‘Clean Air Plants’ or Sansevieria trifasciata in bloom

Sansevieria trifasciata in flower in my living room.  The pungent, sweet citrus smelling, flowers open at night, where in the wild they would be pollinated by moths.

Sansevieria trifasciata, otherwise known as the ‘snake plant’ or ‘mother-in-law’s tongue’, is a common ornamental plant, for a few very good reasons. Like all easy houseplants, it thrives on neglect. Low light and infrequent, irregular, waterings serve this plant well, which is great for those who want the tropical look, without the bright light and humidity to support it.  Many people who have grown Sansevieria for years have never seen it flower because they are ‘too good’ to it.  A rough repotting (or conversely, letting it get too root bound) is often enough to trick Sansevieria trifasciata into flowering (because if it thinks it might die, making offspring is a good idea), although once flowered, new leaves will not grow from that particular rhizome.  While the care is easy and the flowers beautiful, one of its main draws for me originally was its reputation as an “air cleaner”.

Most people, especially plant people, are familiar with the important role that plants (and other photosynthetic organisms) play in our lives by taking in carbon dioxide from the air and producing oxygen. Air quality is about more than just carbon dioxide and oxygen levels though, but thankfully plants play an important role in helping us with air pollution too.

Indoor air quality is often a lot worse than people expect it to be. Second hand smoke, mold, dust mites, radon (from rocks below the foundation or even granite countertops), and volatile organic compounds (like benzene and formaldehyde, from paints and plastics) all can negatively impact our health. Starting in the 1970s, NASA became interested in this issue, and how it might effect astronauts who would have to live in small, poorly ventilated spaces for long stretches of time (hello, city apartments). They found that certain common houseplants could actually remove pollutants like formaldehyde, benzene, and ammonia from a room rather efficiently.

A little Golden pothos (Epipremnum aureum) and Snake Plant (Sansevieria trifasciata) combo I put together for my husband’s office.

So this big NASA push, and the follow up studies in the ’80s and ’90s, give us a list of good house plant choices that are both easy to care for indoors and act as air cleaners.  The usual list you see recommends Mother-in-law’s tongue (Sansevieria trifasciata), Chrysanthemum (Chrysanthemum morifolium), English Ivy (Hedera helix), Spider plant (Chlorophytum comosum), Boston fern (Nephrolepis exaltata ‘Bostoniensis’), a few good palms, a handful of Dracaena, and Golden pothos (Epipremnum aureum) and many other similar members of the Araceae family (for Araceae think pothos, peace lily, and philodendron).  Are these the only good air cleaners? No, NASA actually looked at a wide range of houseplants, of which I’ve compiled some of my favorites from their data into a chart below. If English Ivy and Golden Pothos aren’t your thing, there is no reason you can’t have a house of orchids, bromeliads, and ferns. Not all plants were tested in the same situations so a ‘-‘ doesn’t necessarily mean that ‘Plant X doesn’t remove Y’, in some cases it just means that we don’t know.

I took data from the two main studies quoted, the first from 1989 from NASA on “Interior Landscape Plants for Indoor Air Pollution Abatement”[1] and the second from 1993 in the Journal of the Mississippi Academy of Sciences by B.C. Wolverton and John D. Wolverton[2] (B.C. Wolverton was the principle investigator on the 1989 NASA study). While these are “new” compared to the original work from the 1970s[3], they are still outdated and the methods not “the best” scientifically, as repeatability is a bit of an issue here, but the data is still worth looking at in context.

