The Ultimate Guide to Grow Lights for Indoor Plants

When I first started looking into grow lights for my indoor plants I felt totally overwhelmed. Kelvin, lumens, E27, IP, lux, and whatever this thing is: μmol/s. How could I compare my options when I didn't understand the 'light lingo' in the first place?! 

To save you from the big learning curve I went through, here's the Ultimate Simplified Grow Light Guide for Indoor Plants, explaining what all the different terms mean and answering all the common grow light FAQ's to help you compare and choose the right grow lights for your indoor plants :)

 

 

What does 'full spectrum' mean?

Full-spectrum (when it comes to grow lights), typically means the light produced is white or close to it, and that colours appear similar to how they would in natural light. 

The full spectrum means the light produced covers the entire visible spectrum from roughly 400 to 700 nanometers (nm), including blue, green and red, sometimes including ultraviolet (UV) and infrared (IR), depending on the light.

Full spectrum lights normally appear white or close to it, with the colour temperature in the range of 5000K to 6500K (Kelvin), from a slightly warm-white (but not yellow), to cool white (but not blue). 

 

What does CRI mean?

Some grow lights also tell you the CRI or Colour Rendering Index. 100 is the CRI of natural sunlight, so if a bulb has a high CRI of 90+ for example, it would mimick natural sunlight with 90% or higher accuracy.

What is PAR and Nm (nanometers)?

Nm stands for nanometers, which measures the wavelengths of the light produced. Plants require light in the 400 to 700 nm range to photosynthesize, known as PAR (Photosynthetically Active Radiation). It's not the brightness or lumens (how bright it appears to us), but the PAR that matters to our plants.

What's the difference between full spectrum and pink/purple grow lights?

The 'neon pink' grow lights, sometimes called burple or blurple, produce the red and blue spectrums only, producing a pinky-purple light.

It's the red and blue spectrum that benefit plants the most to support growth, however not everyone likes that pinky glow. It also makes green foliage appear a dark brownish colour and plants do still benefit from the missing colours. The high concentration of blue light can also cause photochemical damage to eyes and cause headaches.
  

Plants use more of the blue and red spectrum, and less of the yellow and green. Full spectrum lights look like sunlight and appear 'white' but give plants the full spectrum, including red and blue.

 

How close or far away from plants should a grow light be?

As a general rule, LED grow lights should be at least 30cms away from plants. You'll often see 60cm to 1 metre recommended.

The younger the plant the further away it should be from a grow light. The higher the output of your grow light, the further away it should be.

The supplier can normally recommend the distance for each bulb. For example, Sansi recommends 30cms to 1.5 metres for their 24 watt LED grow light.

You should also take the plant's light tolerance into account. For example, if a guide says 30cms but your plants are sensitive to bright light, start further away and adjust from there.

The type of light matters also. LED grow lights in general are higher output than fluorescent, so will typically be better hung or placed further away than fluorescents. 

If your set-up means the light will have to be closer than 30cms, look for a light with adjustable brightness, such as Grow Bar or Photo+ or go down in wattage, such as one of two Sansi 10w or 15w instead of one 24w or 36w.

  

How long should a grow light be on per day?

Most plants need at least 12 hours of 'Good Growth' light per day, with at least 8 hours of darkness, however will grow better with 16 to 18 hours of good light. 

You can look up the Good Growth level for different plant in the Plant Species Light Guide coming up below. 

Houseplants in general tend to be more tolerant of fewer hours of light, with 6 to 12 hours being sufficient, provided the light source is strong enough during that time, but will grow better provided with more.

How long a grow light should be on depends on how many hours of 'good light' a plant also gets from natural sources. A grow light can be used to supplement the good light hours, or as the only source of light if it's bright enough (again, depending on the light requirements of the plant).

For example, a plant that prefers bright light and gets good natural light for 3 to 4 hours in the mornings, might have a grow light programmed to come on for 8 to 9 hours a day, starting around midday, every day.

