T5 fluorescent tube autoflower growWhat are fluorescent tubes?

Fluorescent tubes are exactly the same as CFL (compact fluorescent lights) but they have been around way longer.

All the fluorescent tubes are gas discharge lamps that use electricity to excite mercury vapor. This vapor creates excited mercury atoms and when they bump into the bulb’s outer coating then the coating (usually Phosphor) starts to glow!

For a number of years the old T12 bulb fixtures were considered standard when lighting large areas with decent efficiency, but as good as these T12 lights were, they were not good for growing plants and that includes cannabis. They had a relatively low lumen per watt ratio and their only advantage was the low temperature.

After the T12 bulbs came the T8 light bulbs that had a smaller diameter and they are more efficient and can be used in growing regular and autoflowering cannabis plants. These T8 fluorescent light bulbs have a luminous efficacy of about 70 to 80 (lumens per watt) and they are available in different light output colors so they can be used for growing marijuana plants. These T8 tubes are the second generation of fluorescent lighting and they are good for lighting purposes and their luminous efficacy is acceptable for growing. But there is an even better choice and that is the third generation fluorescent tubes or T5.

The T5 tubes can have a luminous efficacy of up to 100 lumens per watt and that means that you can grow your plant with less electricity! If you are growing a decent amount of autoflowering plants under a 200W and 24 hour light cycle of fluorescent lights then (if electricity costs 10 cents for a Kw/h) you would be paying 48 cents every day ((200W*24H)/1000)*0.10$) or about 15 $ every month.

With these 15$ you can grow a decent yield compared with T8 – just look how more efficient the T5 tubes are:

  • T12 = 200W * 60l/Watt = 1200 lumens
  • T8   = 200W * 75l/Watt = 15000 lumens
  • T5   = 200W * 85l/Watt = 17000 lumens

In this comparison, I have taken the average lumen per watt numbers. The actual numbers can be slightly larger, but even at these average numbers the T5 bulbs outperform T8 and T12 bulbs, so they will grow denser and larger buds and your cannabis plants will be stronger and bigger, so your yield will be higher by just buying the right fluorescent tube lights!

After seeing these numbers you should know that you can most definitely grow cannabis with T8 and even T12 bulbs but you will get the best results from the T5 fluorescent lights, so in the rest of the article I will be talking about the T5 bulbs because they are the most efficient and most widely used fluorescent tube grow lights! 

Fluorescent light penetration

First of all, every cannabis grower should know that light penetration strength is a myth and no light source can penetrate the plant canopy more than others if they match in light output.

Fluorescent tubes can penetrate as much as HID lights if they match the HID lamp in their lumen output and this means that 16000 lumens from 150 W HPS light would penetrate as much as 16000 lumens from fluorescent tube lights but the fluorescent T5 tubes give you the added benefit of spreading the light across all the pants if you mount them on the sides and on the ceiling! 

Fluorescent tube light intensity, spread

So now that we know that light penetration is not different from any light source if they match in lumen output, we need to know how intense is a T5 grow light and what is its spread? Autoflowering cannabis growers should know that fluorescent tube light has small intensity but a large spread whereas the HPS has high intensity and small spread.

If you choose to grow with T5 tube lights then you will definitely need a lot of them because they have a low light output but because they have this low light output they produce less heat so ultimately if you match an HPS or MH lumen output you will produce less heat and for small grow operations this is the major selling point! 

T5 light sizes & shapes

T5 fluorescent tubes come in different shapes and sizes but the typical sizes are

  • 54.9cm / 22”
  • 84.9cm / 34″
  • 114.9cm / 46″
  • 144.9cm / 60″

There are also the PL-L fluorescent tubes that are basically the same tube folded in half so it is half of the length but double width. These Pl-L bubs have both connectors on one side and that makes them more compact and all the heat is generated at that one side so it is easier to ventilate it out of the way

There are also some other shapes and sizes like the U shape or the Pl-S but they are not as efficient as the regular T5 tubes so I won’t include them in this article!

T5 fluorescent bulb efficiency

T5 fluorescent light bulb efficiency is a critical thing that every autoflower cannabis grower should know before buying his next grow lights. T5 are the most efficient fluorescent lights and they can get up to 100 lumens per watt but this maximum efficiency is only achieved in the long  tubes that are 114 cm and 144 cm long but that doesn’t mean that the 54 cm and 84 cm tubes don’t have great efficiency. This means that you need to check what is the lumen output of the bulb you are looking to buy, but usually longer tubes are more efficient!

Another great thing about these T5 tubes is these PL-L bulbs that are basically a long tube bent in half, so you can get better efficiency not buying those ridiculously long lights!

T5 light lumens

Fluorescent light Lumens output is the total amount of light that the bulb emits in all directions and because this light is emitted 360 degrees around the bulb many growers will introduce some kind of reflectors or reflective materials to better utilize the light that is coming out in the wrong direction.

