Grow Lighting FAQ

grow lighting

Grow Lighting FAQ

Crop Growth Lighting Fundamentals Explained

Grow lighting can be one of the more mystifying aspects of starting up and operating an indoor garden, urban farm or greenhouse with supplemental crop lighting, so we present grow lighting FAQ to help. 

There are lots of choices for grow lights available to growers—as well as hype, whether from their local hydroponics store or preferred online vendor.  The following is intended to help sort fact from fiction, from the new grower to the master grower, better understand the principles and truths behind the type of crop lighting they are selecting from or cropping with already in order to maximize their gardening return on investment.

What can further confuse matters is the fact that there are very different technologies that can deliver good lighting for growth, fruits and flowers—a whole cart of different fruits, if you will ie Apples versus Oranges. 

So, let’s get started with grow lighting FAQ, shall we?

Grow Lighting FAQ #1 Different Types of Grow Lights

What are the most important differences between HPS (high pressure sodium), MH (metal halide), CMH/LEC (ceramic metal halide)/(light emitting ceramic), fluorescent and LED (light emitting diode) crop lighting?


Tried, tested and true. HPS grow light systems are inexpensive to buy relative to the volume of light energy they produce, however, they are not the most energy efficient, ie the quality of light is not the highest—the sheer output helps make up for any light quality deficiencies, although the compensation is at the expense of the grower with power bills. Most often available as DE (double ended) lamp types (increased efficiency and longevity) in higher wattages, lower wattages are most commonly supplied as single ended lamps (traditional).

TIP: the higher percentage of red and orange in the HPS lighting spectrum is best reserved for flowering and fruiting plants.

Above: Installing Revolution Micro Aerospace Grade DE HPS Lighting Systems in a Commercial Grow Facility


Shares many of the characteristics of HPS lighting although MH lamp types tend to produce a higher quality of light, while not having quite as high an output in terms of sheer intensity per watt of power consumption.  Most growers who produce leafy greens, herbs, etc will prefer MH lighting over HPS grow lighting because the higher percentage of blue in the MH light spectrum  produces nicer leafy or  vegetative growth versus HPS.

Above: MH 10K “Finisher Lamp” for DE Lighting Systems


While still considered a HID (high intensity discharge) source of crop lighting like HPS or MH, CMH grow lights consume considerably less power relative to the amount of crop yield they can produce and also offer other advantages by way of potentially increasing crop quality because of the extremely high light rendering quality for crop growth they generate. A 315 watt CMH grow light is often said to produce the equivalent yield to a 600 watt or 750 watt HPS grow light system, however the trade off is that the initial purchase price per light can be higher—and you will need more fixtures to cover the same sized room versus a 1000 watt DE HPS grow light system.

Learn MORE About CMH Grow Lighting | HERE |

Above: (left) Summit-Lighting 315 watt CMH, (right) 320 watt dual element CMH lighting system


T8 or T12 type fluorescent plant growth light tubes and fixtures have been around for a long time and still hold their place for indoor gardeners.  Fluorescent fixtures and quality lamps can produce excellent light quality for a wide variety of plant types and growth phases because you can mix and match different light rendering colour fluorescent tubes, for example a 6K rating is very “blue” (for growth) while a 3K rating has considerably more “red” (for flowers and fruits).  While they sound ideal, the limitations is that they do not throw light with strong intensity for any type of distance.  A great choice for propagating or cultivating plants that will not be taller than a foot.

Above: Flourescent Lights are ideal for plant propagation or smaller light loving plants


LED grow lighting can be astoundingly efficient and it can also be a waste of money, depending on specifically which LED grow light system you opt for. The reason for huge differences in the results of LED grow lights is the fact that they are not nearly as standardized as HPS, MH, CMH/LEC or Fluorescent technologies.  The differences vary widely from manufacturer to manufacturer—depending on the drivers, diodes, reflector and blend of different spectrum LED chips in the fixture.  If well researched, the crop quality and yield relative to electrical consumption of the fixture makes them a worthwhile choice for growers.  Spend extra time and diligence before making an LED grow light purchase—they aren’t cheap!

AVici LED grow light

Learn MORE About LED Grow Lighting | HERE |

Grow Lighting FAQ #2 Claims of Higher Outputs or Brighter Light Over Others

One HPS grow light company says their 1000 watt HPS DE model will outperform all others by up to 10%—is that likely to be true?

While naturally, it depends on who is saying it, in the end it’s not likely to be true.  That’s because even at the very top end of high quality HID grow light manufacturing there can be 5 to 10% difference from lamp to lamp, even within the same batch marked box go grow lamps.  This being the case, and not even accounting for the reflector, ballast and suggested area of coverage these sorts of claims should be regarded as vaguely possible.

