Reading a window

What a window delivers depends on which way it faces, the time of year, and what's in the way, among other things.

Once you accept that the window is your light source, the next skill is reading what a given window delivers. Three of the biggest factors are which way it faces, the time of year, and what's in the way. The size of the window and the color of your walls matter too (Trinklein, 2017; Shaughnessy & Pertuit, 1999).

The four directions

These descriptions assume you're in the Northern Hemisphere. If you're in the Southern Hemisphere, read north for south throughout.

South-facing windows get the most light (LuEsther T. Mertz Library, n.d.). From late summer onward, south-facing glass receives more and more direct sun, more than east or west windows from fall through spring (Pacific Northwest National Laboratory, 2023). A south window suits plants that need bright light. Plants that don't can sunburn there from late spring to early fall (Trinklein, 2017).
East-facing windows get direct sun in the morning (LuEsther T. Mertz Library, n.d.). East and west windows receive about the same amount of light, but a west window runs much hotter (Baxendale et al., 2013), so in our view an east window is the more forgiving of the two for common foliage plants.
West-facing windows get direct sun in the afternoon (LuEsther T. Mertz Library, n.d.), and plants there are exposed to much higher temperatures (Baxendale et al., 2013). So we'd expect scorch sooner at a west window than the light level alone suggests.
North-facing windows get the least light. One Arizona extension guide puts a northern exposure at about a fifth of a southern one (Schalau, 2024). In summer they catch low sun early in the morning and in the evening, at a glancing angle (Pacific Northwest National Laboratory, 2023). Most houseplants, even ones that tolerate low light, grow best in bright indirect light (Steil, 2023a), but north light suits some plants well: Missouri's extension recommends a north window for African violets (Trinklein, 2010).

Orientation diagram: a floor-plan square with north, east, south, and west windows, the sun's arc across the southern sky, each window labeled for its light character. Walnut line on cream.

Fig. 6. What each window direction delivers across a day, Northern Hemisphere. South gets the most light, east gets morning sun, west gets afternoon sun and more heat, and north gets the least. In the Southern Hemisphere, read north for south.

The seasons change what a window delivers

A window isn't a fixed light source. The sun is higher in the sky in summer and lower in winter (U.S. Department of Energy, 2000), and the days are longer in summer (Stern, 2014). Both change how much light a spot gets through the year (Shaughnessy & Pertuit, 1999).

At a south window the two effects pull in opposite directions. Winter days are shorter (Stern, 2014), but the low winter sun shines more directly onto south-facing glass, and a well-sized roof overhang can keep direct summer sun off a south window while still letting winter sun in (Pacific Northwest National Laboratory, 2023). Missouri's extension guide puts the result plainly: in winter, every houseplant gains from a south window's light (Trinklein, 2017). Indoor light is often very low in winter (Steil, 2023b), so our advice is to move plants closer to the glass or into a south window as winter comes, and from late spring to early fall to pull sun-sensitive plants back from south glass.

Side-view diagram of a window with a high summer sun arc and a low winter arc; the low winter sun reaches deep into the room while the high summer sun and roof overhang light only near the glass. Walnut line on cream.

Fig. 7. The same window, two seasons. The low winter sun shines more directly into a south window; the high summer sun can be kept off the glass by an overhang. Placement changes with the season.

What's in the way

Obstructions decide how much of a window's light reaches your plant. Trees outside the window cut light, though deciduous trees that drop their leaves let most winter sun through (U.S. Department of Energy, 2000). Buildings, roof overhangs and other structures take their cut too (Trinklein, 2017). Closer to home, curtains and sheers can reduce light a great deal (Steil, 2023a), and dirty windows can block up to half of it (LuEsther T. Mertz Library, n.d.). Low-emissivity (low-E) window coatings are used mainly to cut heat transfer, ideally by blocking ultraviolet and infrared while passing visible light (Howell et al., 2002). Glass type matters too: in the Danish apartment study from Lesson 2, two-layer high-transmittance glass let in about 15 percent more visible light than three-layer energy-saving glass (Volf et al., 2024). Pale, light-colored walls can work the other way, bouncing light back into the room (University of Maryland Extension, 2023). Your eye won't register most of these differences. A meter and the plant will.

From memory Tier 1

Strengthen your understanding

Try these in your head first, then open to see.

You have exactly one good window and you grow typical foliage houseplants. Which direction would you most want it to face, and why?

East, in our view. East and west windows get about the same amount of light, but an east window gets its direct sun in the morning and runs cooler than a west window. A south window is brightest but can sunburn plants that don't need bright light from late spring to early fall, and a north window gets the least light.

Your plants sit a few feet back from an east window all summer. As winter comes on, what change is worth making, and why?

Move them closer to the glass, or into a south window if you have one. Winter days are shorter and indoor light is often very low then, while a south window picks up more direct sun as the sun sits lower. In winter, almost any houseplant benefits from a south window's light.

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