Enhance Your Indoors with Tropical Plants

Published in Harrowsmith magazine, Winter 2025

Winter can be a difficult time of year for us passionate gardeners for whom soil humus and fresh foliage supports a regular part of our wellness routine. While digging in the dirt remains a few months away, the benefits of gardening can be successfully brought indoors for the winter months.
Inhaling the humid air of a productive greenhouse is proof-positive of the impact abundant plant material can have on indoor air quality; the smell alone is unmistakable. Leaves, roots, and soil all work together to remove harmful “volatile organic compounds” (VOC’s) such as benzene, formaldehyde, trichloroethylene, and ammonia as well as carbon dioxide from the air and replace it with fresh oxygen.
Nowhere is this more important than in newer buildings, which by virtue of their energy efficient airtightness exchange less fresh air with the outdoors. While this is positive from an energy conservation standpoint, it is important to monitor from an indoor air quality perspective, especially if your home is heated with natural gas. There is nothing more uplifting than abundant plant life to freshen up a stuffy new build.
While the concentration of plant material required to make a meaningful difference on air quality has long been debated, there has been research into what species work most efficiently. The commonly cited 1989 NASA Clean Air Study found the following common houseplants effective at removing benzene, formaldehyde, trichloroethylene, xylene, and ammonia: English Ivy (Hedera helix), Pothos (Epipremnum aureum), Peace lily (Spathiphyllum Mauna Loa), Variegated Snake Plant (Sansevieria trifasciata). Notably, these plants are easy to find and dead simple to grow.
For plants to clean air effectively, they need to be healthy and growing. Here are some tips to get the most out of your living air-purifiers:
– – Light is the big one during this short-day time of year. Placing your tropicals in south-facing, unobstructed windows is ideal but artificial light can be helpful for plant growth. While an agricultural growlight is likely overkill, as it is built for high-intensity food crops, a regular LED bulb will be energy efficient and beneficial. Bear in mind this supplemental LED lighting is lower intensity than the sun, which can be compensated for with longer exposure – up to 14 hours a day won’t hurt, and with the efficiency of LED lights there shouldn’t be too much cause for concern on your hydro bill.
– Climate. Bearing in mind that most indoor plants are either Mediterranean or tropical origin, do what is possible to mimic their native habitat by keeping your indoor temperature above 20c and managing humidity appropriately. Canadian houses tend to be dry in the winter, which suits Mediterranean plants such as lavender and bougainvillea well. Most indoor plants that clean indoor air most effectively tend to be faster growing topicals, like those listed in the NASA Chean Air Study, benefit from higher humidity. You can make these plants happy by spraying their leaves with a fine water mist every couple of days or so.
– Water less, but water well. The biggest cause for houseplant failure is too much love in the form of watering. Waterlogged potting soil will lead to root rot. If your plants are looking yellow or black and wilted, check to see if the soil is too wet by pushing your finger into the soil about 3 cm. deep. Remove the dead foliage and repot into fresh soil if necessary. Allow your tropical plants to become dry at least 2 cm. below the surface of the soil and when you do water, do so generously. When water seeps out the bottom of the pot, you have applied enough to encourage roots to reach deep within the pot.
– New Year Diet Program. Like many of us, January is a good time for indoor plants to cut back their food intake. Pay close attention – if there is new growth, continue fertilizing at a reduced rate. Most common houseplants slow down growth in the winter. We recommend that you hold off fertilizing completely until early spring when you notice growth picking up again.
– In extreme cases, fertilizing in winter can turn soils salty and burn roots however most often it is just a waste – the expensive fertilizer will end up in flowing right through the soil. Also, we tend to eschew synthetic fertilizer in favour of the range of slower acting organic alternatives. The primary reason to choose an organic fertilizer over something conventional is the energy intensity of production. Producing synthetic fertilizers is an energy intensive process which typically consumes a lot of natural gas, whereas most organic fertilizer on the market are concentrated organic matter from various composting processes.
– Air flow. As a follow up to the seminal NASA Clean Air Study, researchers developed an air filter that used forced air flow through a combined carbon filter with plant material. The carbon filter could absorb smoke, organic chemicals, pathogenic microorganisms (if present), and radon which was then absorbed by the plant roots and into the plant tissue. A critical element of this design was forced air movement, which on a lesser level can be accomplished by ensuring proper airflow around the home with any type of fan. Not only will a fan help circulate lower quality air through your plant collection for purification, but the movement of air will also improve plant health by discouraging the settlement of insect problems such as mites.
As the stuffy indoor days of winter wear on, consider how you can maximize the freshness of nature with indoor plants.

When to repot indoor plants

From time-to-time tropical plants require repotting to maintain good health.
The signs that a plant is ready to be moved up one or maybe two pots larger include: premature drying of the soil, where the roots often take up more space in the current pot than soil does. The perfect ratio is about 30% roots and 70% soil by volume.
Lift the plant out of the pot and examine its roots. If they are circling round the inside wall of the pot that is another sign that your plant is ready for an upgrade.
Only upgrade the pot size by 2 to 4 cm., measuring the diameter of the pot at the rim.
Only use quality potting mix. Pull the roots loose, taking the pressure off the main root structure which encourages new root growth. Be ruthless in your approach: you are pruning roots which is never a bad thing.
Make sure the new container has generous drainage. Place the plant in the new pot and push soil down around the roots of the plant using a wooden ruler or paint stir stick to create pressure between the soil and the wall of the pot.
Leave 2 cm. between the top of the soil and the rim of the pot for water to settle.
Water well and place in the sunniest window you have.

When to resuscitate a plant and when to dispose of it.
We are sure you have seen indoor plants that are way past their “best before date”. Some of them can be resuscitated and others should be composted. How do you know the right time for each? To a large degree this is a matter of personal taste. If you have a 3rd generation Christmas cactus that your Grandmother brought over from the old country, you have no choice but to manage it and keep it alive best you can.
Old, straggly plants often just need pruning, increased light, cleaning of the leaf surface using insecticidal soap and perhaps repotting (see When to repot indoor plants)
On the other hand, ask yourself if the space that an old, tire plant is taking up in your living space is adding much to your quality of life. If the answer is no, consider tossing it on the compost and buying a new one. A young plant, like a young person, is very adept at growing in a new home and enhancing indoor living space with new growth and often flowers.