Set the light to 20 hours — "more growth." Basil looks fine the first week, then suddenly starts flowering in week three instead of week six. Cutting the vegetative period in half is not the effect of more light — it is the plant's response to a "night dark period that is too short." For some crops it is not day length that triggers flowering, but the duration of continuous darkness.
Quick Glossary
- Photoperiod — the duration of light and dark periods over 24 hours; plants use the day-to-night ratio to determine season and regulate flowering, growth, and dormancy
- Circadian rhythm — the plant's internal biological clock with an approximately 24-hour cycle; regulates stomatal opening, enzyme activity, hormone synthesis, and other physiological processes independent of external lighting
- Photoinduction — the process of initiating flowering through accumulation of a photoperiodic signal over several cycles; in most plants it requires several consecutive "correct" night dark periods
Short-Day, Long-Day, and Day-Neutral: Three Response Categories
Photoperiodic response is not the same for all plants:
Short-day plants flower when the dark night period exceeds their critical threshold (typically 10–12 hours of continuous darkness). Basil, chrysanthemum, cannabis. For these crops, even 1–2 minutes of light in the middle of the night dark period (a "night break") completely blocks flowering — photoinduction is cancelled.
Long-day plants flower when the dark night period is shorter than the critical threshold (long day, short night). Spinach, dill, radish. For spinach, 14+ hours of light triggers bolting — and the total vegetative mass will be reduced.
Day-neutral plants flower regardless of photoperiod — tomato, cucumber, pepper. For these crops, day length affects DLI and the rate of photosynthesis, but not flowering initiation.
Circadian Rhythm: Why the Plant Knows What Time It Is
The plant has an internal clock — an approximately 24-hour cycle that regulates physiology even without external signals. Stomata begin opening before sunrise — "anticipating" increased light. Synthesis of certain photosynthesis enzymes peaks 2–4 hours after the light period begins. Root activity and nutrient uptake follow their own rhythm that does not coincide with the photosynthesis peak.
The practical implication: a plant grown under an irregular light schedule — 14 hours one day, 18 the next, then a sudden night — becomes desynchronised. Stomata open at the wrong time, enzyme activity shifts, nutrient uptake becomes unpredictable. A stable and repeating light schedule matters not only for DLI, but for the coherence of physiological rhythms.
Optimal Schedules for Common Crops
Microgreens: 14–16 hours per day — day-neutral crops with a short cycle; photoperiodic response is not critical. The goal is to accumulate DLI 12–18 mol/m²/day at the available PPFD.
Lettuce and leafy greens: 16–18 hours — long-day crops where a long day accelerates growth, but for bolting-prone species (spinach, coriander) exercise caution — 16 hours is often a safe maximum.
Basil: 14–16 hours with a mandatory 8–10-hour uninterrupted dark period — a short-day crop where reducing darkness to 6 hours or less triggers early flowering and shortens the vegetative cycle.
Tomato, cucumber, pepper: 16–18 hours — day-neutral crops; longer day = higher DLI = more photosynthesis; the upper limit is set by the minimum dark period needed for recovery.
Strawberry: photoperiod-sensitive — short-day for generative bud initiation. 8–10 hours of light for fruiting, 16+ hours for vegetative runner propagation.
The Dark Period: Why It Is Necessary Even for Day-Neutral Crops
For day-neutral crops, the dark phase does not initiate flowering — but it is critical for other processes. In darkness, the plant converts carbohydrates accumulated during photosynthesis into structural components: cell walls, tissues, roots. Plants under continuous lighting (24/0) often show fatigue symptoms: chlorosis, leaf edge necrosis, reduced turgor.
Minimum dark period for most crops: 6 hours. Optimal: 6–8 hours. Below 4 hours — risk of degradation even in day-neutral crops.
Three Mistakes That Cost the Most
24/0 for "maximum DLI." The plant absorbs more photons — but cannot process the products of photosynthesis fast enough. Structural synthesis, chloroplast recovery, and hormonal cycles require darkness. The result at 24/0: leaves that look active but have reduced photosynthetic efficiency and elevated stress.
Interrupting the night dark period for basil "just to check" — or for irrigation triggered by a timer with lighting. Even 1–2 minutes of white or red light in the middle of the dark period cancels the photoperiodic signal in short-day crops. When working at night in a zone where basil is growing — use only a green torch (green light is not perceived by phytochrome as "day").
Extending day length instead of increasing PPFD when DLI is insufficient. If DLI is inadequate, it is more rational to raise PPFD at the same duration than to extend the photoperiod beyond what is optimal for the crop. 18 hours at 200 µmol/m²/s gives DLI 12.96 — the same result as 14 hours at 257 µmol/m²/s, but without the risk of photoperiodic effects and with lower electrical load.
Signs That the Light Schedule Is Set Correctly
- Basil and other short-day crops do not flower ahead of schedule
- Spinach and coriander do not bolt during the vegetative cycle
- Day-neutral crops show turgid leaves and normal morphology after 2–3 weeks on the established schedule
- The schedule is stable and does not change between cycles without an agronomic reason