THE SHORT DAYS OF JANUARY CAN BE HARD FOR CANADIANS TO STAND AND EVEN HARDER FOR HOUSEPLANTS TO TOLERATE.
Several of our readers have emailed me over the past few weeks asking for advice on how to overwinter tender plants and tropical indoor plants during winter. My friend Mark grows as he puts it "Beautiful Agave, one of my favourite plants."
Agave are grown indoors as houseplants and outdoors in summer as a tender plant. They are meant to be brought indoors during winter but can be difficult to overwinter indoors.
There are many tender plants we grow in our gardens such as geraniums, citrus trees, cannas and many others. Tender plants are usually long-lived perennials, shrubs, vines or trees that are not native to our climate, cannot survive winter freezing and must be brought indoors in October or November to overwinter.
Some plants grown outdoors in summer such as antirrhinum - commonly known as snapdragons (for the nice lady who mailed me a handwritten letter) - marigolds, impatiens, begonias and other plants used in pots and bedding schemes are called "annuals" because they grow, flower, set seed and die within one year. Annuals cannot be overwintered because they only live one year and they should be composted at the end of the summer.
During the winter, we all turn up the heat and the sunlight levels drop, which provides the perfect conditions to make tender plants and indoor tropical plants suffer, drop leaves, grow spindly and sometimes die. Those environmental conditions are usually made worse by peat-based soils, small pot sizes, over-watering and over-fertilization.
Plants grown indoors during the winter need bright light - as bright as possible and preferably direct sunlight to prevent plants from stretching, a condition called etiolation. Energy efficient fluorescent light bulbs or incandescent bulbs that are used to light our homes do not provide the range of wavelengths needed to help plants grow properly.
Lighting alone will not cure all that ails our plants in winter. Plants need light but temperature control is interrelated with light levels. High temperatures and low light will cause stretching and leaf drop. High temperatures and high light levels will provide good conditions for plant growth but as a general rule we do not want our tender plants or tropical indoor plants to grow rapidly during winter because it is out of sync with our climate.
Indoor plants also need a rest period during winter - a break of sorts, like mild dormancy where they maintain growth but do not put on much new growth.
The humidity level of the air is another key factor that must be managed to successfully grow plants indoors. Generally, most homes have forced air heating systems, some have radiant heating but both systems generally make the humidity levels of indoor air low or dry. Most plants prefer some measure of humidity in the atmosphere to help keep their leaves flush with moisture, except desert plants like cactus which prefer low humidity.
Since our homes during winter have warm temperatures, low humidity and low light levels or unnatural types of lighting, plants must exist in environmental conditions that are generally opposite to what they need to grow happily.
There are several ways to keep tender plants and indoor tropical plants growing healthy during winter. Generally, plants grown indoors during winter like cool temperatures, some humidity, some air flow to prevent disease buildup, no fertilizer, restricted watering and bright light. In other words, cool, bright, semi-humid and in a resting phase.
To help your plants survive winter indoors try the following techniques. Increase the light levels by moving plants to south or west facing windows. Provide supplemental light for plants in the form of plant grow lights or full spectrum fluorescent lights to help prevent stretching. Choose plants that can tolerate the specific light levels found in your home.
Provide supplemental humidity by misting plant leaves occasionally. Or place watering trays filled with colourful gravel and water underneath your plants to help raise the humidity level. Control the temperature by moving plants away from direct heat sources like heat vents. Place plants in cooler rooms or near cool windows.
Reduce or eliminate all fertilization to allow plants to rest. Fertilizing during winter will force soft weak growth that will stretch or die. Water only as needed and allow a short dry period between each watering which will force plants to rest. It takes some practice to manage your plants through the long winter but growing houseplants will improve the quality of indoor air and help lift your spirits during winter.
Todd Major is a journeyman horticulturist and chief horticulture instructor at the University of British Columbia Botanical Garden. For advice contact him at email@example.com.