Led Grow Plant Growth Stages & LED Lighting 1 — Questions and Answers
Question 1: What light schedule is typically used to keep cannabis plants in the vegetative stage indoors?
- 12 hours light / 12 hours dark
- 16 hours light / 8 hours dark
- 18 hours light / 6 hours dark (Correct answer)
- 24 hours light / 0 hours dark
Correct answer: 18 hours light / 6 hours dark
An 18/6 light schedule (18 hours on, 6 hours off) is the most common schedule for keeping photoperiod cannabis in the vegetative stage.
Question 2: What light schedule is required to trigger flowering in photoperiod cannabis strains?
- 20 hours light / 4 hours dark
- 18 hours light / 6 hours dark
- 14 hours light / 10 hours dark
- 12 hours light / 12 hours dark (Correct answer)
Correct answer: 12 hours light / 12 hours dark
A 12/12 light schedule signals photoperiod cannabis to transition into flowering by mimicking the shorter days of late summer.
Question 3: What PPFD level is generally recommended for cannabis seedlings in their first two weeks?
- 50-150 µmol/m²/s (Correct answer)
- 200-400 µmol/m²/s
- 400-600 µmol/m²/s
- 700-1000 µmol/m²/s
Correct answer: 50-150 µmol/m²/s
Cannabis seedlings require very low PPFD (50-150 µmol/m²/s) as their underdeveloped roots and leaves cannot handle intense light.
Question 4: How does increasing CO2 concentration above ambient levels (1500+ ppm) benefit plants under high-intensity LED grow lights?
- It reduces the plant's water requirements by 50%
- It allows plants to photosynthesize at higher light intensities without reaching light saturation (Correct answer)
- It eliminates the need for blue light in the spectrum
- It replaces nitrogen fertilizer requirements during flowering
Correct answer: It allows plants to photosynthesize at higher light intensities without reaching light saturation
Elevated CO2 raises the light saturation point, allowing plants to utilize higher PPFD levels (1500+ µmol/m²/s) for increased photosynthesis and yield.
Question 5: What is 'light burn' in cannabis plants, and what causes it under LED grow lights?
- A fungal disease exacerbated by high humidity and light intensity
- Bleaching and necrosis of upper leaves caused by excessive PPFD from lights too close (Correct answer)
- Yellowing of lower fan leaves due to insufficient light penetration
- A nutrient deficiency triggered by light-induced pH changes
Correct answer: Bleaching and necrosis of upper leaves caused by excessive PPFD from lights too close
Light burn occurs when LED fixtures are too close to the canopy, delivering excessive PPFD that bleaches and damages upper leaves.
Question 6: What is the 'stretch phase' in cannabis, and how should LED light intensity be managed during this period?
- The first two weeks of flowering when plants rapidly elongate; maintain or slightly reduce intensity (Correct answer)
- The final two weeks of flowering when buds gain mass; maximum intensity recommended
- The seedling stage before roots are established; very high intensity needed
- The cloning stage after cuttings are taken; maximum blue spectrum needed
Correct answer: The first two weeks of flowering when plants rapidly elongate; maintain or slightly reduce intensity
The stretch phase (first 2 weeks of flowering) sees rapid stem elongation; maintaining or slightly reducing light intensity prevents excessive stretching.
Led Grow Plant Growth Stages & LED Lighting 1
What light schedule is typically used to keep cannabis plants in the vegetative stage indoors?