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AGR-HORT49-05 · Lesson 3 · 8 clock hours · theory

Supplemental Lighting: HPS, LED, and Photoperiod Control

Objective. Students distinguish between photosynthetic (DLI-based grow lighting) and photoperiodic (day-length control) lighting strategies and calculate daily light integral (DLI) for a given crop and location.

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This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Why Greenhouse Lighting Matters

Greenhouse crops depend on light for two distinct biological processes: photosynthesis and photoperiodism. Confusing these two processes — or using the wrong lighting strategy for each — leads to crops that grow slowly, fail to flower on schedule, or miss market dates. This lesson covers both strategies and gives you the math tools to design a supplemental lighting program.

Photosynthetic Lighting and Daily Light Integral (DLI)

Photosynthesis requires a threshold amount of light energy delivered across the whole day. Researchers express this as the daily light integral (DLI), measured in moles of photons per square meter per day (mol·m⁻²·d⁻¹). Every crop has a published target DLI range; tomatoes, for example, thrive at high DLI values, while foliage crops tolerate much lower levels.

Supplemental lighting is added when natural sunlight cannot supply the target DLI — common in northern greenhouses from October through March. The light source intensity is measured as photosynthetic photon flux density (PPFD), reported in micromoles of photons per square meter per second (µmol·m⁻²·s⁻¹).

The conversion formula is:

DLI = PPFD × hours × 0.0036

The constant 0.0036 converts seconds-based PPFD readings into the molar, hour-based DLI unit. Rearranging the formula lets you solve for any unknown: if you know the target DLI and your fixture's PPFD, you can calculate exactly how many hours of supplemental light are needed per day.

HPS vs. LED Fixtures

High-pressure sodium (HPS) lamps have been the greenhouse standard for decades. They produce an amber-orange spectrum efficient for photosynthesis and are relatively inexpensive to purchase. Their drawbacks include substantial heat output (which can stress crops placed too close to the lamp), shorter lamp life compared to LED, and lower efficacy — the ratio of light output to electrical input, expressed in µmol·J⁻¹.

Full-spectrum LED fixtures have rapidly improved. Modern horticultural LEDs offer higher efficacy, much longer rated lamp life, and cooler operating temperatures. Lower heat output allows fixtures to be positioned closer to the canopy, delivering PPFD more efficiently. Initial purchase cost for LED is typically higher than HPS, though operating-cost savings over the fixture's life often offset this. Always check the manufacturer's published efficacy (µmol·J⁻¹) and PPFD distribution maps when comparing fixtures, because marketing claims vary widely.

Safety note: Both HPS and LED fixtures operate at line voltage (120 V or 240 V). Always de-energize the circuit and apply lockout/tagout (LOTO) procedures before installing, adjusting, or replacing any fixture or lamp. Wear safety glasses when handling HPS lamps — these lamps operate under high internal pressure and can shatter if touched with bare hands or dropped. Use gloves when handling HPS outer envelopes to prevent skin oils from creating hot spots.

Photoperiodic Lighting: Day-Length Control

Photoperiodism is a plant's response to the length of uninterrupted darkness, not total light energy. Plants are classified by their flowering response:

Growers manipulate photoperiod using two tools:

1. Night-break (day-extension) lighting — low-intensity light (as little as 2–10 µmol·m⁻²·s⁻¹) applied during the dark period interrupts the long night and prevents SDP flowering, or promotes LDP flowering. Incandescent bulbs were traditional for this; LED red/far-red lamps are now common.

2. Black cloth — opaque fabric pulled over benches or hoops to create artificial long nights for SDPs when natural days are too long (summer production of poinsettias, for example).

The timing of these treatments must be calculated backward from the target market date, using each crop's known days-to-flower under controlled photoperiod.

Safety note: Black cloth traps heat rapidly in warm weather. Check air temperature under cloth frequently to prevent heat stress or fire hazard near heat sources. Store cloth when not in use to prevent mold growth.

Putting It Together

A well-designed greenhouse lighting program separates the two goals: high-intensity, timed supplemental lighting (HPS or LED) meets the photosynthetic DLI target, while low-intensity night-break or black-cloth schedules control flowering time. Neither strategy substitutes for the other. Using DLI math and accurate photoperiod schedules allows a grower to hit market windows reliably and control energy costs.

Key terms8

Daily Light Integral (DLI)The total amount of photosynthetically active light received by a surface over one day, expressed in mol·m⁻²·d⁻¹.
Photosynthetic Photon Flux Density (PPFD)The instantaneous rate of photosynthetically active photons striking a surface, measured in µmol·m⁻²·s⁻¹.
EfficacyA measure of how efficiently a lighting fixture converts electrical energy into photosynthetically useful light, expressed in µmol·J⁻¹.
Short-Day Plant (SDP)A plant that flowers when the uninterrupted dark period exceeds its critical night length; examples include poinsettia and chrysanthemum.
Long-Day Plant (LDP)A plant that flowers when the night period is shorter than its critical night length; examples include petunia and pansy.
Night-Break LightingLow-intensity light applied during the middle of the dark period to interrupt the long night, preventing SDP flowering or promoting LDP flowering.
PhotoperiodismA plant's physiological response to the length of the daily dark period, which governs flowering time in many greenhouse crops.
Black ClothOpaque fabric used to cover plants during natural daylight hours to create artificially long nights and induce flowering in short-day plants.

