Cannabis Basic Needs: Light, Water, Nutrients, Air
Asiannabis Community • Cultivation › Basic Growing Info
Updated: July 2026
Cannabis is a C3 plant — it photosynthesizes via the Calvin cycle, making it highly sensitive to light, CO₂, temperature, and water.
1. Light — Photosynthesis Fuel
Cannabis requires PAR light (400–700 nm). Three key metrics:
- PPFD: μmol/m²/s — light intensity at canopy level
- DLI: mol/m²/day — total light over 24h. DLI = PPFD × hours × 0.0036
- Spectrum: Blue (400-500nm) → compact growth. Red (600-700nm) → flowering. Far-red → +30% photosynthesis. UV-B → trichome production
| Stage | PPFD (μmol/m²/s) | DLI (mol/m²/day) | Photoperiod |
|---|---|---|---|
| Seedling | 200–400 | 13–26 | 18/6 |
| Vegetative | 400–600 | 26–39 | 18/6 |
| Flowering | 600–900 | 26–39 | 12/12 |
| Flower + CO₂ | 900–1500 | 39–65 | 12/12 |
Golden Rule: leaf photosynthesis saturates well below 1,800 μmol/m²/s, but canopy yield does not. Rodriguez-Morrison, Llewellyn & Zheng (2021, Front. Plant Sci. 12:646020) grew cannabis at ambient CO₂ (437 ppm) and dry inflorescence yield rose linearly from 120 all the way to 1,800 μmol/m²/s — 4.5× more flower. CO₂ enrichment buys grams per kWh, not a higher ceiling. One catch: potency per gram did not change across that whole range — you get more total cannabinoids because you get more flower, not stronger flower.
2. Water
Plants use only 3-5% of water for photosynthesis; the rest transpires through stomata.
Proper Watering
- Finger test 3-5cm deep: dry → water, moist → wait
- Water thoroughly until 15-20% runoff
- Measure runoff pH + EC to detect salt buildup
| Symptom | Overwatering | Underwatering |
|---|---|---|
| Leaves | Droopy, stiff, heavy — wet soil | Droopy, thin, crispy — dry soil |
| Leaf color | Dark green, yellowing from bottom | Rapid yellowing, drying from edges |
| Recovery | Slow (several days) | Fast (a few hours) |
| Risk | Root rot (Pythium) | Stress, reduced yield |
3. Nutrients — NPK & Micronutrients
3.1 Macronutrients N-P-K
- N: Protein, chlorophyll. High during veg
- P: ATP, roots, flowers. High during flower
- K: Water regulation, enzymes. Stable throughout lifecycle
| Stage | NPK | Explanation |
|---|---|---|
| Seedling | 1-1-1 (light) | 1/4 dose |
| Vegetative | 3-1-2 | High N for leaves & stems |
| Early Flower | 2-2-2 | Transition |
| Mid-Late Flower | 1-3-2 | High P for flowers |
| Flush | 0-0-0 | Plain water last 1-2 weeks |
3.2 Secondary & Micronutrients
- Ca: cell walls. Deficiency → brown spots on new leaves
- Mg: chlorophyll center. Deficiency → interveinal yellowing on old leaves
- S: terpene synthesis. Deficiency → uniform yellowing on new leaves
- Fe, Mn, Zn, B, Cu, Mo: small amounts but essential
4. pH & EC
| Medium | Optimal pH | EC (mS/cm) |
|---|---|---|
| Soil | 6.0–7.0 | Seed 0.4-0.8 / Veg 0.8-1.4 / Flower 1.2-1.8 |
| Coco coir | 5.8–6.3 | Seed 0.4-0.6 / Veg 0.8-1.2 / Flower 1.0-1.6 |
| Hydro | 5.5–6.5 | Seed 0.4-0.6 / Veg 1.0-1.6 / Flower 1.4-2.0 |
pH should drift slightly within the optimal range (e.g., 6.0→6.5→6.2 for soil). Calibrate your pH meter weekly.
5. Air & Environment
5.1 VPD (Vapor Pressure Deficit)
| Stage | VPD (kPa) | Temperature (°C) | RH (%) |
|---|---|---|---|
| Seedling/Clone | 0.4–0.8 | 22–26 | 65–80% |
| Vegetative | 0.8–1.1 | 22–28 | 55–70% |
| Early Flower | 0.9–1.2 | 20–26 | 45–55% |
| Late Flower | 1.1–1.4 | 18–24 | 40–50% |
| With CO₂ | 1.3–1.6 | 26–30 | 40–55% |
5.2 CO₂
- Ambient: ~420 ppm (2026)
- Supplementing to 1000-1500 ppm → +20-30% yield
- CO₂ supplementation only matters when light + nutrients are already optimized
- CO₂ >1500 ppm = no additional benefit + harmful to humans
5.3 Air Circulation
- Exchange air every 1-3 minutes (inline fan + carbon filter)
- Oscillating fans → thicker stems (thigmomorphogenesis)
- Slight negative pressure → odor control
Summary Table
| Factor | Function | Key Metrics | Common Mistakes |
|---|---|---|---|
| Light | Photosynthesis, trichomes | PPFD, DLI | High PPFD without CO₂ |
| Water | Transport, cooling | Moisture, runoff pH/EC | Overwatering |
| Nutrients | Cell building | NPK, EC, pH | Excess N in flower |
| pH | Nutrient uptake | 6.0-7.0 soil, 5.5-6.5 hydro | Not calibrating |
| Air | CO₂, transpiration | VPD, CO₂ ppm | Poor circulation |
References
- Chandra S et al. (2008). Physiology & Molecular Biology of Plants.
- Chandra S et al. (2015). Journal of Applied Research.
- Eichhorn Bilodeau S et al. (2019). Frontiers in Plant Science, 10:296.
- Rodriguez-Morrison V et al. (2021). Frontiers in Plant Science, 12:646.
- Danziger N, Bernstein N. (2021). Industrial Crops and Products, 164:113351.
- Moher M et al. (2022). GCB Bioenergy, 14(10).
Disclaimer: Educational content for legal cultivation. Compliant with Thailand Controlled Herbs (Cannabis) Notification B.E. 2568 & Ministerial Regulation B.E. 2569.
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