Air-purifying plants

The plants on every air-purifying list come from a 1989 NASA report, Interior Landscape Plants for Indoor Air Pollution Abatement, by B. C. Wolverton, Anne Johnson and Keith Bounds. It looked at how leaves, roots, potting mix and the microorganisms in it could reduce indoor air pollutants such as benzene, trichloroethylene and formaldehyde.

The plants sat in small sealed chambers, and the report measured how much of each chemical was gone after 24-hour exposure periods. A sealed box has no fresh air coming in and no new fumes from furniture, paint or cooking, so it is very different from a home.

The report itself went further than a pot on a shelf. Its proposed air filter combined plants with an activated carbon filter and a fan that pulled large volumes of air through the carbon and roots. That design is not what you get from a few houseplants on a windowsill.

In 2019 Bryan Cummings and Michael Waring of Drexel University reviewed 196 experimental results from 12 chamber studies, published in the Journal of Exposure Science and Environmental Epidemiology. They converted the results into a clean air delivery rate, the same measure used for air purifiers.

A typical single plant cleaned about 0.023 cubic meters of air per hour. To match the air exchange a typical building already gets from outdoor air, they calculated you would need 10 to 1,000 plants per square meter of floor. For a 15 square meter (160 sq ft) bedroom, that is 150 to 15,000 pots.

Waring put it simply: plants are great, but they do not clean indoor air quickly enough. The RHS reaches the same conclusion in its advice on houseplants and health: a real improvement in air quality may need large numbers of plants under specific conditions, which is not practical in most homes.

The US EPA names the two most effective steps: reduce or remove the sources of pollution, and ventilate with clean outdoor air. Filtration, such as a portable air purifier, is a useful extra but does not replace either.

In practice that means opening windows wide for a few minutes several times a day, running the range hood while you cook and the bathroom fan while you shower, and letting new furniture or fresh paint air out before you close up the room.

The RHS agrees that opening windows and ventilating indoor spaces generally does more than relying on plants alone. Keep plants out of a cold draft from an open window in winter, though. Cold air on wet roots makes leaves drop.

The RHS says research points to wellbeing and productivity as the greatest benefits of indoor plants, including better mood and less stress. That benefit does not depend on how much benzene a leaf takes up.

The plants that made the NASA lists are also some of the easiest houseplants there are. They cope with indoor light and dry, warm rooms, and they forgive the odd missed watering. That is the honest reason they sell so well.

Golden pothos: a trailing vine that copes with lower light than most climbers and roots easily from a cutting in water. Toxic to cats and dogs.

Spider plant: the most forgiving of all. It handles irregular watering and moderate light and makes plantlets you can pot up. Safe for cats and dogs.

Peace lily: droops clearly when it is dry and stands up again after watering, which helps while you find its rhythm. Toxic to cats and dogs.

Corn plant (Dracaena fragrans): an upright plant that takes little floor space and copes well with indoor conditions. Toxic to cats and dogs.

Parlor palm: small enough for a shelf and happier away from the window than most palms. Safe for cats and dogs.

Boston fern: the one that needs the most attention. It wants evenly moist soil and dislikes dry heating air, but in bright, indirect light it fills out fast. Safe for cats and dogs.

Areca palm: a larger, feathery palm for a bright room. Safe for cats and dogs.

Plants release water through their leaves. A group of several large plants in a dry room raises humidity around them more noticeably than any filtering effect, while a single small pot changes nothing you would notice.

Choose by light first. Check how much daylight the spot gets and pick plants for it, rather than buying from a list. Put plants with similar watering needs together, so a snake plant does not end up next to a Boston fern.

Many pots in a room that is rarely aired mean a lot of permanently damp potting mix. A white mold can grow on the surface, and fungus gnats breed in the wet top layer.

Match watering to the species and empty the saucer afterward. Many of these plants can wait until the top 2 to 3 cm (1 in) dries, but Boston fern needs water as the surface begins to dry. Do not let its root ball dry out.

Wipe large leaves with a damp cloth now and then. Dust blocks light, and you spot pests early while you do it.