Indoor Cannabis Cultivation 101: Lighting Systems for Every Growth Stage

Replicating the Sun Indoors

Cannabis is a light-hungry, photoperiod-sensitive plant. Indoor success hinges on delivering the right spectrum, intensity, and schedule at each growth phase. Modern lighting tech offers precision the sun can't match—if you know how to deploy it.

Phase 1: Seedlings & Clones (Weeks 1–3)

Tool: T5 High-Output Fluorescent (6500K, "Cool White")

• Low heat—can hang 2–4" above canopy.
• High blue spectrum drives compact, sturdy vegetative structure.
• Energy-efficient for low-light-demand phase.
• Run 18/6 or 20/4 light/dark cycle.

Phase 2: Vegetative Growth (Week 3–Pre-Flower)

Tool: Metal Halide (MH) / Ceramic Metal Halide (CMH) – 4000–6500K

• Strong blue/white output maximizes leaf area and node stacking.
• CMH offers broader spectrum, higher CRI, better efficiency than standard MH.
• Typical: 400–600W for 4'×4' area; 18/6 cycle.
• Transition when plants reach ~20 cm (8") or target height.

Phase 3: Flowering (8–12+ Weeks)

Tool: High-Pressure Sodium (HPS) / CMH (3000–3500K) / Full-Spectrum LED

• Red/far-red heavy spectrum triggers and sustains flowering.
• HPS: Proven yield standard; high heat; requires robust cooling.
• CMH 3100K: Excellent spectrum, lower heat, longer bulb life.
• LED: Tunable spectrum, highest efficiency, minimal heat, 50k+ hr lifespan. Premium upfront cost.
• Switch to 12/12 cycle to initiate flowering.
• Expect 40–110 cm stretch in first 2–3 weeks of 12/12—plan headroom!

Sample 3-Light Rotation Schedule

Weeks	Light	Cycle	Notes
1–3	T5 HO Fluor. (6500K)	18/6	Close to canopy, low heat
3–Veg End	MH/CMH (4000–6500K)	18/6	Vigorous veg growth
Flower Start–End	HPS/CMH 3100K/LED	12/12	Max bud development

Key Metrics to Monitor

• PPFD (μmol/m²/s): 200–400 (seedling), 400–600 (veg), 600–1000+ (flower).
• DLI (Daily Light Integral): Match to strain needs.
• Canopy temperature: 24–28°C (lights on), 18–22°C (off).

Light is the engine of photosynthesis. Invest in quality, match to phase, and measure results. Keep it green.