How Many Houseplants Do You Need for Good Indoor Air Quality?
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Few things enhance a room’s decor more than a healthy green houseplant nestled on a kitchen counter, an arrangement of succulents on a table or a potted palm by a window soaking in the sun. And then there’s the added benefit that these houseplants contribute to the room’s air quality, right? Ah, if only that were true.
For decades we’ve been told that indoor air pollution — the strange brew of toxic emissions created from synthetic materials used in home building and home furnishings, as well as organic substances like airborne mold and viruses — could be improved by the addition of houseplants in the environment. But where did this idea come from?
It was a theory supposedly supported by a 1989 NASA study by scientist Dr. Bill Wolverton. Specifically, Wolverton and his colleagues were looking for ways to remove volatile organic compounds or VOCs from the air, particulates that are unable to be filtered out with an ordinary filter.
They hit on the idea of introducing “higher plants and their associated soil microorganisms” as a way to off-gas some of the synthetic organic chemicals in tightly sealed environments (like labs and spacecraft) and experimented with specific plants to prove or disprove their theory.
There were several problems with the research but the major flaw in the study is that homes (unlike spacecraft) are not hermetically sealed chambers. There are leaks around doors and windows and they aren’t made of the same materials. A 1992 memo on the NASA study by the U.S. Environmental Protection Agency concluded, “to achieve the same pollutant removal rate reached in the NASA chamber study” would require having “680 plants in a typical house.”
Dr. Michael Waring, an associate professor of architectural and environmental engineering at Drexel University’s College of Engineering, reviewed studies, including the NASA research, and determined that while houseplants make great aesthetic additions to the environment, that’s about all they do. Waring and his colleagues at Drexel published their findings in the journal Nature in November 2019.
Now, just because plants don’t make the air cleaner doesn’t mean they don’t make you feel better. A 2015 study says that interaction with houseplants reduces physical and emotional stress, and Kansas State University research says plants and flowers promote healing. So, we checked with our own expert, James T. Murphy, a county extension agent with the University of Georgia’s Cooperative Extension service.
Murphy works in the Agriculture and Natural Resources (ANR) program area of extension services, which traditionally covers crop and livestock production. In suburban counties, where Murphy works, ANR typically handles questions from homeowners and small producers. We asked him what’s the most important thing to know about growing a houseplant.
“For the beginning grower the principle we use when we’re talking about any sort of plant is ‘right plant, right place,'” says Murphy. “In other words, we consider what the space offers so what makes sense there.”
Plant growth is most affected by how well you’re able to balance the necessary light, temperature, humidity (moisture in the air), water (quality and quantity), nutrition (fertilizer) and the appropriate soil or growing medium. Every plant is different and the needs species have within the same plants can vary. Remember: No matter how you acquire your plant — whether it’s a purchase or a gift — get detailed instructions on how to care for the plant, including:
We asked Murphy for a shortlist of houseplants for the novice grower, and some tips to help the plants remain happy and healthy. Here are his suggestions:
Succulents: “If you’ve spent any time on the internet, especially Instagram or Pinterest, you know that succulents are massively popular,” says Murphy. From a maintenance perspective, you’d be hard-pressed to find an easier-to-care-for plant. “They’re probably the easiest return-on-investment for effort,” he adds.
Pro tips: Find the right location. “Light is important for all plants but especially for succulents,” Murphy says. Succulents like at least four hours of sunlight each day. They’re also the sourdough starter of plants. It’s easy to propagate succulents and share them with friends.
Bromeliad: If you live in a climate with high humidity and have a sunporch or enclosed porch, try growing a bromeliad. And if you’re fortunate enough to get a bromeliad to flower – it’s called a bract – the “bloom” can last for several months.
Pro tips: Bromeliads don’t require much watering or fertilizing, and light requirements can range from shade to full sunlight depending on species.
Philodendron: These popular plants can range from the trailing variety with small heart-shaped leaves to the larger philodendron hope selloum or tree philodendron with the distinctive split leaves. These plants can grow up to 5 feet (1.5 meters) tall.
Pro tips: All species of philodendron like humidity and light. They will grow toward the light so make sure to rotate your plant occasionally to ensure even growth. They are sensitive to overwatering; a tree philodendron shouldn’t need to be watered more than once a week.
Spider plants: Murphy had a soft place in his heart for these easy to maintain, sometimes variegated houseplants. “My mother gave me a spider plant that came off her spider plant when I went to college,” he says. “I still have it to this day.” In fact, Murphy learned spider plants have a regional nickname, “mala madre,” which means “bad mother” because the plant grows baby plants called “pups.” The pups look like little spiders, giving the plant its name.
Pro tips: Spider plants don’t like direct sunlight and are sensitive to temperature. “Watch them during the winter and keep them away from the window if it’s too cold,” says Murphy.
African violet: This houseplant has its own growing society, annual convention and website proclaiming it “American’s favorite houseplant.” According to Murphy, people sometimes struggle with the African violet because it requires adequate light (and temperature). “But if you pay attention to it and give it the time, it can bloom beautifully for a long time.”
Pro tips: Keep your African violets in a brightly lit location where the temperature remains between 65 and 80 degrees Fahrenheit (18 to 26 degrees Celsius). Your plants need to stay lightly moist (but not wet), in a light porous potting mix.
Blame it on those Instagram-loving millennials! According to the National Gardening Association, U.S. sales of houseplants grew 50 percent over the past three years to more than $1.7 billion. More than 34.4 million households participated in indoor houseplant gardening in 2018.
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How Many Houseplants Do You Need for Good Indoor Air Quality?
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