11 Core Guidelines To Know Before Buying Grow Lights

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Grow lights are available in a wide range of prices, sizes, and styles.

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11 Core guidelines to know before buying grow lights for urban farming

Now: 

For someone just starting out with grow lights, navigating the process to buy the correct light is not always a simple task.

This post will tell you everything you need to know about grow lights before making a buying decision.

Below is the table of contents to navigate this post. If you would like to download the pdf copy of this post (10,000+ words), click here.

Introduction A: Basic Grow Lights Comparison

In this section, you will learn the basic differences between the primary types of grow lights.

Less Technical Basic Comparison of Different Grow Lights Source

If you are looking for a fast and easy comparison of: 

- different grow lights
- their life span / day rated life
- color temperature (K) 
- approximate appearance
- pros and cons

You can simply refer to the diagram (above).

For a complete breakdown of all factors related to grow lights and your buying decision, keep reading past the diagram below: 

Introduction B: What (exactly) are grow lights? 

Next, lets answer a set of basic questions: 

What are the main types of grow lights? 
How do LED grow lights work? 
How do HID grow lights work? 
How do CFL grow lights work? 

Read below to find out: 

Illustration of the main types of grow lights for indoor growing Source

There are 3 main types of grow lights: 

- HID (High-Intensity Discharge): Includes Metal Halide and High Pressure Sodium
- CFL: Compact Fluorescent (Includes T5, T8, etc) 
- LED: Light Emitting Diode

How Does an HID Grow Light Work? Source

How do HID grow lights work? 

These types of lights "produce light by passing an electric current between two tungsten electrodes inside a glass tube containing gases and metal salts".

This set up powers some of the most powerful lights on the market right now.

To get a basic visual understanding of how a high intensity discharge lamp works, see the diagram above.

How do CFL grow lights work Source

How do CFL grow lights work? 

Some of the concepts relating to how CFL bulbs work are similar to HID bulbs, but with some key differences.

For a visual diagram, see the image (above).

How an LED Grow Light Works Source

How do LED grow lights work? 

Current passes through N and P type alloys, and the movement of electrons across the NP gap generates light.

To see exactly how this works, see the diagram above.

Introduction C: How Much Do Grow Lights Cost? 

For a basic breakdown of the cost of different grow lights, see the diagram below that shows the costs of grow lights both initially and over time: 

Cost Analysis of Different Grow Lights Source

Introduction D: Recommended Grow Lights To Buy

Here are some grow lights that we recommend buying based off of our extensive research, experience, and feedback from our group of over 5,000 grow light enthusiasts and users.

Introduction D2: Recommended Grow Light Accessories

Sometimes grow lights will come with all the accessories needed but many times you must buy the accessories separately. Here are some highly rated and reviewed grow light accessories: 

- suspension cables (get here
- power strips and extension cords (get here
- grow tent (get here
- protective glasses / eye-wear for grow lights (get here

Quick Links To Remaining Chapters

Now, we have covered the basics (intro sections above), it's time to go in-depth.

Following you will find some the most in-depth guide for learning different aspects of grow lights on the internet.

Yes, we said it! 

Skip to: 

1. Distance Between Grow Lights and Plants (Space Available)
2. Heat and Grow Lights
3. Know The Key Grow Light Specs: Lumens, Lux, PAR, and Watts
4. Budget
5. Vendor Quality
6. Common Grow Light Accessories
7. Know the Unit Economics (cost to light per unit area)
8. Scalability
9. Safety
10. Purpose
11. Miscellaneous Helpful Notes & FAQs on Grow Lights

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1. Distance Between Grow Lights and Plants (Space Available) 

LED PAR & Lux Chart vs Distance Source

You may be wondering: 

How far should my plants be from grow lights? 
What height should my LED grow lights be? 
Does PAR change for different grow light distances? 
Does lux change for different grow light distances? 
Do different plant stages require different grow light distances? 

All of these answers will be covered in high depth below: 

How Room Size Affects Wattage and Plant Distance Source


The truth to some of these questions, is yes! 

For others, the best answer without getting into specifics is that it depends.

There are many factors, the first of which is the amount of space you have available.

Lighting Distance Varies Depending on Grow Light Type, Which Change Space Requirements Source

 If you don't have the space to operate a certain type of grow light, you won't be able to use it.

