Houseplants and Indoor Air Quality

by Wendy Diaz


Now that we are spending a lot more time indoors due to the cold weather, I decided to investigate the relationship between our houseplants and indoor air quality (IAQ).  IAQ is defined as the level of indoor air pollutants in a building, including carbon dioxide (CO2), moisture, volatile organic compounds (VOCs), combustion products, and particulates1.  We have houseplants for many reasons:  aesthetic beauty, to continue our garden hobby indoors during the winter season and to protect our tender plants (e.g. bougainvillea); but studies have shown there may be other potential benefits to houseplants other than the well known uptake of carbon dioxide and production of oxygen and natural beauty.

Ever since the publication of the 1989 NASA study2, entitled Interior Landscape Plants for Indoor air Pollution and Abatement, there has been much interest and publicity in the media3 about certain plants removing toxic airborne contaminants from our indoor air.  Three chemicals (benzene, trichloroethylene (TCE) and formaldehyde) with known adverse human health effects were injected into Plexiglas chambers containing plants along with activated carbon filters in the original NASA study.  Some of the study results demonstrated removal of low concentrations of benzene and trichloroethylene from the air inside sealed chambers (with activated carbon filter systems) using golden pothos (Scindapsus aureus) and trace amounts of formaldehyde were removed by the green spider plant (Chlorophytum elatum) and also by aloe vera.  The Peace lily (Spathiphyllum “Mauna Loa”) and English ivy (Hedera helix) were also effective in removing benzene from the sealed chambers.  Benzene is a volatile organic carbon (VOC) emitted as a gas from fuel, paints, varnishes, solvents and inks.  Trichloroethylene is used as a commercial metal degreaser and dry cleaning fluids, adhesives, paints and varnishes.  Formaldehyde-based resins are components of finishes, plywood, paneling, fiberboard, and particleboard, all widely used in mobile and conventional home construction as building materials (subflooring, paneling) and as components of furniture and cabinets, permanent press fabric, draperies, and mattress ticking.  

Subsequent research studied the effectiveness of cleaning indoor air of common pollutants such as formaldehyde, nitrogen oxides, sulfur oxides with the spider plant (Chlorophytum comosum), croton (Variegatum) and Peace lily (Spathiphyllum wallisii)5.  Another study6 demonstrated reduction in ozone by snake plant (Sansevieria trifasciata), spider plant (Chlorophytum comosum) and golden pothos (Epipmnum aureum) using CSTR (Continuously Stir Tank Reactor) chambers.

It is now commonly accepted that houseplants have the potential to clean indoor air and a list of the top 10 ‘houseplant cleaners’ is provided here  Nevertheless, there has been criticism from the EPA of the NASA study7.  The studies were done in sealed experimental chambers and did not directly represent the highly variable environment of an actual building.  Thus, the EPA stated that the ability of plants to actually improve indoor air quality is limited in comparison with common ventilation8; and others have stated that further research is needed.9  


In fact, houseplants can be detrimental to indoor air quality if they are over-watered and result in the growth of microorganisms that can be harmful to allergic persons10.  Another factor is that it is far easier to be preventative when maintaining your indoor air quality by using less toxic products in your homes i.e. source control 11, using an air filter and regular cleaning12.

It may be that houseplants do not significantly clean our indoor air of pollutants or they are not the most effective way of reducing toxic gases in our homes and as such, we should not rely on them for this function.  Alternatively, we should control the sources of the pollution by reducing chemicals and making ventilation improvements in our homes.  Nevertheless, it is well documented that houseplants convert carbon dioxide to oxygen and by looking at green nature we can reduce stress and increase your memory13!  Enjoy your houseplants for their beauty and the connection to nature they provide us during these winter months.

In Summary

  • Several studies have shown that common houseplants reduced levels of contaminants in the air inside closed, sealed chambers.
  • Further research is needed to demonstrate whether houseplants provide the same cleaning effect in the more variable environments found in homes and office buildings.
  • Houseplants convert carbon dioxide to oxygen.
  • Houseplants can negatively affect air quality if overwatering promotes the growth of microorganisms.
  • People can improve air quality in homes and other buildings by using air filtration, cleaning regularly, and choosing less toxic products.



  6.  John Girman’s (EPA) analysis of NASA research:





Holiday Cactus Care

By Andrea Laine

Light and temperature:  This plant does not like direct sun and prefers exposure to medium light throughout the year.  My four currently thrive in front of a large north-facing window and a home thermostat that averages 65°F.    

Holiday cacti need long nights to form flower buds.  Be wary of the length of artificial light exposure during late summer and early autumn.  Two recommended options: 1) Set it outdoors in a SHADY spot during the summer.  When many buds have formed, bring it back indoors, or 2) Set it in a less used, cooler room of the house during the summer.  I’ve had good luck with a laundry room (so long as there is a window to let in natural light) or, a guest bedroom.  When flowers begin to bloom move the plant into a higher traffic room so you can enjoy it.

Pinching back the stems in early June will promote branching and places for more buds to form.

Thanksgiving Cactus
Thanksgiving Cactus

Water and fertilization: Water once weekly to keep the soil evenly moist. When flowering is over, reduce amount of water without ever letting the mixture dry out, and resume as before when the stem begins to grow new segments.  Never let water stand in the saucer beneath the pot.

Fertilize plants monthly from the time new growth starts in late winter or early spring, and throughout the summer using a one-half strength soluble fertilizer.  Stop fertilization during the late summer for greater flower bud production in the fall.

Potting:  The holiday cacti flower best when kept somewhat pot bound. Repotting is necessary only about once every three years and is best done in the spring.

