Grow Lights For Seed Starting– Definitions


If you’re starting seeds inside your house or greenhouse and live in the northern part of the United States, you will need a grow light to be successful.  A sunny, south facing window will not be enough light alone (but will reduce the overall amount of light you need to provide).  Grow lights are an incredibly valuable tool and, although expensive initially, will save you money in the long run if you plan to continue starting seeds indoors.


Shopping for grow lights can be very intimidating.   You may have noticed that there are different types of grow lights:T5, LED, Shop Lights and even HID fixtures.  There is also a lot of different vocabulary used in the grow light industry that can be confusing.  Let’s break down the vocabulary and talk about how much light seedlings need so that you can more easily understand what type of grow light to buy.


Important Vocabulary:

  • Watt: The standard unit of power (energy per unit time).  This is what you see on your electrical bill.   


  • Lumen: A measure of the quantity of visible light that a source emits.  This is a measure of visible light that humans can see and not necessarily how much light a plant will be able to use.  


  • Luminous Flux: Luminous flux is the measure of how much light energy is emitted per unit of time in all directions.  It is measured in lumens.


  • PPF: PPF stands for photosynthetic photon flux.  It measures the total amount of light wavelengths between 400-700 nm.  These are the wavelengths that plants use in photosynthesis.  PPF is expressed as micromoles per second (μmol/s).  


  • PPFD: PPFD stands for photon flux density.  It is the same as PPF but takes surface area into account as well.  PPFD is expressed as micromoles per meter squared per second.  (µmol/m2/s) 


  • PAR:  Photosynthetic Active Radiation (PAR) is the measure of light wavelengths between 400-700 nm.  These are the wavelengths that plants use in photosynthesis.  PAR is what sets grow lights apart from other light sources because it indicated how much light the bulb is producing that the plant can use for growth (photosynthesis). 


  • Spectrum: Spectrum describes the “colors” of light that are emitted from the light.  This is stated in the SI base unit of Kelvin (K). Lower Kelvin values (3500K and below) indicate a “red” light that is typically used for flowering.  A higher Kelvin value (4000K and above) indicates a “blue” spectrum that is typically used for starting seeds and vegetative growth.  Some fixtures may be “Full Spectrum” and contain optimal levels of blue and red light.  


  • HID: HID stands for High Intensity Discharge.  These types of lights are most commonly used in commercial horticulture or for plants that require a higher light intensity than T5 lights can supply.  Seed starters may find these fixtures advantageous as they could be hung farther away from the plants and cover a greater square footage.  These type of fixtures will be more expensive than T5 fixtures.


  • Fluorescent: A fluorescent light bulb uses a low pressure Mercury vapor to produce visible light.  There are several different types of fluorescent lights, some are suitable for growing plants while others are meant for lighting in your garage.  Fluorescent bulbs should be replaced every growing season as they depreciate in efficiency over time.  
      • T5: The most efficient “T” type fluorescent.  The tubes are ⅝ inch in diameter and are most commonly used as grow lights.
      • T8: An older technology that is often used for both seed starting and shop lighting.  The diameter of the bulb is 1 inch.  
      • T12: The least efficient “T” type fluorescent.  The tubes are 1.5 inches in diameter and are an older technology.  These are typically used as shop lights.

  • LED: LED stands for ‘Light Emitting Diode.’ LED grow lights are the most energy efficient type of grow lights.  They use less wattage to provide the same (or more) PPFD.  LED’s tend to be the most expensive option but often come with a 5 year warranty and no mercury bulbs to change and dispose of.


How Much Light Do Plants Need?

You might be thinking, okay cut to the chase! How much light do seedlings need?


  • If you’re going to use a T5 fluorescent light, 30 watts per square foot of growing space is needed.


  • If you’re using an LED light, 2000-3000 lumens per square foot of growing space is needed.


  • Look for a “full spectrum” or “vegetative spectrum.”  These lights will typically be near 6400K.


I hope that these definitions and light requirements help you to choose a fixture that falls within your budget.  Come back next week for more information on seedling grow lights!

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