Worked examples4

1. A grower raises lettuce with a target DLI of 17 mol·m⁻²·d⁻¹. In January, natural light provides an average DLI of 5 mol·m⁻²·d⁻¹. The supplemental LED fixture delivers a PPFD of 200 µmol·m⁻²·s⁻¹ at canopy level. How many hours of supplemental lighting are needed each day?
  1. Step 1 — Find the DLI deficit: 17 − 5 = 12 mol·m⁻²·d⁻¹ must come from supplemental lighting.
  2. Step 2 — Rearrange the formula for hours: hours = DLI ÷ (PPFD × 0.0036).
  3. Step 3 — Substitute values: hours = 12 ÷ (200 × 0.0036) = 12 ÷ 0.72.
  4. Step 4 — Calculate: 12 ÷ 0.72 = 16.67 hours. Round to 16.7 hours per day.
Answer: Approximately 16.7 hours of supplemental LED lighting per day are needed.
2. In March, natural light provides a DLI of 10 mol·m⁻²·d⁻¹ in the same greenhouse. Using the same 200 µmol·m⁻²·s⁻¹ LED fixture, how many hours of supplemental light are now required to reach the 17 mol·m⁻²·d⁻¹ target?
  1. Step 1 — Find the DLI deficit: 17 − 10 = 7 mol·m⁻²·d⁻¹.
  2. Step 2 — Rearrange: hours = DLI deficit ÷ (PPFD × 0.0036).
  3. Step 3 — Substitute: hours = 7 ÷ (200 × 0.0036) = 7 ÷ 0.72.
  4. Step 4 — Calculate: 7 ÷ 0.72 ≈ 9.72 hours.
Answer: Approximately 9.7 hours of supplemental lighting per day are needed in March, significantly less than in January.
3. A grower runs an HPS fixture at 400 µmol·m⁻²·s⁻¹ for 6 hours each day. What DLI does the supplemental HPS lighting contribute?
  1. Step 1 — Apply the formula directly: DLI = PPFD × hours × 0.0036.
  2. Step 2 — Substitute values: DLI = 400 × 6 × 0.0036.
  3. Step 3 — Calculate: 400 × 6 = 2400; 2400 × 0.0036 = 8.64 mol·m⁻²·d⁻¹.
Answer: The HPS fixture contributes 8.64 mol·m⁻²·d⁻¹ to the daily light integral.
4. A grower needs poinsettias to be in full flower by November 1. Poinsettias are a short-day plant that typically require about 8 weeks of short-day treatment (nights longer than roughly 12 hours) to reach flowering. Working backward, on what date should the grower begin applying black-cloth short-day treatment?
  1. Step 1 — Identify the target date: November 1.
  2. Step 2 — Count back 8 weeks (56 days) from November 1.
  3. Step 3 — November 1 minus 56 days: October has 31 days, so go back to October 1 (30 days back), then back 26 more days into September: September 1 + (30 − 26) = September 5. More carefully: 56 days before November 1 = 31 days brings us to October 1, then 25 more days back = September 6.
  4. Step 4 — Confirm: September 6 + 56 days = October 6 + 26 days = November 1. The treatment start date is approximately September 6.
Answer: Black-cloth short-day treatment should begin on approximately September 6 to achieve flowering by November 1.

The math in this lesson1

Learners rearrange DLI = PPFD × 0.0036 × hours to find daily supplemental lighting hours.
A-CED.4
Brush up in the math halls at newSchoolWorks.com:
In class: DLI calculation practice: students are given average natural light values (mol·m⁻²·d⁻¹) for two seasons and a crop DLI requirement; they calculate how many hours of supplemental HPS or LED lighting are needed to meet the target, using the formula DLI = PPFD × hours × 0.0036. · Lamp comparison chart: students complete a structured comparison table for high-pressure sodium (HPS) and full-spectrum LED fixtures covering efficacy (μmol·J⁻¹), heat output, lamp life, and initial cost, using manufacturer spec sheets provided by the instructor. · Photoperiod control scenario: given a list of five crops (poinsettia, chrysanthemum, petunia, pansy, kalanchoe), students classify each as short-day, long-day, or day-neutral and write the lighting or black-cloth schedule needed to achieve flowering for two crops on a specified date.
Materials: PPFD/DLI calculation worksheet, HPS and LED spec sheets, photoperiod classification reference (e.g., Greenhouse Crop Production textbook), calculator, black cloth sample
Check: Students complete DLI calculations with no mathematical errors and classify all five crops correctly; pass mark is 80% on a combined 10-question quiz covering DLI math and photoperiod classification.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Greenhouse Production and Environment Management
1. A greenhouse grower uses the formula DLI = PPFD × hours × 0.0036. If an LED fixture delivers 250 µmol·m⁻²·s⁻¹ and runs for 8 hours, what supplemental DLI does it provide?
Greenhouse Production and Environment Management
2. Which of the following best describes the unit µmol·J⁻¹ as applied to greenhouse lighting fixtures?
Greenhouse Production and Environment Management
3. A student needs to classify poinsettia for its photoperiod response. Which classification is correct?
Greenhouse Production and Environment Management
4. Natural light provides a DLI of 8 mol·m⁻²·d⁻¹ in February. A crop requires a total DLI of 20 mol·m⁻²·d⁻¹. How many hours of supplemental lighting at 300 µmol·m⁻²·s⁻¹ PPFD are needed?
Greenhouse Production and Environment Management
5. Which crop from the list below is correctly classified as a long-day plant?
Workplace Safety
6. Why is lockout/tagout (LOTO) required before installing or replacing an HPS greenhouse lamp?
Greenhouse Production and Environment Management
7. A grower wants to delay flowering of chrysanthemums (a short-day plant) during summer. Which technique is most appropriate?
Greenhouse Production and Environment Management
8. Compared to HPS fixtures of similar wattage, modern horticultural LED fixtures typically offer which combination of characteristics?
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