Late summer and early autumn are the perfect time to consider how the seasonal shifts may affect our indoor plants. While we welcome spring with open arms and fawn over every new leaf, autumn brings yellowing leaves and some expletive utterances in this hobby. But don't panic; you can get through winter with your plants in the Pacific Northwest. A lot of us do!
What changes?
As the seasons shift into winter, it brings changes to light intensity, light duration, temperature, and humidity. The Pacific Northwest begins it's rainy season, which is characterized by gray skies that significantly reduce the intensity of light our plants receive through a window. As we get close to the winter solstice, the hours of daylight wane to a mere 6 hours on the darkest days. When the temperature drops, our heaters kick on and that forced air can be very dry.
Many of our indoor plants are tropical or sub-tropical and do well indoors because they are suited for growth underneath thick rainforest canopies that provide spotty, filtered light (e.g., philodendrons, hoyas, syngonium, etc.) that is similar to the light they receive through a window in our homes. However, close to the equator they still receive this filtered light for 10-12 hours daily. When that duration of light begins to decrease, we start to see the oldest leaves begin to yellow and die as the plant starts to conserve it's energy. The plants stop growing.
Plants that grow in partial to full sun (in their native environment), however, will have the hardest time in PNW winter because they need far more intense light to survive (e.g., gingers, tradescantia, sempervivum succulents, adenium "desert rose", etc.). We may notice these plants quickly decline or go dormant. I have many houseplants that go dormant for winter and will return again in the spring, much like my outdoor perennials. For example, this will be the second year with my Plumeria as a winter-dormant stick.
The temperature changes, too. Most houseplants need to be kept between 55-90 degrees (F). We are generally sensitive to temperature changes during the day, but the nighttime temperature shifts affect a lot more plants than we usually realize. Some plants require a fairly consistent temperature between night and day, whereas some need a 10 degree (or more) drop at night to be content. The plants that require temperature consistency start getting thrown for a loop as our night temperatures can be very chilly each night as we approach winter. Keep your house a consistent temperature and keep sensitive plants away from drafts (such as windows, doors, etc.) to keep them content.
As the heater kicks on to keep those plants, humans, and pets warm, the warm air is very drying. If you have ever felt chapped lips and dry skin in the winter months this is likely why! Your plants feel it, too. Some plants are more sensitive to humidity changes than others. Plants that are showing stress from low humidity start to get crispy edges, deformed leaves that curl or twist, or brown spots.
How do I mitigate these changes?
Glad you asked! Your focus should be to supplement light (intensity and duration), keep the temperature above 55 degrees (F) and consistent (warmer for some plants), adjust your watering routine, supplement humidity, and acknowledge that your plant's metabolism is slowing down and will most likely be stagnant during winter. Your plant probably won't grow. And that's ok! Your goal is to maintain health until spring when your plant can explode with happy new growth all over the place!
Adjust your watering routine
Less light (intensity/duration), cooler temperatures, and no growth usually means your plant needs less water. More light (intensity/duration), warmer temperatures, and active growth usually means your plant needs more water. The golden rule is to always check the potting soil's dryness before watering again and never assume your plant needs water on a schedule. As daylight wanes and temperatures cool, you usually need to make some pretty significant adjustments to your watering routine!
Some plant hobbyists find their plants need very little water in the winter, opting to let the soil dry out for longer in between watering. I'm in this camp. It's typical for my aroids to go two weeks without water in winter, even in an airy potting mix that dries quicker than typical potting soil. I let my hoya dry out even longer, as they store so much water in their leaves it would be disastrous to give the plants more water than they need. This is a huge difference from summer when everything is growing and getting watered every few days or so! If I see wilting or wrinkling and the soil is dry, I know I waited too long and increase my frequency. If the soil is damp and I see this, the plant has probably developed root rot and I will start immediate resuscitation!
However, some plant hobbyists find their plants actually need more water in the winter because of the warm, dry air coming from central heating units in their home. Or, if you are flooding your plants with artificial light and the plants are still actively growing you may not make any watering changes at all. It really just depends on your specific situation! Check the soil first and err on the side of caution, opting to let things dry just a little more than you would in the growing season.
Supplementing light
There are so many lighting options! Look for lights that are a minimum of 5,000 kelvins in the "daylight white" spectrum for optimal results. Something advertised as T5-equivalent (or T8) should suffice. Artificial lights are still extremely poor compared to that big 'ole round ball in the sky, so any lighting you choose should be kept on for 12-16 hours a day. Plants need a dark cycle, though, so be sure to give your plant darkness for at least 6 hours every day.
I use plant grow bulbs in desk lamps and pendant lights, T5 garage workshop lights in an indoor greenhouse, under-cabinet "daylight white" LED strip lights on shelves, and more. I do not use the purple/red/blue spectrum lights as the color gives me a raging headache and doesn't do much for me aesthetically. At the risk of repeating myself, I'm going to refer you to my Toolbox blog post, in which I list all of the artificial lights I use in my own home. I even use outlet timers to control the hours in which my lights turn on and off (3am on, 4pm off)- I'm a huge fan of automation!
Supplementing humidity
Humidity is simply the amount of moisture in the air, measured as "relative humidity." The lowest I'll let the humidity in my home get is 50%, but specific "zones" of my home are set up closer to 70% humidity by using a continuous-run humidifier. Other humidity-boosting ideas include: cloches and terrariums to trap humidity for a select few plants, humidity boxes as I blogged about here, trays filled with pebbles/stones and water with plants set on top of them (not super effective).
I generally advise against misting unless your water is very clean and your airflow is very good; misting does not boost humidity for very long and is fairly ineffective if the surrounding air is still very dry. Additionally, water droplets sitting on leaves can spread fungus and bacteria fairly quickly.
Bringing plants indoors from outside
If you're like me, you let a few of your favorites go to "summer camp" on an outdoor patio. When you bring these plants inside as the temperatures start to drop, it's important to treat for pests, even if you do not visibly see any. My best practice is to wash everything thoroughly- the plant, the pot, the drip trays- and don't rush.
I will give each plant coming back inside a shower, sure to wash the leaves of dirt and grime. Then I spray everything down, including the brim and outside of the pots with my DIY neem soap or some kind of horticultural oil. Let it sit usually overnight then shower everything again the next day. This helps prevent any live pests from entering your home. The more thorough you are the better results; it's a good idea not to rush this process. Focus on the undersides of leaves, the nooks and crannies, and the stems. This is where bugs like to hang out the most!
It's possible there are pest eggs in your soil from being outdoors, so it's good practice to apply systemic insecticide granules to the soil and continuing applying every 8 weeks to prevent pests indoors. This cannot be used on edible plants. Other pest control products I recommend are located in my Toolbox blog. When you eventually bring your plant inside to rejoin others, it is advisable to quarantine your plant by keeping it at least three feet away from other plants for a week or so. If you find visible pests in that week you probably just saved yourself a pest disaster!
Plants that are likely to bring some pretty hefty pest problems inside include vegetables, outdoor perennials you are attempting to over-winter, and citrus trees. I would recommend keeping these plants in their own space away from your houseplants, to avoid spreading pests to your collection. Some plant enthusiasts build cold frames and small lean-to greenhouses to over-winter vegetables and other traditionally "outdoor" plants.
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