Understanding the physics and plant biology behind light penetration helps you optimize your setup.
🔬 Light Gradient in the Canopy
In dense canopies, PPFD drops exponentially as light passes through leaves. A typical 3-foot tall plant might show:
- Top (0-12"): 1000-1400 µmol/m²/s — optimal for flowering
- Mid (12-24"): 400-700 µmol/m²/s — suboptimal, leading to stretched, less-dense growth
- Bottom (24-36"): 200-400 µmol/m²/s — below photosynthetic saturation, producing larf
This 40-70% drop is why top-only lighting leaves so much potential yield on the table.
📐 Inverse Square Law & Why More Top Light Doesn't Fix It
Light intensity follows the inverse square law: doubling the distance from a light source reduces intensity by 75%. But there's a second problem: leaf shading.
Each leaf absorbs 80-90% of photons that hit it. By the time light travels through 3-4 leaf layers, almost nothing reaches the bottom. Adding more top light doesn't help—it just over-saturates the upper canopy (wasting energy and creating heat stress) while lowers remain starved.
Solution: Place light sources below the canopy to bypass upper leaf shading entirely.
🌿 LAI (Leaf Area Index) and Light Interception
LAI measures total leaf surface area per square meter of canopy. Dense canopies (LAI 4-6) intercept 90%+ of incoming light in the top 12-18 inches. This is great for photosynthetic efficiency at the top but terrible for lowers.
Under-canopy lighting increases total light interception by targeting previously dark zones. You're not adding more light to already-lit areas—you're lighting new photosynthetic surface area.
🧬 How Plants Allocate Energy Based on Light Signals
Plants use light as a hormonal signal to allocate resources. Areas receiving high PPFD get more:
- Auxins: growth hormones promoting cell elongation and bud swelling
- Cytokinins: cell division hormones increasing bud density
- Carbohydrates: energy for resin and terpene production
When lower buds receive adequate light, the plant treats them as priority sites, allocating resources that would otherwise go only to the top colas.
🔍 Photomorphogenesis: Why Buds That Receive Light Develop Differently
Photomorphogenesis is the process by which light quality (spectrum) and quantity (intensity) shape plant development. Key effects:
- Red light (600-700nm): Promotes flowering and bud swelling
- Blue light (400-500nm): Triggers trichome production and terpene synthesis
- Far-red (700-750nm): Increases flower size but can reduce density if overused
Under-canopy bars with full-spectrum or red-heavy output stimulate lower buds to develop as if they were top colas—dense, resinous, and marketable.