PAR vs Distance Chart Source

The reality is that different grow light types will require different amounts of spacing in between the bulbs and fixture and the plants.

EPPFD vs Spacing Chart Source

If your plants are too close to the lighting, they will at the very least have lower yield and your plants may even be burned.

Different sources will give slightly different advice when it comes to how far grow lights should from plants. For example, lampsplus.com suggests: 

Lighting Distance Visualization Source

"Incandescent light bulbs should be at least 24 inches over your plants. Fluorescents and LEDs have a lower heat signature, so they can be placed 12 and 6 inches over plants respectively"

Lighting Distance Visualization Source

 When space is efficiently used with grow lights, vertical farming stacks can also be feasible by creating multiple layers of plant beds and grow lights.

To get an approximate idea of the space you will need vertically, see the tables below for LED (light emitting diode) and HPS (high pressure sodium) lighting.

At the end of the day, the spacing necessary will have a lot to do with the strength of the light - the stronger the light, the more space needed is a fair rule of thumb for urban farming beginners who don't want to over-complicate planning.

LED approximate lighting distances. Source
HPS / MH approximate lighting distances. Source

What does this involve? 

Once you have a general idea of what type AND strength of lights you are interested in buying, you can now use a grow light distance chart to optimize your grow light spacing even further.

Source

Grow light distance charts can also be referred to as PAR vs distance charts (see example below).

Sample PAR vs Distance Chart for fluorescent grow lights Source

Although it is beyond the scope of this article, once you know your grow light type and desired grow light spacing, you can increase your yield by changing grow light spacing at different stages of plant growth.

Grow light distance may vary depending on plant maturity for growers who want to maximize yield Source

For commercial growers trying to perfectly optimize yield, there is even grow light planning software that helps optimize layout of a grow room.

Professionals Use Grow Light Layout Planning Software To Maximize Yield Source

By now you should have a better idea of what grow light may be best for your needs, given the amount of space you have available.

With that, let's move onto guideline #2: heat emitted by grow lights, and how this affects your buying decision.

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2. Heat Generated By Grow Lights

Heat Output Can Vary Significantly Between Different Grow Lights Source

You may have wondered before: 

How much heat do grow lights generate? 
Do LED grow lights give off heat? 
Which grow lights have low heat? 
How do BTUs compare for different grow lights? 

To answer these questions, let's start with the basics: 

All lights generate heat.

Why?

As outlined in this Quora post, "Do all sources of Light Produce Heat?"

- There are 3 types of radiation or "light"

(1) infrared
(2) visible
(3) ultra-violet (UV) 

- What most people describe as "heat" is actually infrared  "non-visible" light
- The process of converting electrical energy into light generates both "visible" and "non-visible" infrared light (heat) 

Infrared, Visible, and Ultraviolet Regions of Electromagnetic Spectrum Source

So, what is this important? 

Simply because: the efficiency of a grow light to convert electrical energy into visible light results in less heat produced from the light.

Why is less heat good for a grow light?

There are at least 2 major reasons: 

1) Excess heat can burn plants and otherwise reduce yield.

2) Excess heat means less visible light output which your plants can use for every $ spent.

Now, how do different grow light heat outputs compare? 

How different grow lights compare across several areas, including power draw and efficiency Source

Heat From Fluorescent Grow Lights: 
CFL, T5, and T8 will typically have modest power draws and efficiency. Will generate more heat than an LED but less than HID lights.

Heat From HID (High Pressure Sodium / Metal Halide):
Will generate significant heat, may need to use with fans or other tools for ventilation. To get the power benefits of HID lights while reducing the heat creation, ceramic metal halide (CMH) grow lights offer less heat output but are more expensive than HPS or MH.

Heat From LEDs (Light Emitting Diodes):
Minimal heat, high efficiency. Heat output decreases typically as price & quality increases.

Quantifying Heat Generation:  How do BTUs compare for different grow lights? 

BTU's also known as British Thermal Units, is a thermal metric to help measure temperature in your growing space.

Example BTU Calculator Source

In recent years, BTU calculators (example above) have been popular tools used to make sure that the heat generated by your grow light does not cause your room to overheat.

