As the temperature drops, most of us are getting out sweaters and snow boots, but what should we do to help prepare our indoor plants for winter? Just like humans, plants experience the changing environment around them. The seasons affect light, temperature, and even humidity. 

How do plants react to changing light availability?

The earth rotates on a tilted axis. The direction of that tilt determines the seasons. In the northern hemisphere, the earth is tilted towards the sun during summer (June-August) and away during winter (November-March). As we approach winter, the average day length will begin to decrease. Winter Solstice is the shortest day of the year, where the average day length in the lower 48 states is only 9 hours long. Most indoor plants are from tropical regions where weather and sunlight availability are consistent all year. All plants can detect changes in day length through special light receptors in their leaves. This can make the transition into winter a little confusing for a plant from the tropics, where changes in daylength do not accompany the seasons. But with the right care, your plants can easily survive even the most brutal winters.  

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How do the seasons affect light intensity?

Even in moderate climates, sunlight will be less intense during the winter months due to the sun passing the earth at a lower angle. It will become vital to keep your plants near a South or West facing window to utilize as much light as possible. It's important to remember that light passing through your windows is up to 50% less intense than the sunlight outside due to reflection off the glass. Make sure you give your windows a quick cleaning; this will assist with optimum light penetration. If you notice that your plants are getting tall and leggy, you might need to add supplemental light to your plants. 

Don't worry about buying fancy grow-lights. Any LED or fluorescent bulb will do the trick; just make sure to place it approximately 6 inches from your plant. You can make up for the lack of natural light by increasing the exposure time. Keep your lamp on a timer set to provide 12-18 hours of light during the daytime. Let your plant rest in the dark at night. 

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How will winter temperatures affect my indoor plants?

In general, house plants and people enjoy the same temperature ranges. Most house plants are happy in temperature between 65-75 Fahrenheit, which tends to be the range we heat our homes. Plants can tolerate slightly cooler or warmer temperatures than this range, but it's important to introduce those conditions gradually. Keep your house plants from getting below 50 Fahrenheit, which is when portions of the leaves could begin to die from cold stress. If you keep your plants on the ledge of a window, check that the air directly next to the glass does not get below 50 degrees at night. This can become especially problematic if your plant becomes sandwiched between the window pane and curtains, where the air will quickly drop to a lethal level. Instead, close your curtains between your plant and the glass to protect them against the freezing temperatures. Avoid keeping plants in areas with dramatic temperature changes, such as doorways. Additionally, avoid places that can get very hot like near ovens, heaters, vents, or fireplaces. Bursts of hot air can quickly dry and burn plants, just like people in a desert environment.

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How does indoor heating affect my plant?

Plants have small pores in their leaves where they can take in carbon dioxide to create sugars. Dry air draws water out of the environment, which is why we get chapped lips and dry skin during winter. Similarly, dry air can draw moisture out of the pores in plants through a process called transpiration. Much of the water we give our plants end up lost through this means. Plants enjoy air between 40-60% humidity because moist air doesn't pull as much water from the plants. More humid conditions give the roots time to slowly drink up the liquid in the soil and utilize it to grow. The humidity levels in a heated building during winter can drop to as low as 10%! Help your plants stay humid by aggregating them. Each plant gives off water vapor from its leaves, and if you group them together, they can sit in a mini water vapor cloud, which helps prevent excessive transpiration. If you keep a humidifier in your rooms, consider placing your plants nearby it.

If the air is dry, should I water my plants more?

Because the air in heated buildings can become very dry, it is logical to think that plants will need more water. And while it is true that water from the surface of the soil and the leaves will evaporate quickly, it is essential to remember that plants slow their metabolism during the winter. This means that the rate of growth is going to decrease dramatically. Although more water will be lost due to transpiration, less water will be used by the plant. To determine if your plant is ready for water, check out the soil. If the surface of your soil looks dry, stick your finger into the soil 1-2" below the surface and feel the soil. If the soil feels moist, wait a little longer (a few more days) and check again. Unless you have ferns or citrus varieties that require staying moist at all times, this is a good method to avoid overwatering. Another way to test if your plant is ready for water is to pick up the pot. A pot of dry soil is much lighter, and you will be able to feel a big difference before and after watering.

What are some things I should not do during winter?

During winter, imagine your plant as a hibernating bear. Indoor plants will not grow much during this period. Avoid repotting, pruning (unless you have woody deciduous plants), and fertilizer application. All of these activities are best served during Spring and Summer when your plant will be actively growing.