Propagation:  In spring or summer take cuttings of 2-3 joints or more. Pinch them off at the joint and place them in a clear glass of water. Once there are a few roots equal to an inch or so (happens within a few weeks), place in potting soil and you have another plant for your home or to gift to the hostess of the next holiday party.

This Christmas cactus is more than 50 years old.
This Christmas cactus is more than 50 years old.

References and Resources

See the first part of Andrea’s spotlight on holiday cacti here:

Holiday Cacti

By Andrea Laine

Holiday cacti have had a place in my indoor garden for as long as I’ve been gardening.  (That’s 30 years.)  Some in my current collection have been in my husband’s family for close to 50 years!  Hundreds of cultivars are available blooming for several weeks in shades of red, pink, and purple, as well as white.

There are three types that look similar to one another but bloom at different times of year, to which they owe their common names: Thanksgiving, Christmas and Easter.

All have gracefully arching stems composed of succulent segments, which are botanically called phylloclades.  Subtle differences in the shape of the segments and the color of the anthers help distinguish one species from another.  

  • Thanksgiving cactus (Schlumbergera truncata) has saw-tooth or pointed projections on the margins. Its anthers are yellow.  
  • Christmas cactus (Schlumbergera bridgesii) has flat, jointed segments with smoothly scalloped edges. Its anthers are purplish-brown.
  • Easter cactus (Rhipsalidopsis gaertneri) has flat oblong segments with bristles at the edges. Flowers are more star-shaped than the other cacti.   

Note how the segments differ between these two species of holiday cacti:Christmas CactusThanksgiving Cactus

In their native habitat, these cacti grow on trees in the forests of Brazil. Here in North America they are common holiday hostess gifts and excellent year-round houseplants. Best of all, it is much easier to coax a holiday cactus to bloom again than it is a poinsettia!

Coming soon: Holiday Cactus Care

References and Resources

Scouting for Insect Pests, June Edition

Squash Bugs



Squash bugs, which have overwintered as adults, are actively laying eggs now. Egg clusters can be found on the undersides of leaves. Remove and crush the eggs to reduce the population of squash bugs.

photo: Whitney Cranshaw, Colorado State University,

Nymphs and adult squash bugs suck sap from plants and can quickly destroy squash vines. Adults and nymphs frequently hide under damaged leaves and at the base of plants. Keeping debris and mulch away from the base of plants reduces cover for the bugs and may also reduce damage. In fact, Clemson University suggests the following method of trapping adults and nymphs: “The secretive nature of squash bugs can be used to your advantage in controlling these pests. Place a small, square piece of old shingle or heavy cardboard under each squash plant. As bugs congregate under it for protection, simply lift the trap and smash them with your hoe (or shoe).”




These pests can be a problem on ornamental plants in the landscape as well as on houseplants. Female mealybugs are oval shaped, soft-bodied, wingless, and covered with a fluffy wax. Males are gnat-like and have wings and a waxy tail. Eggs are contained in a fluffy, waxy mass. Mealybugs feed on sap from plants. They excrete waste called honeydew, which promotes the growth of sooty mold. Small infestations can be controlled by wiping away the insects with a cotton swab dipped in isopropyl (or rubbing) alcohol or nail polish remover or with insecticidal soap.

Spotted Wing Drosophila



Photo: Jesse Hardin, North Carolina State University

The Spotted Wing Drosophila damages fruit crops by cutting a slit into healthy fruit and laying eggs inside. Spotted Wing Drosophila prefers softer fleshed fruits like berries, cherries, and grapes. These insects are less than 4mm in length, and their larvae can be found inside fruit, causing softness and holes.

How can I manage SWD (Spotted Wing Drosophila) in my garden?
While SWD infested fruit may be unpalatable, larvae are not harmful if consumed. Ripening and ripe fruit are susceptible to SWD attack, but flies do not appear to be attracted to unripe fruit. Good cultural management can reduce SWD damage. Good cultural control includes:
1.Excellent sanitation: fruit should be harvested frequently and completely. Any unmarketable fruit should be removed from the field and either frozen, “baked” in clear plastic bags placed in the sun, or hauled off site to kill or remove any larvae present. When you done harvesting for the season, strip any unwanted fruit from plants and destroy it.
2.Canopy and water management: Prune plants to maintain an open canopy. Do not overwater plants. Leaking drip irrigation should be repaired, and overhead irrigation should be minimized.
3.Exclusion: Fruit can be covered with fine mesh bags or paint strainers prior to ripening to exclude flies. Bags should be tightly sealed. Placing a foam plug between branches and the sealed based of bags is useful to prevent plant damage and maintain a tight seal.
4.Regular fruit sampling: Fruit should be observed regularly for infestation before and during harvest.

While cultural control may be sufficient to reduce SWD infestation below damaging levels, insecticides are currently the most effective tool to reduce or possibly prevent SWD infestation. Insecticides can only be applied to plants for which they are labeled. The label is the law! County extension agents and university
specialists can assist with selecting effective, appropriate insecticides. There are some organically acceptable insecticides available for SWD, but they are less persistent than conventional materials and may need to be applied more frequently. (per

This video shows how to make a Spotted Wing Drosophila trap to assess the presence of these pests in your garden.

Finally, Japanese Beetles are active. Beetle traps are more effective at attracting Japanese Beetles than controlling them. If you do use traps, locate them far from plants you wish to protect. Per NCSU: Homeowners can take advantage of the beetles’ aggregation behavior by shaking plants to dislodge beetles each morning. Without beetles already on a plant, it is less likely that beetles will aggregate there later in the day. Picking beetles off by hand will also reduce the accumulation of beetles that results in severe damage. They can be easily knocked into a widemouth jar of soapy water.  In some settings, flowers or plants can be protected with cheesecloth or other fine mesh.