With LED grow lights, heat generated will be less significant, however for HID grow lights, here are example values for BTUs generated for different strengths of grow light: 

250 W HID = 855 BTUs
400 W HID = 1368 BTUs
600 W HID = 2052 BTUs
1000 W HID = 3420 BTUs
1500 W HID = 5130 BTUs

Why are these numbers important? 

Knowing the exact quantity of heat generated by your grow light system will help you determine what type (if any) of heat reducing tools you need.

These may include tools like fans or pumps (image below).

Note: 

BTU Calculations are meant to be general guides. Factors such as insulation, ambient temperature outside, and ceiling height can affect the thermodynamics of any given indoor growing space.

In many cases, you will not know the best way to address over-heating from grow lights until you do some experimentation.

Example of fan use in an indoor growing environment Source

By now, you should have an understanding of how both...

(1) Spacing / space available, and
(2) heat generation

affect your decision when deciding to purchase a grow light.

Next, you will learn about key grow light technical specifications to look out for when looking at different types of grow lights.

Ready? Let's go! 

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3. What is your target grow light intensity, color, and day-length / rated life? 

Grow lights have 3 primary attributes you should consider:

1. Color
2. Lumens, Lux, and PAR Metrics
3. Day Length / Rated-Life

You will learn about each in depth below: 

LED Grow Light Color (Also Known as Spectrum or PAR Coverage) 

Spectrum Comparisons of LED (Top) vs (Below) Leading High Pressure Sodium and Metal Halide Light Sources Source

Refresher:

Typically what humans refer to as "light" is actually only the visible area of the light spectrum (~400 - 750 nanometers).

This visible light spectra range covers most of the range of wavelengths needed to grow plants, known as the PAR or photosynthetically active radiation.

Photosynthetically Active Radiation Source

Certain color spectra within the PAR are better for growing plants than others.

"Red" and "Blue" light spectra are ideal for plant growth, with red being associated more with the flowering stage of plant growth and blue with the vegetative stage. color is measured by wavelength (in nanometers).

Plant Stages of Growth Source

One of the biggest advantages of LED lighting is that it has highly customizable wavelength capabilities without changing fixtures.

This results in the ability to custom-tailor the light regiment for a plant in order to create optimal qualities like flavor, texture, or even plant color.

Wavelength based on growth stage Source

Therefore, when evaluating an LED grow-light (or any grow light) for use, (as far as color spectrum goes) you should look for a grow light which offers: 

1. the complete PAR spectrum (400 nm - 700 nm), with an emphasis on the red and blue wavelength ranges.

2. Wavelength coverage in areas of the spectra that help you reach your growing goals (depending on your growing goals, you may want to focus on different wavelengths) 

Using Wavelength To Grow For Specific Plant Qualities Source

As seen above, each wavelength of light promotes specific effects on plant growth.

Within plant categories, these guidelines may change. For one example, see the wavelength / plant growth effect chart for basil below: 

Using Wavelength To Grow For Specific Plant Qualities Source

Next, we will cover: 

a) Lumens, Lux, PAR, and Watts Metrics
b) Day-length / Rated-life.

Lumens, Lux, and PAR Metrics

Bulb Type vs Brightness in Lumens Source
What are lumens? Why are lumens important? 

Lumens are defined as,  "a unit of measure describing the total quantity of light from the visible spectrum that is emitted from a source of light (in all directions)".

Lumens are important because they determine the maximum amount of light your plants can absorb. Without enough brightness (lumens), you will not be able to grow your plants as effectively as you like (if at all).

Lumen outputs for different types of grow lights Source
What is lux? Why is lux important? 

Lux is simply lumens per meter squared. It is important because the amount of light which reaches your plant canopy is a more useful metric to know than the amount of light which your grow light emits.

Visualizing 1 Lumen Source

Keep in mind however, that lux is a measure of light visible to humans, NOT plants. In order to get a better concept of light visible to plants, you must use PAR measurement instead of lux.

How is PAR Quantified? Why is this metric important? 

PAR photon irradiance (often abbreviated as just PAR) is a metric which gives allows the best understanding of the light which your plants are getting from your grow light.

The recommended units of measurement for PAR, µmol m-2s-1.

The most accurate instrument for measuring a light’s output is a spectroradiometer.

Spectroradiometer Example Source

Knowing spectroradiometer patterns will help you make a decision between different grow lights.

