Grow Light

Grow lights are artificial light sources that provide plants with the wavelengths and intensity needed for photosynthesis. The relevant photosynthetic spectrum is roughly 400-700 nanometers (PAR - photosynthetically active radiation), peaking in the blue (450-500nm) and red (620-700nm) wavelengths. Green wavelengths are reflected rather than absorbed, which is why plants appear green.

For seed starting under lights, the goal is providing enough light to produce compact, stocky seedlings rather than the elongated, pale plants that result from window-sill germination in low light. For full indoor production of fruiting crops, the light requirements are substantially higher.

LED vs. Fluorescent vs. HID

Fluorescent T5 and T8 fixtures: Traditional standard for home seed starting. Inexpensive ($30-80 for a 4-foot shop light), widely available, adequate for seedlings and leafy greens at close range (2-4 inches from canopy). Not powerful enough for fruiting crops. The technology is mature and reliable; T5 fixtures provide more light per watt than T8.

LED grow lights: The current standard for serious indoor growing. Modern full-spectrum LEDs (using white phosphor diodes or mixed red/blue diode arrays) are more efficient than fluorescent, last longer (50,000+ hours), and produce less heat per unit of light output. Quality varies enormously. Look for PPFD measurements at specific heights rather than wattage claims; wattage is a meaningless quality indicator.

HID (high-intensity discharge) - HPS and MH: High-pressure sodium and metal halide lights. Very high light output; the traditional standard for commercial production. Generate substantial heat requiring ventilation. Largely supplanted by LED in new installations due to LED efficiency improvements.

Light Quality Metrics

PPFD (Photosynthetic Photon Flux Density): Micromoles of PAR photons per square meter per second (µmol/m²/s). This is the instantaneous measure of light intensity reaching the plant. A seedling starting setup might target 200-400 µmol/m²/s; fruiting crops need 600-1,000+ µmol/m²/s.

DLI (Daily Light Integral): Total PAR photons delivered per square meter per day (mol/m²/day). DLI integrates PPFD over the photoperiod. This is the most useful metric for plant production.

Crop type	Recommended DLI
Seedlings (vegetable transplants)	12-20 mol/m²/day
Leafy greens (lettuce, spinach, herbs)	12-17 mol/m²/day
Fruiting crops (tomato, pepper)	20-30 mol/m²/day

To calculate DLI from PPFD: DLI = PPFD × photoperiod (hours) × 3.6 / 1000

Example: 400 µmol/m²/s × 16 hours × 3.6 / 1000 = 23 mol/m²/day. This is adequate for seedlings and leafy greens but at the lower end for fruiting crops.

Seedling Starting Setup

The most common home use: a shop light or dedicated seedling fixture on an adjustable chain, kept 2-4 inches above the seedling canopy, on a timer for 14-16 hours per day. Raise the fixture as seedlings grow to maintain distance.

A 4-foot T5 shop light costs $30-60 and provides adequate light for one 1020 seedling tray directly beneath it. A 4-foot LED fixture (quantum board style, ~200W equivalent) costs $80-200 and covers 2-4 trays with better efficiency and lower heat output.

Spectrum

Plants use red (660nm) for flowering and stem extension and blue (440-490nm) for compact vegetative growth. Early LED grow lights were purple/blurple arrays of specific red and blue diodes; these work but are unpleasant to work under and make plant color diagnosis difficult. Modern “full spectrum” or “white” LEDs add green and white phosphor, making the light appear white or slightly warm-white while maintaining adequate red and blue content. Full-spectrum LEDs are preferable for environments where you spend time.

Heat Management

All grow lights produce some heat. In a small seedling setup, this may be negligible. In a closet or tent with multiple fixtures, heat buildup requires ventilation. LED fixtures generate significantly less heat per unit of light output than HID; this is a meaningful practical advantage in small indoor spaces.