In the 1989 study, all “plants tested were obtained from nurseries in [the] local area. They were kept in their original pots and potting soil, just as they were received from the nursery”. Because plant size was so variable, pollutant removal measurements are normalized against “total plant leaf surface area”. Tests were preformed in climate controlled Plexiglass chambers for a period of 24 hours only. In the 1993 study, plants were all “standard nursery stock” in various sized pots (which is why I used pot diameter to normalize the data a bit, as leaf surface area wasn’t available in this study) and the μg/hour particulate removal measurements were averaged over a five month period inside a sealed plastic chamber (with 12 hours of grow lights a day, fans, and temperature and humidity settings). All tests were repeated “three or more times” (although if that was with the same individual plants each time, I don’t know). In both studies, the chambers had artificially high levels (higher than normal room levels) of the pollutants in questions pumped in. Only for formaldehyde do I have an overlap in numbers, but because the tests were so different and specimens so few, you can’t compare numbers between the two studies meaningfully.

Now some data:

	Ammonia Removed (μg/hour/pot diameter in cm)[2]	Benzene Removed (μg/hour/leaf surface area in cm²)[1]	Formaldehyde Removed (μg/hour/leaf surface area in cm²)[1] (μg/hour/pot diameter in cm)[2]	Xylene Removed (μg/hour/pot diameter in cm)[2]
Silver-Vase  Bromeliad/”Urn Plant” (Aechmea fasciata)	–	–	15.395 μg/hour/cm[2]	–
Chinese evergreen (Aglaonema modestum)	–	.196 μg/hour/cm²	.096μg/hour/cm²[1] 37.11μg/hour/cm[2]	–
Flamingo lily (Anthurium andraeanum)	162.165μg/hour/cm	–	13.228μg/hour/cm[2]	10.866μg/hour/cm
Spider plant (Chlorophytum comosum ‘Vittatum’)	–	–	0.175 μg/hour/cm²[1] 27.586 μg/hour/cm[2]	12.167 μg/hour/cm
Garden Mum (Chrysanthemum morifolium)	239.539 μg/hour/cm	.758 μg/hour/cm²	95.395 μg/hour/cm[2]	13.224 μg/hour/cm
Dendrobium Orchid (Dendrobium sp.)	–	–	–	25.000 μg/hour/cm
Cornstalk Dracaena ‘Janet Craig’ (Dracaena fragrans ‘Janet Craig’)	–	.071 μg/hour/cm²	.133 μg/hour/cm²[1] 53.583 μg/hour/cm[2]	6.063 μg/hour/cm
Striped Dracaena (Dracaena fragrans ‘Warneckei’)	–	.225 μg/hour/cm²	29.921 μg/hour/cm[2]	11.614 μg/hour/cm
Fragrant Dracaena (Dracaena fragrans var. ?)	–	–	46.207 μg/hour/cm[2]	13.498 μg/hour/cm
Dracaena marginata (Dracaena reflexa v. augustifolia)	–	.167 μg/hour/cm²	.113 μg/hour/cm²[1] 38.030 μg/hour/cm[2]	–
Golden Pothos (Epipremnum aureum)	–	–	.138 μg/hour/cm²[1]	–
Banyan Ficus (Ficus benghalensis)	97.368 μg/hour/cm	–	61.842 μg/hour/cm[2]	17.829 μg/hour/cm
Guzmania “Cherry” Bromeliad (Guzmania lingulata × wittmackii)	–	–	–	11.496 μg/hour/cm
English Ivy (Hedera helix)	–	0.301 μg/hour/cm²	0.408 μg/hour/cm²[1] 55.172 μg/hour/cm[2]	6.453 μg/hour/cm
Neoregelia Bromeliad (Neoregelia sp.)	–	–	–	3.701 μg/hour/cm
Boston Fern (Nephrolepis exaltata v. Bostoniensis)	–	–	91.773 μg/hour/cm[2]	10.246 μg/hour/cm
Kimberly Queen Fern (Nephrolepis obliterata)	–	–	52.283 μg/hour/cm[2]	12.717 μg/hour/cm
“Moth Orchid” (Phalaenopsis sp.)	–	–	15.789 μg/hour/cm[2]	–
Snake plant/mother-in-law’s tongue (Sansevieria trifasciata)	–	.417 μg/hour/cm²	.454 μg/hour/cm²[1] 12.434 μg/hour/cm[2]	10.329 μg/hour/cm
Peace Lily (Spathiphyllum sp.)	83.487 μg/hour/cm	0.217 μg/hour/cm²	0.079 μg/hour/cm²[1] 61.776μg/hour/cm[2]	17.632 μg/hour/cm