What you want to avoid, is irregular patterns of PAR photon irradiance (Fixture 3, below).

Grow Light Fixture Uniformity and Efficiency Source
What are watts? Why are watts important? 

A  watt is defined as a unit that shows the rate at which electrical power is used by the grow light.

Watts are important because grow light manufacturers will commonly display Wattage in their product specs as a proxy for the power capability of the light.

Keep in mind that with led grow lights, efficiency goes down as watts increase.

The light strength of an LED grow light is not a direct function of Wattage.

It is influenced by the quality of components and design feature, including the particular LED chip used, chip density, optics used, heat sink, and the driver.

Parts of an led light chip become less efficient with higher wattage due to heat Source

In reality, most LED grow lights are less efficient as wattage goes up.

Why? 

Higher temperatures in the semiconductor circuit board containing the LEDs cause significant decrease in LED efficiency.

Despite this fact, the lumens per watt in an LED, regardless of the wattage, will still be higher than many other types of light bulbs (see image below).

Lightbulb Efficiency Comparison Chart Source

Day Length / Rated Life

‍A last important spec to consider is rated life of your lighting and how that coincides with not only your budget but your "day-length", or how long you are using your grow lights on a day to day basis.

While some types of grow lights have rated lifetimes of over 10 years, warranties on lights often will fall into the 3-5 year range. The durability of your light will be determined primarily by how you use it.

‍A last important spec to consider is rated life of your lighting and how that coincides with not only your budget but your "day-length", or how long you are using your grow lights on a day to day basis.

While some types of grow lights have rated lifetimes of over 10 years, warranties on lights often will fall into the 3-5 year range. The durability of your light will be determined primarily by how you use it.

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4. Budget

example cost breakdown for different TX fluorescent lighting. source

One of the most important metrics to consider when buying your first grow light is the price of the grow light.

The good news for grow lights is this: there are a lot of options at all price points.

$: CFL bulbs: can be found on amazon for less than 10 USD

$$: T5, T8, T12 fluorescent: good value for less than 100 USD. A 4 tube T5 grow light system can generate multiple hundreds of dollars worth of produce in a year (1-2 year pay back period with all other costs factored in besides lighting).

$$$: LED and High Pressure Sodium (small sizes): most LED solutions will be on the pricier side relative to fluorescent, if you are going to spend on LED's, your best option is to invest in a light from a well established LED vendor (full explanation in the next section "Vendor Quality") 

$$$$: Most heavy duty commercial style systems will involve high pressure sodium/metal halide (collectively referred to as HID or high intensity discharge)and high priced options are increasingly now appearing for LED systems. These systems will range in the thousands of dollars+ (USD) and will often be part of larger systems involving grow tents, grow "boxes", air filters, fans, controlled environment features, etc.

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5. Grow Light Vendor Quality

How light quantity, duration, and quality are related source

There are over 57,000 results on Amazon.com for the search term "grow lights". This vast array of options is not equal as far as quality goes.

For grow lights, ratings and reviews on sites like Amazon, Alibaba, or even niche sites for grow lights may not guarantee you are receiving the product quality you paid for.

How so? 

With grow lights, the technical specs in reality often are not as advertised, even for a brand new product.

In this case study from Albopepper.com the true wattage of brand new LED grow lights varied wildly (one light had 34.2% lower wattage than advertised).

When you are paying for grow lights, a lot of what you are paying for is wattage, so this is a serious risk to consider when buying grow lights. This sort of issue could cost you hundreds if not thousands of dollars, and, most importantly, time, if undetected.

Solution: trusted brands like Lumigrow may have more expensive products but also have a long track record of shipping consistently performing products. International products from countries like China should be reviewed thoroughly.

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6. Common Grow Light Accessories

The concept of a grow light kit is to provide everything you need to get growing, fast! source

One of the best ways to make sure you are not missing any key grow light components is to get a grow light kit.

What do you need to know about grow light kits? 

Keep reading to find out! 

What you need to know about grow light kits

If you are looking for a grow lights kit a simple search online will help you several options. We recommend looking on Amazon, Home Depot, eBay, Gardeners Supply, or other well-known vendors to get an idea of the options available.

What does a grow light kit include?

