I recall visiting a local conservatory as a child. I was infatuated with the banana palms and exotic, tropical plants–my winter oasis. If you, like me, live in a frozen wonderland for months of the year, you will know the desire to see, smell, and taste plants while you wait for spring.
Winter’s challenge is that plants grown inside are rarely adapted for it, and those that are usually show it by promptly shedding their leaves. If you want plants to thrive you must provide a growing environment that the plants will enjoy. In this article I will explain the temperature and humidity issues I consider when growing an indoor winter garden.
If you live where winters are cold
In a cold environment, heating the air also dries it. Imagine air to be a glass of water. The colder the air is, the smaller the glass is. Air at -30ºC (-22ºF) is like a shot glass, it can only hold a tiny amount of water. As the air heats up, it becomes like a large water glass. It can hold much more liquid. The relative humidity tells us how full that glass is. In the picture below, the small glass is 100% full which is similar to 100% relative humidity (RH), but if you were to pour that same liquid into the larger glass, it is only 20% humidity. This is important because when the air has a low humidity, it wants to fill up, so it sucks water from everything around it. That’s why it feels dry to us (and to our plants).
To think of it another way, lets say that the outside air is -25ºC (-13ºF) and 100% humidity.
That means the air is holding 0.6 g of water and cannot hold any more. That air makes it into your house and is warmed up to +20ºC (68ºF). But air at +20ºC is able to hold up to 17.3g of water. Because it is only actualy holding 0.6g of water, it feels very dry, in fact it is about 3% relative humidity (there is barely enough water to cover the bottom of the glass). So it feels very dry, it is sucking up moisture from everything around it including your plants, but your plants have adaptations to keep them from loosing moisture.
Plant leaves are covered in small holes (or pours) called stomata. Stomata allow gas exchange (which is necessary for photosynthesis) and also permit water to exit the leaf (which is surprisingly critical for a plant to drink). When the air is too dry, the plant closes its stomata to prevent loosing too much water to the air and dehydrating. On the downside, this also reduces photosynthesis and prevents the plant from drinking much. All the functions of the plant will slow down, including growth.
So, unless you have a cactus or another plant adapted to a very dry climate, your plant in the 3% air is very stressed, has its stomata tightly closed, and is not really growing.
Make sure that your plant has enough water. I am reluctant to recommend this because I believe more people hurt their plants from over-watering than under-watering, but in dry heated air your plants will dry out more quickly. The trick is to keep the plant moist but not wet. If you cannot always be there to water it when needed, consider setting up something more automatic such as a wick system.
Actively humidify the air near your plant. You could humidify your whole house, but equipment to do this is fairly costly (as is the electricity to run it). You could humidify around your plant by closing it off from the whole house and humidifying just the area where the plant is.
Passively humidify the air near your plants. Fill a tray with pebbles and then cover the pebbles with water. Place the tray underneath the plants (but make sure that the plant roots and media are not in contact with the water). Keep the pebbles covered with water for as long as you need the area around the plant to stay more humid.
Mist your plant. This is simple and inexpensive but be careful not to overdo this because water sitting on leaves (especially for long periods) invites disease problems. If you do mist, do so early enough in the day to make sure that the plant dries off fully before sunset.
If you live where winters are wet
In a more mild but wet environment you’ll struggle with high indoor humidity. High humidity creates a host of possible disease problems, some of which are more prevalent when it is cool and humid, others when warm and humid. Many mould spores sit dormant until the relative humidity rises above 85%. Ideally you ‘ll keep the environment around your plant lower than 85%. Here are four ways to help this situation.
Actively dehumidify. You will need a dehumidifier to make this work, but it is an effective method of dehumidifying. Of course, buying and running a dehumidifier is expensive, but it will help prevent fungal diseases on your plants.
Good air movement. Plants care about the humidity right above the plant leaf surface, not how humid the whole room is. Having fans that move air across the surface of the leaves will take moist air away from the plants will reduce outbreaks of moulds. A ceiling fan can do this effectively if positioned correctly. You can also use a household fan. This is usually most effective if you place the fan so that it blows away from the plants (and draws air from the plant area).
Increase the Temperature. Air at higher temperatures is a lower relative humidity than cooler air with the same actual moisture. This is usually a temporary measure, but it can be very effective.
You can spray it with fungicides. There are a variety of biopesticides (such as fungi and bacteria that kill other bacteria and fungi) that are quite effective. Most, though, only work if they are sprayed before a problem occurs. In a greenhouse, especially, one sprays when conditions look favourable for mould to grow (such as when there is a problem and humidity climbs too high). Laws and regulations, as well as trade names, are different in different locations, so talk to sales people or authorities in your region before spraying.
Day vs. Night temperatures.
Changes in temperature between daytime and nighttime are significant for plants. If your average night temperatures are warmer than your days, it causes some stress for most plants. If this happens occasionally it is not a big problem, but over time their leaves will become paler and eventually die. For some reason, this temperature difference is more pronounced in plants during short days (Nov, Dec, Jan) than during long days. It also affects some families of plants more than others. Be aware that if you have your thermostat set to keep your house cooler in the dayttime than the nighttime, this could affect your plants.
Winter brings certain challenges to your indoor plants. Your plants want Goldilocks’ just right humidity, and mild temperatures that are akin to the natural cooler-at-night-time, but the outdoor environment’s impact on your indoor garden space makes that difficult. For optimum growth, you will have to figure out what the problems unique to your situation are and mitigate them.
This is part 1 of 2. Click here for the second part of your winter indoor garden.