However, that said, there ARE differences in the AVERAGE light output one brand may produce over another, as well as how reliable and safe they are to operate—so it can be true that one manufacturer produces a better grow light system than an other.

Click HERE to learn more  about deceptive light rating practices

Grow Lighting FAQ #3 Do I REALLY Need More Than One Type of Light

There are recommendations for using different types of lamps or light sources for different growth phases.  Is there any validity to this, or do people just want me to buy more gear or is there one type of lighting that can do it all?

Most growth lights will allow you to grow a crop from start to finish with just one fixture and light type. However, if you want to factor in things like plant health (which influences potential insects or other problems), harvest weight, crop quality, fruit or flower density, essential oil content, time required for cropping cycles, etc.  This makes fixtures that can allow you change light colour (Kelvin  ratings) by switching various lamps for different phases a good choice.  For an even higher level of versatility, some lamps can have two different lighting elements in the same glass jacket click HERE.

General Rules: blue for leaf and stem growth, more red for flower and fruit production and increased UV spectrum for finishing crops with higher yields of essential oils.

Grow Lighting FAQ #4 Grow Light Safety

How safe are grow lights to operate?

Always make sure that the grow light you are selecting carries an appropriate rating for the application, for example CSA or UL approved is a must.  In some instances, for example for use in a greenhouse, it should also be suitable for use in damp locations.

From there, it’s just like the stove in your house—if used as directed by the manufacturer and appropriate certification with care and common sense it’s a very useful thing to have in your home.  Used without caution or care, accidents or injury can occur.


-never overload circuits

-avoid any cords on floors where moisture may occur

-hang lamps securely and at safe distances from ceilings or walls

-always wear eye protection when working around grow lights that are operating

-having any additional wiring that may be needed performed by a qualified person

Grow Lighting FAQ #5 How Much Light Do Plants Need

How much light do I need to give my plants, what are ways I can save on my purchase and operating expenses?

In most types of plants, there is a very direct relationship between the amount of light energy plants can receive to the final weight and quality of the harvest they produce.  Keep in mind some types of plants prefer slightly lower levels, ie shade plants while most of the veggies, greens, flowers, fruits and herbs we enjoy at hoe do prefer higher light levels.

An inexpensive light meter can help you get the most out of your grow light—without one, you are only guessing what levels are making your plants happiest.  However, light meters don’t tell you the whole truth and should be taken as a point of reference and certainly not an absolute.  For example, a light meter can tell you that an LED flashlight from a few inches away is going to produce more light for photosynthesis than full sun during the afternoon.  It’s about how you use the light meter.

For high light level plants, 800-1000 uMols is a good range for flowering and fruiting or if using a lumens/lux meter would read 5000-7500 from most light sources.  Again, we can’t stress enough these are tools for reference and not absolutes—beware of claims.

When you think of saving on your purchase, think in terms of a year or longer—not just the day you buy it.  For the 100 bucks you save that day on buying a fixture, you can spend considerably more money than that on electricity running it for a year, so efficiency does count in your actual bottom line. Also consider lamp replacement frequency recommendations—how often will you need to replace the lamp to keep getting lots of light? Many lamps decrease output significantly after a year of use while still drawing the same level of electricity.

Above: A Light Meter can help you understand and duplicate results from your crop lighting practices

Grow Lighting SAQ (SHOULD Ask Question)

What about Green Light, there’s a lot of conflicting information out there?

It IS true that true green light will not interrupt the plants critical dark cycle, for example having pure green work lights in a grow room or black out greenhouse that allow you to work in the garden while the crop “sleeps”.  However, a coated incandescent or fluorescent light is not a true green lamp—these lamp types emit red and far red wave lengths that DO interrupt the critical dark cycle, and green coatings do not filter this out.  True green LEDs are your best choice for grow room night lights.

SUPER PRO TIP: Green light also stimulates increased insect activity, ie they scurry around more, so if treating your crop for pests, run your green lights for several hours before any applications to plants because you will hit more of your target pests which makes your treatments more effective per application.

About Erik Biksa 233 Articles
Erik Biksa has been writing about and discussing hydroponics growing, related technologies and cropping methods since 1999 in a variety of professional publications and platforms globally Erik has travelled the world learning and teaching modern growing techniques and technologies and is appreciated by many growers for his informative yet hands on approaches. Presently, he is the Editor at Grozine Hydroponics Mag.

Be the first to comment

Leave a Reply

Your email address will not be published.