Grow Lights 101: Understanding Light Spectrum

Natural light is best, but modern windows block UV, and winter days are short. Grow lights are the secret to keeping finicky plants like Ficus and Hoyas happy year-round. But the terminology can be confusing.

Do I Need “Purple” Lights?

No. The old “blurple” (blue + red) lights are outdated for home use.

• Red Light: Stimulates flowering and fruit production.
• Blue Light: Stimulates vegetative growth (leaves and stems).
• Full Spectrum (White): Modern LEDs combine red, blue, and green to look like white sunlight. This is better for your eyes and excellent for plants.

Recommendation: Always buy “Full Spectrum” white LEDs (3000K-6000K) for living spaces.

Key Terms to Know

1. Foot-Candles (fc) / Lux

A measure of brightness as seen by the human eye.

• Low Light: 50-250 fc (Snake Plant, ZZ)
• Medium Light: 250-1000 fc (Pothos, Philodendron)
• High Light: 1000+ fc (Fiddle Leaf Fig, Succulents)

2. Kelvin (K) - Color Temperature

• Warm White (2700K-3000K): More red. Good for flowering. Looks like a standard living room bulb.
• Cool White (5000K-6500K): More blue. Good for leafy growth. Looks like daylight.
• Best Balance: 4000K-5000K is a great sweet spot for general houseplant growth.

3. PPFD / PAR

This measures the actual light particles (photons) plants can use for photosynthesis. Professional lights list this. For hobbyists, a high-quality LED bulb is usually sufficient without needing a PAR meter.

Distance Matters

Light intensity drops off drastically with distance (Inverse Square Law).

• 6 inches away: Very intense. Good for succulents.
• 12 inches away: Good for most foliage plants.
• 24 inches away: Weak. Only low-light plants will survive here.

How Long to Leave Them On?

Grow lights are weaker than the sun, so they need to run longer.

• Foliage Plants: 12-14 hours per day.
• Flowering/Succulents: 14-16 hours per day.
• Dark Period: Plants need darkness to rest and respire. Do not leave lights on 24/7. Use a cheap timer plug!