Winter light in Bozeman, and what your windows do with it
On the shortest day Bozeman gets under nine hours of daylight, and the noon sun barely clears 21 degrees. Why the low light favors south windows, and what clean glass can and cannot do.
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Houseplants in a Montana winter: which window, and how close
Distance from the glass and the direction a window faces matter far more than dirt. What the extension services recommend, and where clean glass helps.
If your houseplants sulk from November to March, dirty windows are rarely the main reason. Where the plant sits matters far more. But in a Montana winter, when every plant indoors is short of light, it all adds up.
How much light houseplants need
| Light level | Foot-candles | Where you find it |
|---|---|---|
| Low | 50–250 | A north window, or a dim corner |
| Medium | 250–1,000 | An east window, or near but not in a west window |
| High | Over 1,000 | A south or southwest window |
Distance matters most
Light falls away fast as you move back from a window: Mississippi State University Extension notes it can drop from 1,000 foot-candles to 100 within a few feet, and that plants struggle eight to ten feet back. The University of Missouri puts it more bluntly: past ten feet from an average window, even low-light plants usually do not get enough.
Colorado State University Extension says the high-light spots are within two feet of a south window from October through March, and within two feet of an east or west window all year. For a Montana winter, that is the whole plan: move the light-hungry plants close to the south glass.
Where clean glass comes in
University of Maryland Extension lists “the cleanliness and type of window” among the things that change how much light a plant gets. It is a small factor next to distance, but not nothing. Greenhouse growers have found that, for crops short of light, 1% more light brings roughly 0.5 to 1% more growth. That is a greenhouse finding, and a winter houseplant is short of light in the same way.
A winter plan
- In October, move the plants that need the most light to within two feet of a south window.
- Turn them every week or two so they do not lean.
- Get that south glass cleaned in the fall, inside and out.
- Keep leaves off the cold glass itself on the coldest nights.
Montana State University Extension’s MontGuide on houseplant care has more on matching plants to windows.
Sources
- Lighting for indoor plants — University of Minnesota Extension
- Interior plants: light (PlantTalk Colorado 1352) — Colorado State University Extension
- Care and selection of indoor plants (P1012) — Mississippi State University Extension
- Lighting indoor houseplants (G6515) — University of Missouri Extension
- Lighting for indoor plants — University of Maryland Extension, 2023
- Houseplant selection and care (MontGuide) — Montana State University Extension, 2020
- Light and yield in greenhouse crops: testing the "1% rule" — Marcelis et al., Acta Horticulturae 711, 2006
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