Top end grow light kits will typically include:

- Light
- Fans / Filters
- Grow Tent
- Ducting / Clamps

Often times a grow light will be the only part of the "package" or "kit" you are purchasing. Here are other potential additions besides the grow light that you should know about: 

Ballast/fixture: grow light systems require an electrical component called a ballast to regulate the power supply in the grow light. if you are buying just a bulb, of course you will also need a ballast. this is a necessary component for lights. ballasts can be purchased independently from grow lights but grow lights can also have the ballasts incorporated into unit. 

Grow Tents: grow tents are another feature mainly seen with more expensive systems. Tip: It's worth noting that many of the benefits of a grow tent that help create a more controlled growing environment can be replicated cheaply DIY (not all the benefits).

Ducting and Clamps: components to help make sure the grow tent is sealed properly and any wiring is handled in safe and organized fashion.

Fans / Filters: Compenents that help ensure proper air flow, air quality, and temperature within the grow tent or grow room.

Now, whether you are just getting started with a grow kit or building an entire grow room, you will need to understand the unit economics of what you are doing.

Read on to learn more about the unit economics of grow lights.

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7. Know the unit economics

The analysis below shows the long term outlook of purchasing different types of grow lights.

This particular grow light case study is for a theoretical 4' x 4' growing space focused on a daylight spectrum (running 16 hours/ day).

One of the major LED advantages, as you can see from this case study, is the durability of the LED fixture.

In Depth Case Study: Cost of LED 340 W vs HID vs T5 over 8 years

So: 

How much will it cost you to operate a 340 W LED grow light for 8 years, all costs added? 

See the case study below: 

source

Now: 

Generally speaking, understanding the unit economics of powering your grow lights requires a 2 simple steps: 

1) Know the rate paid for electricity: this will typically be expressed in cents / kWhr (kilowatt hr). Your kWhr price can be provided by your electricity provider. Keep in mind that the full cost of your electricity is not just the kWhr cost.

There will always be fees + taxes added onto the base kWhr price.

For example, if your kWhr price is $0.08/kWhr, your total electricity cost will be closer to $0.12/kWhr after taxes and fees from your provider. (+50%).

source

2) Know the wattage of your bulbs. Given the kWhr prices above, if you are using 2 40w fluorescent bulbs, then your total wattage is 80 w (40 * 2). To find the cost of operation, divide 80/1000 (1000 because the unit is kilowatt hrs), then multiply by $0.12/kWr.

The resulting equation results in the cost to run these 2 40w bulbs for 1 hour: 

80 / 1000 * 0.12 = $0.0096

If you plan to run the lights for 24 hours / day, multiple the above total by 24 to get the daily cost, and by 24 * 30 to get the monthly cost: 

$0.0096 * 24 = daily cost = $0.23
$0.0096 * 24 * 30 = monthly cost = $6.91

Now, the issue is, with two 40w fluorescent bulbs you will not have enough lumens to effectively grow most crops.

Lets use the same example, with a 600 W HID Discharge bulb.

600 / 1000 * .12 = $0.072

Plan to run the lights for 18 hours / day, multiply by 18 to get daily cost, multiply by 18 and by 30 to get monthly cost: 

$0.072 * 18 = $1.296 = daily cost
$0.072 * 18 * 30 = $38.88 = monthly cost

As you can see, more wattage typically means more cost to operate on a unit scale. The upside is that your grow will have better results! 

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8. Scalability

Cost per square foot decreases as quantity of lighting shelves increases in this example. Source

As of 2019, the 3 main options for grow lights are: 

1. CFL (Fluorescent) 
2. HID (High Intensity Discharge) 
3. LED (Light Emitting Diode) 

Per Sol Tech Solutions, here is how each of these compare from a scalability perspective: 

1. CFL is best for small grows by non professionals, typically do not scale well niether from a cost or yeild perspective.

Pros: 
Inexpensive
Good for small grows
Available in many stores
Works in any standard light fixture
Available in various color temperatures     

Cons:
Low light output compared to other types of bulbs
Need to be close to plants
Need to use several
Not meant for bigger growsInefficient
Not for intermediate/experienced growers

2. HID is currently still the best at scale for most commercial applications, although with the rapid advance of LED's grow light technology the advantages over LEDs at scale are less and less significant.