 

Can you run grow lights at night?

Yes, you can run grow lights day or night, but having grow lights mimic daylight hours will give you better results. However if your plants are in a dark area with little to no natural light, by all means create your own artifical schedule for them. 

About 12 to 16 hours light with a rest period of at least 8 hours darkness is an ideal balance for most indoor plants. Take temperature into account also as combining light and warmth will get better results. 

 

Can you keep grow lights on 24/7?

Plants need both light and darkness but how much of each they need varies by species. To grow, photosynthesis has two biochemical processes, light reaction and dark reaction. Keeping lights on 24/7 can cause stunted growth or plant tissue to die in some plants.

As a general rule for indoor plants, aim for at least 12 hours of good light a day and around 8 hours of darkness. 

What does the beam angle tell you?

Another factor that helps determine the distance a grow light should be from plants, is the beam angle. Beam angle means how wide an area the light covers (without a shade).

Wide beam lights are around 45 to 120 degrees, and narrow beam vary from 5 to 45 degrees. 

A narrower beam angle is more directional so can be further away from plants, with a guide of 60cms to 2 metres depending on the intensity and brightness.

A wider beam angle is a good option for using in lamps and desktop lights, as it can go closer to plants, around 30 to 120cms away.

Wider beam bulbs also spread out and covers a wider area, so if you combine a wider angle with a brighter, higher intensity light, they become good options for bigger areas like small rooms and greenhouses.

 

What is Lux?

Lux tells you how intense a grow light is, which helps you know how close or far away your grow light should be from your plants. Lumens and lux both refer to light intensity as it is perceived by our eyes. It's PAR that tells you about the quality of light as plants perceive it (more about PAR coming up).

What does Kelvins tell you?

Kelvins measure the colour temperature. The higher the Kelvin, the whiter or more blue the light becomes. A lower Kelvin will be more yellow, orange or red. A cloudy sky measures around 6,000K. Direct Sunlight is 4,800K.

Light bulbs below 2000K would appear similar to candlelight, are dimmer and more yellow.

2000 to 3000K is a soft white with a yellow appearance like you might use in a bedroom or lounge.

3100 to 4000K is better for task lighting like kitchens and offices.

4600 to 6000K is closer to the blue-white light of daylight, where bright illumination is needed, or when you want something lit naturally similar to the way it would be outside.

6500K and higher is a cooler, more blue-tinged light, more often used in commercial locations for very bright task lighting.

Can you use LED grow lights with power point timers?

Yes, if your grow light does NOT come with a built-in timer, you can usually get yourself a power point timer, also called a plug-in timer, and set when your lights turn on and off automatically.

These start around $10 and are easy to find at hardware stores like Mitre 10 or Bunnings. Most LEDs cannot be used with dimmer switches. 

What does E27 mean and what lamps do E27 bulbs fit?

If the light you're looking at is a bulb rather than a plug-in light, then the fitting matters. The E in E27 means Edison Screw and the 27 tells you the diameter (27mms). E27 is the standard screw fitting in New Zealand. 

E27 gives you the biggest range of lamps and other light fittings compatible with your grow light. However keep in mind how wide the shade is matters too, because grow lights tend to be wider than household light bulbs. 

The list of available lamps that fit E27 grow lights changes all the time, especially with the cheaper lighting that retailers don't have as permanent stock items. You also need to check the wattage rating is at least the same or higher than your bulb's wattage.

What is Lumens?

Lumens measures visible light from a light source and tells you the intensity of the light. One lumen equals one candle. One foot candle (FC) is 10.76 lumens. More lumens isn't always better.

The brighter the light is, the higher the lumens. However lumens alone doesn't tell you much about grow lights as far as your plants are concerned. Lumens is more helpful for us, to measure how bright a light is to the human eye, however PAR is what plants care about. 

What do watts tell you?

