G-01 Field guide
Start with placement.
Match the plant to the room before you adjust watering, feeding, or humidity. Light is the variable everything else responds to.
The four light tiers
Indoor light falls into four usable bands. Low light (0-80 PAR) is the back of a north-facing room or a shelf far from any window -- survival light for shade-tolerant plants, not growth light. Medium (80-200 PAR) reaches a few metres from an east or west window and suits most common houseplants. Bright indirect (200-400 PAR) sits beside a large window without direct sun on the leaves -- the sweet spot for most tropicals. Direct (400+ PAR) means sun on the leaves for part of the day, which succulents and Mediterranean plants prefer.
Room brightness is not the same as plant-usable light. A bright room can still be low light for a plant on a shelf three metres from the window.
How to read a room
Stand where the plant would sit and look at the window. If the sky is fully visible and light lands directly on the pot, that is bright indirect or direct. If you can read comfortably but the sky is partially obscured, that is medium. If you need to switch on a light to read -- or you would not choose to sit there -- that is low light. Test it at different times of day: a windowsill that gets direct sun at noon is direct light, even if it is shaded in the morning.
Seasonal adjustment
Light intensity drops 50-70% between summer and winter at northern latitudes. Move plants closer to glass in winter, but keep them off cold panes and away from hard radiator drafts. In summer, move plants back from south or west windows to prevent scorch -- bright indirect becomes direct when the sun moves higher and shines more directly through the glass. A plant that was fine in spring may need moving in July.
What happens in the wrong light
Too little light: slow or no growth, pale or yellowing leaves, long spindly stems reaching toward the window, potting mix staying wet for weeks. Too much direct light on a plant adapted to shade: bleached or scorched patches on leaves, crisp or papery texture at the edges, soil drying out very quickly. In both cases, move first before adjusting anything else.