By Laura Gardner, Master Gardener in Training

We turn to searching for information online or in books on how to care for our plants. Unfortunately there are a lot of inaccuracies surrounding certain beliefs and practices concerning them. Since most of our attention is now focused on indoor gardening, let’s look at five questions concerning houseplants.

1. Are Poinsettias Poisonous?

You may have been gifted with a Poinsettia (Euphorbia pulcherrima) and you may be concerned because you have heard that they are poisonous. The plant, while not edible, would need to be consumed in large quantities to be harmful. That being said, it still is wise to keep the plants out of reach of pets and children as some consumption may lead to digestive distress. Like other members of the Euphorbiaceae family, Poinsettias produce a milky sap and so handling the plant without gloves may affect those with a latex allergy.¹

2. Can Houseplants Filter or Absorb Pollutants?

Recently I read in a book on houseplant care that plants draw in harmful volatile organic compounds (VOCs) such as formaldehyde and benzene and clean the air for us. Two examples mentioned included the Peace Lily (Spathiphyllum) and the Spider plant (Chlorophytum comosum). It stated that the former can filter out formaldehyde and the latter both formaldehyde and benzene. While it is true that plants have the ability to absorb pollutants, large numbers are required to have a significant impact. The belief that plants clean the air may be associated with a single NASA study² from the 1980s that was conducted in an air-controlled laboratory setting and not in an open home or office setting. More recent research has determined that a building’s air handling system or open windows is more effective at reducing pollutants than plants. One would require 10-1000 plants per square meter in order to be comparable.³

3. Will Misting and Pebble Trays Increase Humidity?

We may struggle with maintaining optimum humidity levels in the winter for our tropical houseplants. Many books and online sources recommend increasing relative humidity around plants through misting and placing the plant pots on trays filled with pebbles and water. Misting is generally ineffective because the water evaporates so quickly. It would have to be done constantly to have an impact. Concerning pebble trays, an experiment published in the American Orchid Society Bulletin found that in the winter (set in a home in Minnesota), relative humidity levels were raised only slightly. The RH at 40 mm above the tray was measured at 3%; at 110 mm 2%; and at 300 mm it measured 0%.⁴ Using a humidifier is really the only method that has the ability to increase humidity levels significantly.

4. Should you Use Ice Cubes for Watering Orchids?

Recently I read a comment on the Master Gardeners of Ontario Facebook site that someone said that their Moth Orchid (Phalaenopsis) came with a tag advising them to use ice cubes as a method of watering it. It seems that this advice is really intended to help prevent new Orchid owners from overwatering their plants. While there is a study that found that using ice cubes did not negatively affect the health of the orchids, it was only conducted for a period of 4-6 months. A problem with using ice cubes is that it has the potential to accumulate salt build-up (from water and fertilizer) that can affect the plant’s longevity in the long term.⁵

5. Should you use Leaf Shining Products or Oils?

Some sources advise using commercial shine products or oils to help make a plant’s leaves shiny and reduce build-up of dust. However, there is research indicating that the use of leaf shine on Weeping Fig (Ficus benjamina) caused the plants to have a reduced tolerance to low light stress, resulting in three times as much leaf loss as untreated plants.Treated plants required higher light levels.⁶ Another problem is that they can block the pores or stomata in the leaves of some plants, resulting in reducing the plant’s ability to photosynthesize and respire.

References

1. Newman, S.E. and B.E. Edmunds. Poinsettias. Colorado State University Extension. Online: https://extension.colostate.edu/topic-areas/yard-garden/poinsettias-7-412
2. Wolverton, B.C. et al. Interior Landscape Plants for Indoor Air Pollution Abatement. Online: https://ntrs.nasa.gov/citations/19930073077
3. Cummings, Bryan E. and Michael S. Waring. Potted plants do not improve indoor air quality: a review and analysis of reported VOC removal efficiencies. Journal of Exposure Science & Environmental Epidemiology. Volume 30, pp. 253–261 (2020). Online: https://www.nature.com/articles/s41370-019-0175-9
4. Kohl, Douglas. A Study in Humidity: Douglas Kohl Evaluates the Effectiveness of a Common Method to Raise Humidity around Orchids Growing in the Home. American Orchid Society Bulletin, 63(8). 1994. pp. 916-917.
5. American Orchid Society. Greenhouse Chat Webinar. June 2021. Online: https://www.aos.org/All-About-Orchids/Webinars/chat/Greenhouse-Chat-June-2021.aspx
6. Steinkamp, K. et al. Acclimatization of Ficus benjamina: A Review. Online: https://mrec.ifas.ufl.edu/foliage/resrpts/rh_91_5.htm