With fluorescent lights wattage was a good way to judge brightness. A 100 watt florescent is brighter than a 60 watt. But with LEDs lower wattage can produce more light than a higher wattage fluorescent. In an LED grow light the wattage really only helps you know how much electricity you'll use.

A better guide to help you judge brightness is the PPFD. Most suppliers don't tell you the PPFD for a light, however you will often be given the PAR. If you see a measurement given in μmol/m2/s or μmol-m-s that's the PPFD.

 

What is PPFD?

Plants require light in the 400 to 700 nm range to photosynthesize, known as PAR (Photosynthetically Active Radiation). PPFD and PPF both tell you the amount of PAR a light source produces that your plants can use for photosynthesis to grow. PAR by itself tells you the quality of light only, not the quantity. 

PPFD and PPF are measured in micromoles. PPFD stands for Photosynthetic Photon Flux Density and is measured in 'micromoles per square meter per second' which is μmol·m-s or μmol/m2/s. PPF stands for Photosynthetic Photon Flux and is measured in micromoles per second, which looks like μmol-s and is sometimes shortened to μmol.

 

How much do grow lights cost to run?

Watts alone won't tell you how much power a grow light will use, as how energy efficient the light source is also factors into energy use. LED lights are much more energy efficient than other light sources. Some general costs follow to help you get a sense of power use and cost.

One 60 watt bulb left on for 24 hours will cost around 20 cents per day depending on your rates. Used for 8 hours a day, 7 days a week, a 60 watt bulb works out less than 50 cents a week, or around $24 a year.

An oven on medium to high heat, run for one hour, will use around 2,400 watts, costing you around 60 cents an hour depending on your power rates (all costs used are based on average rates in New Zealand). 

A computer used around 3 hours a day, and unplugged when not in use, costs around $55 over the course of a year. 

 

What light levels do indoor plants require?

Each indoor plant has a different range of light it can handle from the 'Minimum for Maintenance' where they may stay alive, but may not do much in the way of grow; right up to commercial growth light levels that a grower might use to achieve maximum growth.

In the middle is the 'good growth' level that we aim for with indoor plants, often with the help of a grow light. This guide below includes some of the most popular houseplants and the light levels they require.

How to read the Plant Species Light Guide (below)

The first set of numbers is the Minimum for Maintenance (MM). The middle set of numbers achieves Good Growth (GG), and the third set of numbers is for Commercial Growth (CG). 

Plants with a higher Minimum Maintenance level (the first set of numbers), are more likely to require a grow light when grown indoors, unless you can give them plenty of bright light for 12+ hours a day. 

If the Minimum for Maintenance is low, this indicates the species is more low-light tolerant. If the Commercial Growth level is low, this indicates a species than won't tolerate very bright light or direct light well. 

For most indoor settings, aim for at least the Good Growth level. To maintain a Commercial Growth level of light requires a more controlled setting such as a greenhouse, along with control over and close maintenance of other factors such as soil pH, airflow, temperature, humidity and fertiliser.

You'll need a good light meter to test the light levels, however this chart is still helpful to get an idea of how tolerant of low light and bright light your plant is.  

FC stands for Foot Candle and µmol is the symbol for a micromole, used to count the number of photons in a plant grow light system. More about both terms coming up. 

Note that FC varies based on the type of light source. The chart below is based on sunlight requirements. Also keep in mind this guide gives you the average requirements per species. There will be individual varieties within each species that may fall outside of these averages.

 

Plant Species Light Requirements Guide 

KEY: See above for how to read and use this guide. The first number is MM, the second is GG and the third is CG (see the definitions above).