An interesting quirk of taxonomy is that plant species move around quite a bit, naming wise, and it can sometimes be confusing knowing what is what. Sometimes genetic testing reveals two plants to be more or less related than they were thought to be, and other times people just realise the exact same thing has been named more than once. Dracaena fragrans received its named from the famous 18th century English botanist John Bellenden Ker Gawler. Later, the famous 19th century German botanist, Adolf Engler, named it again, Dracaena deremensis. Unfortunately for Engler, Ker Gawler got there first, so officially D. fragrans is the name. In the above study, three Dracaena fragrans cultivars were used in the 1993 study, although they were named as Dracaena deremensis ‘Warneckei’, Dracaena deremensis ‘Janet Craig’, and Dracaena fragrans. Which cultivar the later was, is anyone’s guess. What is nice though is that now we have three test subjects, within a single species, so we can compare a little data.

For our three Dracaena fragrans, looking at formaldehyde, we see removal levels of 53.583 μg/hour/cm, 29.921 μg/hour/cm, and 46.207 μg/hour/cm. Does this mean that one cultivar is better than another at removing formaldehyde? No, not necessarily. Without 100s, if not 1000s, of tests on unique plants, we can’t say one variant of Dracaena fragrans is strictly better than another. We can probably say that, in general, Dracaena fragrans is better at removing formaldehyde than Sansevieria trifasciata and, in general, worse at removing formaldehyde than Chrysanthemum morifolium or Nephrolepis exaltata v. Bostoniensis, but without a larger sample size, we can’t say with certainty.

What we can say, though, is that clearly some houseplants remove volatile organic compounds from the air, and that’s a good thing.

A good question to ask at this point is how are these plants filtering this stuff out of the air? The answer isn’t actually all that straightforward.

In the 1993 study, the scientists decided to play with a few variables, and repeated the test with a few plants where the potting soil was covered in sterile sand. Looking at the Boston fern (Nephrolepis exaltata v. Bostoniensis) they found that, averaged over five months, 1027 μg/hour of formaldehyde and 208 μg/hour of xylene were removed when the soil was exposed and only 409 μg/hour of formaldehyde and 103 μg/hour of xylene were removed when the soil was covered in sterile sand. By looking closely at the soil, they determined that, for the Boston fern, 60% of formaldehyde was actually being removed by the microbes in the soil and the other 40% taken in by the leaves (50.5% of xylene was taken in by microbes in the soil and the other 49.5% went to the leaves). In 1989, when the tests were repeated without plants at all, and just open containers of soil, the results weren’t nearly as good. 20.1% of benzene in a sealed container was removed over a 24 hour period with just soil, while 89.8% of benzene was removed when that soil also contained English ivy (Hedera helix)( 77.6% for Dracaena fragrans ‘Janet Craig’, 70.0% for Dracaena fragrans ‘Warneckei’, 73.2% for Epipremnum aureum, and 52.6% for Sansevieria trifasciata (hey, it’s better than a pot of dirt)). The combination of these two results suggests that microbes in the soil, in connection with the roots of the plants, not in isolation, take in the majority of pollutants.

So, to remove pollutants, plants need leaves, roots, and microbe rich soil. Pretty basic stuff, actually.

The next big question I think one should ask is, “Okay, so the microbes+roots take in the lions share of pollutants, what are they doing with it?” Again, that’s a question that doesn’t have an easy answer. Bacteria like Dechloromonas can anaerobically metabolize benzene into carbon dioxide, but that’s not one of the bacteria that we expect to find in potting soil. How and why each particular microbe-root system is taking in and using these pollutants depends entirely on the system and in some cases, we honestly don’t know.

