Temperatures are expected to drop below freezing all across the Midwest this week. If you haven’t already brought your precious potted plants indoors for the winter… what are you waiting for? But if it’s too late, and your pretty patio tropcials like mandevilla or hibiscus have already moved on to the great garden in the sky, there’s always next year to try, try again. Done right, there’s something of an art to safely transitioning plants from indoors to out (and back again). And it begins by understanding that all sunlight is not created equal. Here’s the how and why…
WINDOW LIGHT ISN’T VERY BRIGHT
Glass may appear to be thin and transparent, and you’d think it would let all the sunlight just shine right through. But the truth is, glass is pretty tough stuff, and it acts as a filter. The sunlight that shines down on us is actually made up of many different ‘types’ of light, known by their different wavelengths. Not all wavelengths are visible to the human eye, but it’s these visible types of light that are used by plants for photosynthesis. And that beneficial, visible light is weakened as it passes through glass, resulting in lower light levels. Screens on windows then reduce light levels even more.
The light that is filtered out by glass doesn’t just vanish into thin air; it’s converted to heat energy. Glass is also a fairly efficient insulator, so that heat then gets trapped inside once light has passed through. This is known as the green house effect, and anyone who has left their car windows closed on a summer afternoon has experienced the results.
Glass also has a nasty habit of letting harmful ultraviolet rays pass right through, unmolested. More specifically, cancer-causing UVB rays shine through at 100% intensity, while more beneficial wavelengths are reduced. Put another way: window light offers the full impact of sunlight’s harmful effects without all the benefits. Plants that require full, intense sunlight (such as tomatoes) don’t usually fare well when grown indoors next to a window. And more delicate plants can easily become damaged by this unnatural mix of heat and direct window light.
BUT BACK TO POTTED PLANTS…
So let’s say you have a plant sitting inside by a window for a few months. Hopefully it has grown accustomed to it’s environment and it’s thriving and looks great. Now take that plant and put it outdoors in direct sun. If it doesn’t just up and die after a few days, then chances are pretty likely it will be seriously shocked by the sudden change in climate and exposure. Plants can get a sunburn too, you know. Leaves tend to turn brown and fall off, and the entire plant may wither and look scraggly. It takes days, weeks or even months for them to fully recover.
The better practice is to slowly transition plants when moving them from indoors to outside. Those that will find their way into full sun should first be placed in an open, shaded area for at least a few days. This allows them to build up their tolerance to increased light and UV rays.
The opposite is also true: just as humans can suffer from Seasonal Affective Disorder as a result of winter’s shorter periods of daylight, plants are also affected in much the same way. Rather than wait till the weather turns cold to suddenly bring your potted plants inside, it’s better to slowly transition them to lower light levels. We do this by moving them from full-sun to open shade. Again, a few days or weeks of this will help them adapt to their new environment indoors.
It’s possible that your outdoor potted plants will lose some color over winter, or even stop growing completely once they’ve been brought indoors. And that’s okay. A period of rest or dormancy is often required by many species, as part of their normal growth cycle. After all… these aren’t always houseplants we’re talking about. Sometimes we’re just trying to keep outdoor potted plants alive until spring and summer weather can rejuvenate them again. And doesn’t that sound better than spending hard-earned money to buy the same plants you already bought last year?