Grow lights

Lamp types and LEDs

• For small-scale growers, UMN lists LED and fluorescent lamps as the most common options. Its comparison also includes incandescent bulbs and high-pressure sodium (HPS). ^[1]
• UMN describes LEDs as energy-efficient, long-lasting and wide-spectrum, without producing too much heat, but with a higher up-front cost. Fluorescent lamps cost less to buy but use more energy and do not last as long. HPS emits substantial light but also a lot of heat and is better suited to large-scale systems. ^[1]
• Missouri Extension: HPS has the lowest initial cost but is the least energy-efficient and has the shortest life. LEDs cost more at first but are the most efficient and last longest. Fluorescent lamps sit in between. HPS efficiency is about 0.9–1.7 µmol/J; current LEDs are over 2.5 µmol/J. ^[2][3]
• Oklahoma State (HLA-6450): LED advantages include energy efficiency, durability, a long life (up to 50,000 hours, dimming to about 70 percent) and low heat from the surface. Drawbacks include higher initial cost, weight on some fixtures, and less coverage area. Heat leaves through a heat sink, so the lamp can sit close to the plants. ^[4]
• OSU distinguishes three LED structures: lead-wire, surface-mounted and high-power LED. Missouri advises buying a fixture whose manufacturer provides a spectrum graph and PPF or PPFD data. Do not choose a grow light by lumens, lux or Kelvin: those describe light for human eyes, not the PAR plants use. ^[4][2]
• UMN: a balance of blue and red light is best for growth; blue alone can be enough if you only grow leafy greens. Red light helps plants form thicker stems needed for flowers or fruit. Bulbs marketed as white or full spectrum produce both. Cornell’s home hydroponics guide: choose an LED mainly for efficiency (PAR per unit of electricity), not marketed spectra; in general plants grow well under light similar to the sun. ^[1][5]
• In stacked or close-canopy systems Missouri would not use HPS, because radiant heat can burn the plants. LED and fluorescent lamps can be placed closer. ^[2]

PPFD – light intensity at the canopy

• PPFD is how much PAR reaches a square metre each second (µmol/m²/s). It depends on the fixture and the distance: it is usually higher when the lamp is closer to the plants. ^[2]
• Missouri Extension says lettuce needs a PPFD between 250 and 350 µmol/m²/s. Above 350 µmol/m²/s the plants may still look healthy, but the extra energy does little to raise yield or shorten time to harvest, and excessive light can cause photoinhibition (photosynthesis is reduced). ^[2]
• Oklahoma State gives a general recommendation of 400–800 µmol/m²/s “for improved plant growth”. That is a general range, not the lettuce target; for lettuce use Missouri Extension’s 250–350 µmol/m²/s. ^[4][2]
• Missouri: avoid lights if the manufacturer does not provide a spectrum graph and PPF or PPFD figures. A PPFD map at different heights, or a quantum sensor, helps you check the value at the canopy. ^[2]

DLI – daily light integral

DLI is the PAR accumulated over a day, in mol/m²/day. Virginia Cooperative Extension (SPES-720, Table 3) gives the example ranges below. Fruiting crops need more light than leafy crops. ^[6]

• DLI is calculated from PPFD and hours: DLI = PPFD × hours × 3,600 / 1,000,000. Virginia Extension’s example: 200 µmol/m²/s for 16 hours gives a DLI of 12 mol/m²/day. ^[6]
• Cornell’s home hydroponics guide generally wants 12–17 mol/m²/day for leafy greens, which matches Virginia Extension’s lettuce range (12–17). Too much light often shows up as tip burn. ^[5][6]
• Oklahoma State gives a rough split: DLI 5–10 for low-light plants, 10–20 medium, 20–30 high and 30–50 very high light. Species needs vary; use the Virginia table below when the crop is listed. ^[4]
• If plants look stretched and thin, they need more light (higher intensity or a longer period). Both Cornell and UNH Extension give this advice. ^[5][7]

Photoperiod by plant type

• Photoperiod is the part of the day when plants receive light. Missouri Extension says hydroponic plants need about 12–16 hours of light per day. Light sensors and timers can improve energy efficiency. ^[3]
• UNH Extension advises running lamps for 14–18 hours per day and adjusting as needed. In winter, UNH says a windowsill does not provide enough light even for lettuce at a reasonable rate, and tomato, pepper, eggplant and cucumber need supplemental lighting. ^[7]
• UMN’s example runs a small 9-watt LED for 14 hours a day. That is an example, not a general rule for every crop. ^[1]
• Illinois Extension notes that fruiting plants such as tomatoes and peppers need 8–10 hours of sunlight a day, and that most food crops indoors need artificial light. That 8–10 hours refers to sunlight, not to how long lamps should run. ^[8]