Now, houseplants may take in ammonia, benzene, formaldeyhe, xylene, toluene, tricloroethyne, and likely others (I didn’t include everything in the chart above for brevity), but they also give back. A 2009 study in the American Society for Horticultural Science‘s journal looked at the reversal of the above studies. They closed plants into controlled glass container, and they monitored what came out (not including emissions from the plastic pots). Nicely, this study had an overlap with some of the plants we looked at above (and a final tie-back to my Sansevieria trifasciata flowers).

Table 1 from “Volatile Organic Compounds Emanating from Indoor Ornamental Plants” by Dong Sik Yang, Ki-Cheol Son, and Stanley J. Kays. HortScience April 2009 vol. 44 no. 2 396-400. Click to expand and read.

Looking at Spathiphyllum wallisii, Sansevieria trifasciata, and Ficus benjamina we see all sorts of volatile organics being emitted day and night. While Sansevieria trifasciata may not have been the best at cleaning the air, it also doesn’t make nearly as big of a mess as others.  What are all the chemicals? Some of these compounds help attract pollinators, some deter pests, some are flavors, some are odors, some help regulate temperature, and some are still kind of a mystery at this point.

One interesting feature of the data is that day time emission of volatile organic compounds is much greater than night time emission. Most plants require their stomata (the pores on the leaves that allow gases in and out) to be open during the day to facilitate photosynthesis and carbon dioxide exchange.  An open stomata means water loss however, so at night, when photosynthesis doesn’t occur, the stomata close, to conserve resources.  A closed stomata doesn’t just mean less water vapour escaping, it almost means fewer volatile organics escaping.

Now here is my favourite aspect of this study:  their Peace Lily (Spathiphyllum wallisii) was flowering at the time.  Because of the flowers being open (and most people like Peace Lilies for the flowers after all), a lot more was being released than in the other cases.  The biggest contributor was α-farnesene, both an insect pheromone and a distinct floral scent.  Spathiphyllum wallisii was producing 100 times more α-farnesene than everything Sansevieria trifasciata was producing.

Are the farnesene compounds harmful? Probably if you’re in an enclosed space with a lot of them.  From the Material Safety Data Sheet for a mixture of Farnesene isomers:

Inhalation of vapours may cause drowsiness and dizziness. This may be accompanied by narcosis, reduced alertness, loss of reflexes, lack
of coordination and vertigo.

Basically, if your Peace Lily is in flower, open a window.  Is this unique to Peace Lilies? No, there just aren’t studies on floral emissions from all flowering houseplants.  If it has a strong smell, it likely is producing a large quantity of some volatile organic, whether you want to be in a room with it entirely depends on what it is.

While I couldn’t find on a study on volatile emissions from Sansevieria trifasciata  flowers, I know that the smell, however sweet, was strong enough for me to chop the flower stalk off after a few days.

Beautiful, but the headache wasn’t worth it for me.

References:

[1] B.C. Wolverton, Anne Johnson, Keith Bounds. “Interior Landscape Plants for Indoor Air Pollution Abatement”. NASA, September 15th, 1989. [Open Access PDF]

[2]B. C. Wolverton and J.D. Wolverton. “Plants and Soil Microorganisms: Removal of Formaldehyde, Xylene, Ammonia From the Indoor Environment”, Journal of the Mississippi Academy of Sciences. 1993 [Open Access PDF]

[3] National Aeronautics Sciences and Space Administration. 1974. Proceedings August  Symposium, TM-X-58154. 27-29, 1974. NASA of the Skylab Johnson Life Space Center. 161-68.

[4] Dong Sik Yang, Ki-Cheol Son, and Stanley J. Kays. “Volatile Organic Compounds Emanating from Indoor Ornamental Plants” American Society for Horticultural Science. HortSci. April 2009 vol. 44 no. 2 396-400. [Open Access HTML]