Light Schedules for Leafy Greens vs. Fruiting Plants
The number of light hours per day isn't one-size-fits-all - leafy greens and fruiting plants have fundamentally different photoperiod needs. This guide explains the biology behind light schedules, gives specific recommendations for common crops, and covers how to use DLI to optimize results with the light you have.
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"How many hours of light do my plants need?" is one of the most common hydroponic questions - and the answer depends almost entirely on what you're growing. Plants evolved in specific photoperiod regimes, and mimicking those signals indoors determines whether they grow vegetatively or shift to flowering.
The three photoperiod categories
Plants fall into one of three photoperiod response types:
Short-day plants flower when nights are longer than a critical threshold. They need long dark periods to initiate flowering. Examples: chrysanthemums, poinsettias, some cannabis strains. For food growing: strawberries initiate fruiting on short days.
Long-day plants flower when days are longer than a critical threshold. Examples: lettuce (bolting), spinach (bolting), cilantro (bolting). Most leafy greens are long-day plants - which means giving them very long photoperiods will eventually push them toward bolting.
Day-neutral plants flower based on age and environmental cues other than photoperiod. Examples: tomatoes, peppers, most modern strawberry varieties, cucumbers. These can receive any photoperiod without triggering or blocking flowering.
Leafy greens: the bolting tension
Lettuce
Lettuce is technically a long-day plant but in practice is quite tolerant of extended photoperiods at cool temperatures. The key variable:
- Temperature below 68°F + 16–18 hours light: Lettuce grows productively without bolting for 8–10 weeks
- Temperature above 75°F + 16 hours light: Bolting initiated within 3–5 weeks
- Optimal schedule: 16 hours on / 8 hours off for most indoor growing environments
Running 18 hours of light on lettuce in a cool room accelerates growth without causing early bolting. In a warm kitchen at 74°F+, reduce to 14 hours and prioritize cooling the environment.
Spinach
A classic long-day plant that bolts under extended photoperiods more readily than lettuce. Strict 14 hours on / 10 hours off is the recommended schedule for productive spinach. At 16+ hours, spinach in warm conditions bolts within 4–6 weeks.
Cilantro
Among the most photoperiod-sensitive common herbs. At 16 hours in a warm environment (above 70°F), cilantro bolts within 3–4 weeks. Reduce to 12–14 hours for the most productive cilantro in kitchen growing conditions. Alternatively: grow as a successive batch crop and harvest the entire plant before bolting begins.
Kale and Swiss chard
Both are biennial plants with minimal bolting tendency in a single growing season under artificial light. Run 16 hours comfortably. These are among the most light-tolerant leafy greens for extended photoperiods.
Mint and basil
Mint: Day-neutral for practical purposes; grows productively at 14–18 hours. Run 16 hours as a sensible default. Mint grows aggressively regardless of photoperiod.
Basil: Long-day tendencies but primarily responsive to temperature and age. Flowering in basil is triggered more by stem maturity than by photoperiod. Pinch flower buds regularly regardless of light schedule. Run 16–18 hours for maximum leaf production.
Fruiting plants: more light, different priorities
Tomatoes and peppers (day-neutral)
Neither tomatoes nor peppers require a specific photoperiod to initiate flowering. They will flower and fruit at any photoperiod from 8 to 20 hours. This means:
- More light hours = more photosynthesis = more growth and yield potential
- The practical upper limit is 18 hours - 24-hour lighting causes leaf chlorosis and reduced carbohydrate translocation in some varieties
- Recommended: 16–18 hours on / 6–8 hours off
For fruiting plants, light intensity matters far more than light duration. A tomato plant under 18 hours of weak light (100 µmol/m²/s) will produce far less than one under 16 hours of strong light (500 µmol/m²/s). Invest in a higher-intensity LED before extending photoperiod hours.
Strawberries
Modern everbearing and day-neutral strawberry varieties (most home-growing varieties) are not photoperiod-sensitive for fruiting. Run 14–16 hours. Some June-bearing varieties do require short days to initiate fruiting - check the variety before purchasing.
Cucumbers
Day-neutral; flower based on node count and plant age. Run 16–18 hours.
Using DLI to work with the light you have
If your light is lower intensity than ideal, you can partially compensate by extending the photoperiod to increase DLI (Daily Light Integral = the total daily photon dose). This works up to the photoperiod limits of each crop.
Example: You have a light delivering 200 µmol/m²/s. For lettuce needing DLI of 15 mol/m²/day:
- At 16 hours: DLI = 200 × 16 × 0.0036 = 11.5 mol/m²/day (below target)
- At 20 hours: DLI = 200 × 20 × 0.0036 = 14.4 mol/m²/day (close to target)
- But: 20 hours may cause bolting issues in some crops
The right long-term answer for insufficient DLI is a brighter light. Extending photoperiod indefinitely has crop-specific limits and increasing risks for long-day bolting crops.
Quick reference: recommended photoperiods by crop
| Crop | Recommended hours | Max before stress | |---|---|---| | Lettuce | 16 h | 18 h (at cool temps) | | Spinach | 14 h | 14 h | | Cilantro | 12–14 h | 14 h | | Kale, chard | 16 h | 18 h | | Basil | 16–18 h | 18 h | | Mint | 16 h | 18 h | | Tomatoes | 16–18 h | 18 h | | Peppers | 16–18 h | 18 h | | Strawberries | 14–16 h | 18 h | | Microgreens | 12–16 h | 18 h |
This guide is reviewed against published horticultural research. It is not a substitute for advice from a certified horticulturalist.