The mental model
Now pull it together as one system. Light reaches a spot with three properties: intensity, quality and duration. The leaf turns what arrives into sugar, as you saw in Module I. Indoors, light intensity is hard to adjust, so the University of Florida's advice is to choose plants that suit the light you have (UF/IFAS Gardening Solutions, n.d.), and in our view where they go is the other decision you control. Grow lights are the other option. Missouri's extension guide calls supplementary electric light usually the simplest and cheapest way to give plants enough light when a window can't (Trinklein, 2017), but this module has stuck to windows. Everything here has been teaching you to read the supply and match it to the plant.
Fig. 13. Light as the first input to the system you learned in Module I. Intensity, quality (the spectrum) and duration are the three dials, and the leaf turns what arrives into sugar. Indoors, placement is the main way you set them.
Read the signal, name the cause
Tap a symptom on the left, then tap the explanation on the right that fits it. Diagnosis is the skill, so you do it, you don't just read it.
The symptom
What's actually happening
How this connects to the rest of the curriculum
Light isn't a standalone topic. It sets the pace for much of what you'll learn next:
- Module I (How a Plant Works) is the engine this module feeds: light is the energy that runs the photosynthesis you already met.
- Module III (Water, Properly) reads light and water together. Plants in high light lose more water through their leaves than plants in low light (Pennisi, 2022), so in our view a change in light can change how often a plant needs water.
- Module VI (Feeding) follows growth. Indoor plants in low light need less fertilizer (Shaughnessy & Pertuit, 1999), and a plant's need rises when light increases and it's growing actively (Pennisi, 2022). Feeding more than a slow-growing plant can use can lead to pale or spindly growth (Steil, n.d.). Module VI covers when feeding helps and when it makes things worse.
- Modules VII and IX (Reading a Plant, Diagnosis) build on the symptom-reading you just practiced. Stretch, lean, bleach and scorch are signs you'll learn to read alongside the others.
Nine things to remember and use
- Window light is the input you can change least at home. Short of adding grow lights, the decisions you control, as we see it, are which plants you choose and where they go.
- Light has three properties: intensity (how much), quality (which colors) and duration (how long). On a windowsill, intensity is the one that usually falls short.
- Your eyes adapt to the light around them, so they're poor judges of indoor light. Use the shadow test or a light-meter app instead.
- Light drops as you move away from the window, often steeply. "Near a window" and "in the same room as a window" can be very different places for a plant.
- Foot-candles and lux measure light the way your eye sees it; PPFD and DLI count the light a plant uses. "Bright indirect" means bright enough to cast a distinct shadow, without sun falling on the leaves.
- Read a window by its direction, the season and what's in the way. In the Northern Hemisphere, south gets the most light, east gets morning sun, west gets hotter afternoon sun, and north gets the least.
- Every plant has a light budget. Match the plant to the spot; there's no good spot in the abstract.
- Stretched, leaning, pale growth usually means too little light; bleached or scorched patches usually mean too much, too suddenly.
- When you increase a plant's light, do it gradually, over a week or two.
Map the light in your home
You can't place plants well until you know what your own rooms offer. Walk your home once, around midday on a reasonably clear day, and check every spot where you'd realistically keep a plant. Fill in the grid as you go. The example row is done for you.
For each spot, note the window direction (your phone has a compass), what's in the way of the light, a shadow-test reading at the glass and a few feet back, hours of direct sun, and your verdict.
| Spot / room | Window faces | Obstructions | Shadow at glass | Shadow ~5 ft back | Hrs direct sun | Your verdict |
|---|---|---|---|---|---|---|
| ExampleLiving room, left of TV | East | Large oak, sheer curtain | Soft | Barely visible | ~2 (morning) | Medium at glass, low past 3 ft; bright-indirect plants only |
Shadow key: crisp = direct sun · soft = bright indirect · barely visible = low. If you used a light-meter app: note the midday reading in the verdict column too. Using the University of Florida's bands (UF/IFAS Gardening Solutions, n.d.): 25–100 foot-candles (about 270–1,100 lux) is low, 100–500 (about 1,100–5,400 lux) is medium, 500–1,000 (about 5,400–10,800 lux) is high, and over 1,000 is direct sun.
Keep this sheet. When you're tempted by a plant, you'll already know whether you have a home for it, and when winter shortens the days, you'll know which spots to check again.
"I've read this module, and I've mapped the light in at least one room of my home."
This marks Module II complete and carries your progress forward. The work is done by your own honest account. No points, no badge, no leaderboard.