxide is absorbed and oxygen is released during the day and not at night.

What I'd forgotten about from my college biology studies is that carbon dioxide and oxygen aren't the only gases that move through plant tissues. Water vapor also escapes through the pores in plant leaves. This is an important function because evaporation from leaves is what causes suction in the roots, drawing that water up from the soil through the plant's vascular system.

In extreme environments, too much water can be lost through transpiration. Desert plants like cacti, euphorbia, agave, and crassulaceae succulents (jade plant, echeveria, sempervivum, kalanchoe, sedum) employ a modified form of photosynthesis to minimize water loss. These plants only open their gas exchange pores at night. Photosynthesis still happens during the day in the presence of light, using the carbon dioxide that the plant stored in its tissues from the previous night. Once the sun goes down, the plant releases the oxygen that built up inside its tissues during the day and collects carbon dioxide for the next day's photosynthesis. This is called crassulacean acid metabolism or CAM photosynthesis, in honor of crassulaceae succulents.

Some epiphytes such as orchids also employ CAM photosynthesis. These plants don't grow in soil, but on the surface of other plants, typically trees. Therefore, they absorb water from the humidity surrounding them and they use CAM photosynthesis to conserve every precious drop.

Ok, so some plants are capable of releasing oxygen at night, but why would that be particularly desirable in a houseplant? Perhaps it is believed that these plants will do more to purify the air at night, thereby making it easier to sleep. But as I also discussed in my garden center FAQ post, plants are not the magical cure to sleep disorders. The air we breathe is not pure oxygen anyway, just around 20%. A much greater proportion of oxygen in the air is actually harmful to human health. Don't worry, houseplants don't produce that much oxygen anyway, not when compared to the entire biomass of plants on Earth.

When choosing houseplants, you'll be the most successful by considering your home environment and your skill level to care for them. Remind yourself that what you are doing is actually difficult (but that doesn't mean you can't do it!) because no houseplant actually evolved to survive indoors after all. Wealthy Victorians started the houseplant craze little more than 100 years ago, constructing solariums and conservatories in order to recreate the environments from where the plants were collected. Sorry, but those beauties featured on #succulentlove won't last too long in a low-light basement apartment, but a pothos will love you back.

It's important that we use critical thinking to evaluate articles and advice, especially when making decisions that affect our wellbeing and the environment. I love this video from PBS Studio's Above the Noise on how to evaluate the validity of scientific claims. Again, my intent is not to shame anyone, but to educate so that we can all make the best decisions.