African Violet

200 FC (40 µmol)  |  400 FC (80 µmol)  |  1000 to 1200 (200-240 µmol)

Alocasia

200 FC (40 µmol)  |  400 FC (80 µmol)  |  2000 to 5000 (400-1000 µmol) 

Anthurium

100 (20 µmol)   |  400  (80 µmol)  |  1500 to 2000 (300-400 µmol)

Begonia 

200 FC (40 µmol)  |  400 FC (80 µmol)  |  2000 to 2500 (400-500 µmol) 

Calathea 

200 FC (40 µmol)  |  400 FC (80 µmol)  |  1000 to 1200 (200-240 µmol)

Hoya 

100 (20 µmol)   |  400 (80 µmol)  |  1500 to 2500 (300-500 µmol)

Ficus 

400 (80 µmol)  |  800 (160 µmol)  |  2000 to 6000 (400-1200 µmol)

Syngonium

100 (20 µmol)  |  200 (40 µmol)  |  1500 to 3000 (300-600 µmol)

Monstera

100 (20 µmol)   |  400 (80 µmol)  |  1000 to 2500 (240-500 µmol)

Peace Lily

50 (10 µmol)   |  200 (40 µmol)  |  1500 to 2500 (300-500 µmol)

Peperomia

100 (20 µmol)  |  200 (40 µmol)  |  1500 to 3000 (300-600 µmol)

Phalaenopsis

200 FC (40 µmol)  |  400 FC (80 µmol)  |  1000 to 1500 (200-3000 µmol)

Philodendron (vining, eg: Heartleaf, Brasil)

100 (20 µmol)  |  200 (40 µmol)  |  1500 to 3000 (300-600 µmol)

Philodendron (climbing, eg: Pink Princess)

200 FC (40 µmol)  |  400 FC (80 µmol)  |  1500 to 2500 (300-500 µmol) 

Pothos (Epipremnum, Scindapsus)

100 (20 µmol)  |  200 (40 µmol)  |  3000 to 5000 (600-1000 µmol)

Sansevieria

100 (20 µmol)  |  200 (40 µmol)  |  1000 to 6000 (200-1000 µmol)

ZZ Plant

100 (20 µmol)  |  200 (40 µmol)  |  1000 to 2000 (200-400 µmol)

Source: For the full list see House Plant Journal >

 

Can you use LED house lights to grow plants?

Yes and no. Provided it's a new generation LED, at the higher brightness and intensity you might use to light a shop or for task lighting like a kitchen, then yes, some LED lights can be enough to grow low-light plants like seedlings, Peace Lily, ZZ and other low-light tolerant species. Use the Plant Species Lighting Guide above to get an idea of low-light tolerant plants.

Even if they aren't enough to support growth, some LED house lights can still meet the minimum levels required for plants to stay alive (that's the Minimum for Maintenance set of numbers in the plant species light chart above).However  in general the answer is no, for most of our tropics-loving indoor plants, you'll need a grow light to support growth.

 

What are optics?

Optics direct light towards your plants, increasing light intensity. Imagine optics a bit like a hose. Turning the nozzle can change both the area the water covers and also how strong the spray is. Turn one direction and you might get a very fine spray that covers a broad area. Turn the other way to get a single jet with much stronger water pressure that wets a smaller area. Optics work in a similar way to change the coverage (beam angle), and intensity of the light.

LEDs with no optics would have a broad 180 degree beam, light up more plants, but with weaker light intensity. Primary optics reduce the beam angle, with 90 degrees being common. In the past only primary optics were used, however some of the newer generation of lights now use secondary optics or secondary lens.

Secondary optics create a narrower beam, focusing the light to a smaller beam angle, meaning more light reaches the plants, and the light reaches further down. Light intensity increases, but how wide the coverage area is reduces.  

Wide beam angles are more popular to cover more plants from further away, such as for greenhouses. More narrow beams are preferred for medium distances for better depth, such as use in a grow tent or vertical cabinet. 

 

What does the IP rating tell you?

IP stands for Ingress Protection which basically tells you What can get in? A light's IP rating will give you 2 numbers, such as IP20. The first number tells you the protection against solids. The second number tells you protection again liquids.

