Dear Master Gardener: I’ve heard that having plants in one’s home helps purify the air. Is this true and if so, are there certain types of plants that are more beneficial than others?
Answer: According to NASA research, many common houseplants and blooming potted plants help fight indoor air pollution. Pollutants in homes and offices can come from synthetic carpeting, fabrics, laminated counters, plastic-coated wallpaper and other man-made materials. Through photosynthesis, plants are able to scrub considerable amounts of harmful gases from the air. As part of the photosynthesis process, plants absorb carbon dioxide and release oxygen; but researchers have discovered that many houseplants also absorb benzene, formaldehyde and trichloroethylene. No plant is beneficial for removing tobacco smoke. Most likely all plants are beneficial to some degree; however, testing has shown that some are more efficient cleaners for certain types of toxins than others. It cannot be assumed that all harmful pollutants can be removed by plants.
In a NASA study, plants were placed in sealed chambers, which were injected with chemicals. This study discovered that philodendron, spider plant and golden pothos were most effective in removing formaldehyde and gerbera daisies and chrysanthemums were very effective in removing benzene from the chamber atmosphere. Other effective air purifiers were bamboo palm, peace lily, ficus, Dracaena, Sanseveria, English ivy and Chinese evergreen species. The research also showed that plant leaves, roots and soil bacteria all remove trace levels of toxic vapors. The recommendation from the NASA studies regarding how many houseplants would be needed to improve air quality of an average house (1800 square feet) were 15-18 plants in 6-8 diameter containers. The plants NASA tested are:
• Hedera helix English ivy
• Chlorophytum comosum spider plant
• Epipiremnum aureum golden pothos
• Spathiphyllum `Mauna Loa’ peace lily
• Aglaonema modestum Chinese evergreen
• Chamaedorea sefritzii bamboo or reed palm
• Sansevieria trifasciata snake plant
• Philodendron scandens `oxycardium’ heartleaf philodendron
• Philodendron selloum selloum philodendron
• Philodendron domesticum elephant ear philodendron
• Dracaena marginata red-edged dracaena
• Dracaena fragrans `Massangeana’ cornstalk dracaena
• Dracaena deremensis `Janet Craig’ Janet Craig dracaena
• Dracaena deremensis `Warneckii’ Warneck dracaena
• Ficus benjamina weeping fig
Dear Master Gardener: Two of my houseplants, a croton and aloe, have shiny, sticky leaves and some of the leaves of the croton are turning yellow. Although I don’t see any insects, I was wondering if my plants have aphids and if so how I get rid of them.
Answer: Aphids are not commonly found on houseplants and are easy to see. They are about 1/16 to 1/8 inch long and have pear-shaped soft bodies with conspicuous legs and antennae. The ones found on houseplants are usually green. Your problem is most likely scale insects, which don’t look like typical insects. Scales, which congregate on the undersides of leaves along the main veins, look like oval spots about 1/8 inch long, but their yellowish or greenish brown color makes them hard to see until the infestation is severe. Juveniles or “crawlers” are so tiny they are barely visible without magnification. Scales use needle-like mouthparts to feed on plant sap, secreting sticky honeydew as an end product of that process. If the infestation is severe the scales encrust the stems and leaves like lumpy blisters, and plants may yellow and die. Susceptible plants are aloe, aralia, croton, dracaena, ferns, India-rubber tree, ivies, and palms.
To control scales, gently scrub them off the leaves, using warm soapy water and a small brush, then rinse the foliage with clear tepid water. Treat severely infested plants with bifenthrin, permethrin, resmethrin, insecticidal soap, pyrethrins, disulfoton, imidacloprid, or plant oil extracts. You will need to apply at least two to three applications sprayed once every 10-14 days. Because their waxy covers are so impervious to insecticides, add a few drops of liquid soap or detergent to help the material slide under the edges of the “shells.”
Crow Wing County Master Gardeners are trained and certified volunteers for the University of Minnesota Extension Service. All information given in this column is based o research and information provided by the university. To ask a question, call the Master Gardener Help Line at218-824-1000, extension 4040, and leave a recorded message. A Master Gardener will return your call.