Indoor Plant Light Requirements: The Shadow Test & Supplemental Lighting

Human-comfortable light levels are often far too dim for plants. Use this simple shadow test to match plants to your windows—or add artificial light.

The 12-Inch Shadow Test

Place a sheet of white paper where the plant sits. Hold your fingers 12 inches above. Observe the shadow:

• Dim, fuzzy shadow: Low light. Only Aspidistra, peace lily, snake plant, philodendron tolerate this.
• Medium, distinct shadow: Moderate light. African violet, corn plant, pothos, spider plant thrive. Too dim → brown spots, leaf drop.
• Sharp, crisp shadow: High light. Hibiscus, geranium, coleus, ficus, cactus, succulents, bromeliads, Norfolk pine need this to keep leaves in winter.

Boosting Light with Fluorescents

1. Buy a clamp-light fixture (shiny aluminum reflector).
2. Screw in a CFL equivalent to 100W incandescent (warm white/daylight spectrum).
3. Position fixture as close to leaves as possible—fluorescent light drops off sharply beyond 4 feet.
4. Run 14–16 hours daily on a timer.

For a permanent setup, build a PVC light stand (plans linked).