Pros: 
Costs less than other high-quality lights
Produces great results
Easy to setup and operate     

Cons:
Produces a lot of heat, requiring ventilation and exhaust
Limited lifetime
Does not plug into a normal socket
Requires special hood & ballast
Can significantly increase electric bill

3. LED is the grow light technology that has gained the most ground in the past decade. It offers many scalability advantages that neither CFL or HID offer.

PROS:
The most energy efficient grow light
Barely produces heat
Plugs into standard outlet
Long lifetime   

 CONS:
High initial costs

Verdict: If you are interested in scalability and are willing to invest capital in the long term growth of your grow light system, upper end LED's will once again be the best way to go.

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9. Grow Light Safety

Grow Light Safety Is Important, but there is more to grow light safety than just eye protection Source

As detailed in 7 tips for vertical farming beginners, not all grow lights have the same safety risks.

Two key areas to evaluate with grow light safety are:

1) how fragile the light is 
2) the materials that are present in the light itself.

In both of these categories, LEDs have a clear advantage.

1) For household growing projects where pets or children may be disturbing the lights, LEDs are much less likely to break when knocked down or jostled, due to the small size of the diodes and the way they are situated in the fixture.

Other lighting systems like fluorescent tubes and HPS lighting can be very fragile and will shatter or even explode if they are knocked over with enough force.

2) The following are factors you should consider for any indoor grow light with respect to safety: 

2a: Fire - it is a common misconception that fires started by grow lights are caused simply by heat creation. Many fires casued by grow lights are electrical fires that result from incorrect electrical sources not designed for grow lights.

Large grow light fixtures typically require 220 V "dryer" outlets. Overloading your electrical outlet combined with long periods of running time can pose a risk for indoor growers.

2b: Mold

When grow lights cycle on and off, the moisture level in the growing spaces changes. This continuous changing of moisture (lights on = low moisture, lights off = condensing of moisture), can increase the risk of mold or mildew which can be harmful to humans.

2c: Mercury: 

Many types of grow lights, with one exception being LEDs, have a significant amount of mercury present. The main problem is if these lights break, they can pose a health risk - anyone in the growing room can potentially inhale the mercury which can cause braind damage.

Across many of these areas, LED's, while priciest, have the best safety profile.

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10. Purpose

At this point, you should define what exactly you are trying to accomplish in your search to buy a grow light: 

Do you eventually have commercial intentions? 
LED or HID are your best options.

Do you just want to start a hobby? 
CFL or (some) LED's will be your best options.

Are you trying to optimize your yield? 
HID or upper end LED's are your best options.

Do you have a smaller budget (
CFL's will be your best option.

Still have some questions about evaluating grow lights? 

Check our section below! 

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11. Miscellaneous Helpful Notes and FAQs on Grow Lights

Can You Use "White" Household LED Lights To Grow Plants? 

As noted above, plants need about the same wavelengths of light that humans see in (‘the visible spectrum’).

White LED lights, also known to many as common LED light bulbs, do NOT emit light throughout this necessary spectrum range (380 - 720 nm).

The white LED spectrum is not complete enough for growing most plants Source

White LED lights in reality only emit very specific peaks within that range of spectra which alone are not enough to grow indoors effectively.

To give a specific range, ‘white LEDs’ will typically emit wavelengths only in the 450 - 550 nm range, or approximately 29.4% of the necessary photosynthetically active range (PAR) (550 nm - 450 nm / 720 nm - 380 nm).

Grow Lights In Greenhouses

Do you need grow lights in your greenhouse? 

Most plants need 12-18 hours of light exposure per day.  

Adding lights to your greenhouse can be a good option if you are not getting at least 6 hours of direct sunlight on your greenhouse and are not getting total of 12-18 hours.

Keep in mind however, that in a greenhouse, lighting should stay 20-40 Watts of light per square foot, and that light should be evenly distributed among leaves.

When using grow lights in your greenhouse you may not be able to get uniform light on all of your plants, and you may have difficulty getting uniform exposure for all of your crops.

How Long Should Grow Lights Be On?

Most vegetables and flowering plants by default will need a maximum of 12-16 hours of light per day. This means that your grow light will often be “on” for a similar period of time.

However, if you are growing totally indoors, you may be interested or curious about 24 hour, around the clock artificial lighting.