The rating for solids goes from 1 to 6, with 6 being the highest level, also called 'dust-tight' which offers protection against even very fine particles like dust.

The rating for liquids goes from 0 to 8. The highest liquid rating of 8 means the light can be submerged under water deeper than a metre and for long periods of time, also called 'water-proof'. A pool or pond light would need a liquid rating of 8.

Grow lights for indoor use tend to be rated from IP20 to IP40. That's a 2 to 4 for protection against larger solids only, and 0 protection against water. Keep in mind grow lights need airflow. Running a light in high-humidity without adequate airflow is not good for lights, or plants, or us, and can shorten the lifespan of your light or void the warranty. 

Safety first: Always turn lights off at the wall BEFORE touching a bulb. Never touch a wet bulb or light fitting without first turning the electricity off in your house.

Even a bulb rated waterproof should not be touched if the electricity is on. To be safe, If you notice any bulb or light fitting is wet, always turn off your electricity in your house (not just at the wall), to avoid an electric shock.

If a bulb is waterproof, the last number in the IP rating will be a 6 or above. Here's what the second number tells you (eg: the 5 in IP65).

0 has no special water protection. This rating is common for household lights in general, for the lounge, kitchen and in bathrooms where the bulb is not directly above water (you should have a higher rating above the sink, bath and shower for example in case of water spray).

1 can resist vertical drips.

2 can resist water at a 15 degree angle or less.

3 can handle water sprays up to 60 degrees.

4 means the product is water resistant to splashes from any direction (this is the rating you might have for a bulb above your bathroom sink).

5 means the bulb can handle a low-pressure water jet spray.

6 can withstand high-pressure, heavy sprays of water (this would be the rating you'd need for above a shower).

7 can be submerged in water up to 1 metre

8 can be submerged in water deeper than 1 metre (look for the maximum depth it can go to on the packaging)

The IP rating for water, does not relate to humidity or moist environments. When using any bulb in a high humidity environment, ensure there is adequate airflow. If you are concerned your bulb is wet, turn off the power in the house before handling. Handling dry bulbs (with dry hands), just requires turning the power off at the wall.

I do sell grow lights but I am not an electrician, so do recommend you get professional advice when choosing lights for different uses. This advice does not replace professional advice from your electrician.

 

Can LED grow lights damage plants?

It's a common myth that LEDs can't damage plants. This comes from comparing fluorescent lights to LED lights. Fluorescent lights certainly do run much hotter than LED, and run a higher risk of burning plants if they are too close, which isn't likely to happen with LEDs.

However LED grow lights still produce some heat. Not typically enough to cause damage to plants, but running hot can still damage the light itself. LED grow lights are normally designed so heat exits to the back or top of the light, away from plants. This is why airflow is important when using an LED in a lampshade, not so much for your plant's sake, but for the life of the bulb (and for your wallet's sake!).

If you have your plants too close to your LED grow light, you may see yellowing or browning on leaves. How close is 'too close' depends on your plant's tolerance for bright light, which is why there isn't one rule for all lights or all plants. Look up the Plant Species Light Requirements Guide below to compare low-light and high-light tolerance.

Because LEDs run cooler than fluorescents, heat-burn is of little concern, but because LEDs are typically much brighter than fluorescents, you can still get 'photo-bleaching' which looks similar to heat burn, but is caused by the light being brighter than what the plant requires. 

Follow the distance guide the supplier tells you, or start at least above 30cms away from plants, then adjust from there according to your plants individual needs. If you already know your plants are less tolerant of high light, start at 60cms and adjust from there.

 

Should I get 1 or 2 bulbs?

Just like the way sunlight changes direction over the course of the day, two bulbs are better than one if the only or main source of light to provide full coverage, rather than light from only one direction. 

Where can I buy grow lights for indoor plants in New Zealand?

From me here at Love That Leaf. Shop indoor plant grow lights here >