Every houseplant that spent the summer on a Missoula porch needs to come back inside before the nights get cold, and September is the month to do it. The common mistake is waiting until the first frost warning and hauling everything in at once, in the dark, the night before. Bringing houseplants indoors works a lot better as a two week process than a panic, and the plants show the difference all winter.
Why the Timing Matters More Than You Think
Most common houseplants are tropical, and they need protecting long before frost. Below about 50 degrees, many of them can suffer chilling injury, and sensitive types, including many Chinese evergreens, can be hurt below about 55. Nights in the 40s will not kill a pothos or a monstera outright, but they stress it, and a stressed plant dropped into a heated house is a plant that sheds leaves for a month.
In Missoula, nights in the 40s are common by early September, well ahead of the median first fall frost in late September (about September 26 at the airport, per NOAA’s 1991 to 2020 normals). In about 1 year in 10, that first frost arrives by mid September. Missoula summer nights already dip into the 40s now and then, and once those cooler nights become the norm in late August and early September, it is time to start moving plants in, sooner for cold-sensitive plants and in low spots where cold air pools. If your plants are still outside past mid September, you are borrowing time.
There is a second reason not to wait. The gap between outdoor light and indoor light is enormous, far bigger than it feels to human eyes. A shady porch is still several times brighter than a bright window. The smaller the temperature and light change on moving day, the less shock, which is why early September, while the house and the porch still feel similar, beats waiting until October.
Check for Hitchhikers Before Anything Comes In
Plants that summer outdoors often bring pests back in with them: aphids, scale, and spider mites on the leaves, and fungus gnats in the potting mix. Outside, predators and weather help keep these things in check. Inside, nothing does.
So inspect before you carry anything through the door. Look under every leaf, along stems, and around the drainage holes. Sticky residue means scale or aphids. Fine webbing at leaf joints means spider mites. Then hose the whole plant down, top and bottom of the leaves, with a decent spray of water. For anything that showed actual pests, a thorough coating of insecticidal soap is worth it, repeated once about a week later.
The soil matters as much as the leaves. Fungus gnats lay eggs in moist potting soil outdoors, and slugs, earwigs, and ants all take up residence in pots over a summer. Let the pot dry out well before moving day, and if a plant has been sitting on bare ground, lift it and check the drainage holes. Some people also soak small pots in a tub of water for about fifteen minutes to flush soil insects out. That is optional, not a standard step. It will not catch every pest, it leaves the root ball saturated, and it is a bad idea for succulents, snake plants, and ZZ plants. If you try it, do it a few days ahead so the pot can drain. Once the plants are inside, keep them apart from your other houseplants for a few weeks if you can, so anything you missed has time to show itself.
Move Them in Stages if You Can
The gentlest version of this is a gradual step down. Move plants from open sun to a shaded porch for a week, then indoors to the brightest window you have. Each step shrinks the light gap the plant has to adjust to.
If that is more fuss than your September allows, at least land every incoming plant in the brightest spot in the house for its first few weeks, even if that is not where it will live all winter. Let it adjust to indoor life at the top of the light range, not the bottom.
Expect some leaf drop anyway, especially from ficus types, and do not panic about it. Some yellowing and dropped leaves in the first couple of weeks indoors is usually the plant adjusting to less light, not dying. The bigger risk is a worried owner overwatering it during that adjustment.
Placement indoors deserves a minute of thought too, because the winter house has hazards the summer porch did not. Keep incoming plants off cold windowsills where leaves touch the glass, and out of the blast path of heat vents, which will be switching on within weeks of moving day. A spot a few feet inside a south or west window, warm and bright but out of the airstreams, is the winter home most tropicals want. If two plants have to compete for it, give it to whichever one cost more to replace.
This is also the natural moment for a quick grooming pass: snip off summer’s sun-bleached or tattered leaves, pull weeds that seeded into the pots outdoors, and top up any soil that washed low. A tidy plant is easier to inspect all winter, and the first inspection is the one that matters most.
Cut the Water Back Once They’re In
Outdoors in summer, a potted plant might drink water every day or two between the sun, the wind, and active growth. Indoors in fall, all three of those drivers are gone. Growth slows, air movement stops, and light drops. The same plant might need water once every ten days, sometimes less.
Keep watering on the summer schedule and you will drown roots by October. The fix is simple: stop watering by the calendar entirely. Push a finger two knuckles into the soil, and water only when it comes out dry. This one habit prevents a lot of winter plant deaths, because heated indoor air fools people. Dry air does not mean dry soil.
Skip the fertilizer too. Feeding a plant heading into winter dormancy just builds up salts in the pot. Resume in March when the light comes back and you see new growth.
If the plants in question live at your business instead of your house, Garden City Plant Care can take this off your plate. Our office plant care service handles the pest checks, placement, and winter watering adjustments on a regular schedule, to help the plants in your lobby look as good in January as they did in June.
Sources
- NOAA NCEI, U.S. Climate Normals 1991 to 2020 (Missoula International Airport)
- Iowa State University Extension, How and When Do I Bring My Houseplants Back Indoors Before Winter?
- University of Illinois Extension, Time to Bring Plants in for Winter, Minus the Insects
- University of California IPM, Pest Notes: Houseplant Problems