If that is what you are interested in, here’s the deal:

Some plants will actually do quite well when exposed to 24 hours of light, growing and blooming even faster than usual, whereas some plants will not do so well, and may fail to bloom or have stunted growth.

It simply depends on the plant.

Why is this the case?

Photosynthesis depends on two biochemical processes, “light reaction” and “dark reaction”. Generally speaking, the light reaction is when the plant cells convert light to energy and the dark reaction is when the plant cells store the energy for later use.
Now:

Dark reactions, contrary do the name, many times do not actually require the plant to be exposed to just darkness. Some plants can undergo “dark reaction” storage of energy from light while exposed to light, and these are the plants that grow amazingly well in 24 hr light exposure situations.

As a rule of thumb, winter blooming plants which require darkness to bloom will not be candidates for 24 hr light exposure.

Perennial plants like cacti and the larger group of succulents, typically thrive under 24 hr light conditions.

What type of light is best for growing plants indoors?

As covered above, even incandescent bulbs can be used for growing plants indoors, but are not the best option. For most people, fluorescent, Metal Halide (MH), High Pressure Sodium (HPS), and LED lights will be the best options. For more in-depth details, re-visit the areas above!

Can any light be used as a grow light?

For pretty much anyone besides the highly experimental grower, you will want to buy lights specifically designed as grow lights for indoor growing applications. Beyond the simple difference in effectiveness, you may be putting yourself at a safety risk by treating lights not designed to be grow lights in a grow light application.

10 Mistakes  To Avoid Once You Buy Your Grow Light

The sun can burn your plants, and so can LEDs Source

For all of this talk on buying grow lights, there are many pitfalls to watch out for once you make the decision on which grow light to buy.

Here is a short but handy list to keep in mind: 

1. Burning or Overheating Your Plants

This is one of the most common complaints our audience members e-mail in with feedback about. If you buy a grow light for the first time, odds are you will experience the burning of your plants.

How will you know you are burning your plants? 

The first signs are thin dark lines appearing on the outer parts of the leaves, which will develop into darker brown areas and physically curling of the leaves. This is most likely to occur on the top of the plant. This is also reported to occur more commonly at higher elevations.

Commercial flower farm using shade cloth in a greenhouse Source

How to avoid: 

- Adjust the height of your lights. Just because there is a recommended distance does not mean you should stick to the "guidelines" if you are burning your plants.

- Some readers have experimented with shade cloth if they have space constraints limiting how high they can hang lights. Be careful with this strategy as it can increase fire hazard.

- Ventilation and airflow. Most sites will suggest airflow as the solution to preventing overheating. This is a good first step to take if you are experiencing light to moderate browning or curling of leaves.

2. Providing Not Enough Lighting

Commercial flower farm using shade cloth in a greenhouse Source

The opposite of #1 above, providing insufficient lighting will stunt the growth of your plants.

Make sure that, given the amount of plants you want to grow, you have enough lighting ready.

3. Placing The Lamps at Improper Distance

4. Using Low Quality LED Grow Lights

5. Selecting the Wrong Light Spectrum

6. Using the Wrong Light Schedule

7. Forgetting to Adjust the Lights

8. Not Protecting and Maintaining Your Lights Properly

9. Overwatering Plants When Switching from HIDs to LEDs

10. Not Considering All of the Factors

Other Helpful Notes and Tips

Here are a couple more tips for you, if you're still looking for more information. we will update this list and add on tips in the future: 

- fundamentally, all grow lights are replicating the sun. however grow lights are not as powerful as the sun, plan to run grow lights for 12-18 hours per day. Timers can help you manage this cycle.

- Fluorescent lights will not typically have color temperature / Kelvin rating listed on packaging in normal stores, instead they will be more loosely categorized into categories like "warm glow", "soft white", "bright white", "daylight" and "cool white" that each correspond to ratings

- When looking for fixtures / ballasts look for ones that have reflectors. Reflectors will help you significantly increase the efficiency of your lights. As an example, fluorescent lights emit light in all directions with equal intensity. Without a reflector, up to half of your light will not be shining where it needs to shine: on your crops.

- more on cost guidelines:

- Roughly 2/3 of the your grow light expense should be the fixture/ballast and 1/3 of the expense should be the bulbs. other one time costs may involved for things like mounting.

- A reasonable payback period for a non-commercial grow lighting system should be around 1-3 years.

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