T5 grow lights produce low amounts of lumens compared to other light sources but they are very efficient and the lumen count can be anywhere from few hundred (the minimum I have seen is 200 lumens) to thousands (7200 is the biggest that I have seen) but I believe that with a fair amount of search someone can find lower and higher lumen outputs!

T5 reflectors

As I mentioned above, T5 fluorescent tubes give off light in all directions so the above plant gets direct light only from the side that is facing down and that is only 1/4 of the lumen output and the rest of the light is spread to the ceiling and sides! This inefficient light output can become more efficient if we introduce some reflective materials or reflectors!

Imagine if a single reflector on a T5 grow light would reflect 100% of the light that is coming from the sides and the top, then this grow light would become 300% more efficient and no light would be wasted and all of it would shine on the autoflower plant canopy! These T5 reflectors are not 100% reflective and some light intensity is lost because it needs to be reflected multiple times. Some light is lost from this reflection, but they can still dramatically increase the yield and if you have reflective material on all of your walls then they can reflect even more light and your grow lights can become even more efficient!

If you are making your setup by yourself you should buy a T5HO bulb with a reflector pre-installed and at the bottom of the page you will see autoflowering-cannabis.com best suggestions!

But if you would like to buy a pre-made set up we recommend going with 6 or 8 lamp fixtures that will give you a decent amount of light in a small space and the reflectors are already included, and the best ones you can also find at the bottom of the post!

T5 Lux

Lux is the measurement of light that is shining to the surface (one meter) at any given time!

This measurement states that if for example, the bulb shows it has 400 LUX that means it can transfer 400 lux to a square meter at 1 meter distance. This measurement is not as popular as the overall luminescence or Lumen output but sometimes you will see this measurement on fluorescent grow lights!

T5 autoflowerT5 light color temperatures

Autoflowering marijuana needs a certain amount of light to grow and produce buds but not all light is equal to the cannabis plant.

Cannabis plants produce food by photosynthesis and there is basically two light spectrum where the weed plant absorbs the most amount of light and they are the red and blue colors of the spectrum. So now we know that autoflowering cannabis plants don’t use all light equally and now is the time to tell about the PAR or the Photosynthetically Active Radiation that is basically the measurement of how much light a plant can absorb for the photosynthesis process. Almost none of the light sources will give you the PAR ratio but you can suspect that light with more red or more blue spectrum will have a bigger PAR than light that has an average spectrum somewhere in the middle of those two.

Here comes in the color temperatures of the fluorescent grow lights! Basically for the vegetative growth of autoflowers you need to supply more of the 6400K temperature bulbs that have the output spectrum more similar to the daytime SUN and will encourage more steam and leaf growth but when the automatic ganja plant goes into flowering it needs more of the evening red light that can be supplied with the 3000K fluorescent light bulbs!

There are also 5000K and 4000K  bulbs and they work as well but you will have better results if you add some 2700K and 6400K lights as well!

The best scenario is to supply your plants with an equal number of 3000K and 6500 K and even the middle range T5 lights because then you don’t need to change the bulbs and your plants will get an optimal spectrum of light for the best growth!

You can find the best 2 feet and 4 feet 3000K and 6500K bulbs at the end of the article! 

T5 bulb life spanT5 cannabis grow

Autoflowering plants can be ready to harvest in two to three months from seed and if the lights are on 24 hours a day that means about 1500 to 2000 hours of light for one crop. At this rate, you can have up to 10 harvests before you need to change your fluorescent tubes because their lifespan can be up to 20000 hours and more! The average lifespan of these T5 fluorescent grow lights is around 10000 to 15000 that will give you from 5 to 7 harvests but there are many lights with 20000 and even more hours lifespan. There are also some not so good bulbs with 7500 and less hour lifespan. So before buying any fluorescent tube for your autoflowering plants check the average life span on those bulbs and do some math how many harvests can you get from them! 

High output fluorescent tubes (T5HO)

High output T5 tubes are basically the same as the regular T5 fixtures but they have higher lumen output and higher input currency that make them more usable to cannabis growers but that also makes them more inefficient! These HO bulbs usually have double Watt input and almost double-lumen count but they also need special ballasts so you can’t fit a High output fluorescent tube in a regular T5 socket! If you are considering T5 lights for your autoflower or regular cannabis grow you should know that these High output bulbs are the most common used T5 grow lights but you can grow weed with regular bulbs more efficiently! With T5HO bulbs you won’t get that 100 lumens per watt but you can get up to 80-90 lumens per watt and in the long run you will lose some money but you will need fewer bulbs to illuminate the same amount of autoflowering plants!

Now that you know all this detailed information about T5 fluorescent tubes you can make an educated decision if they are good for your setup but you should also know that they are perfect for autoflowering plants in small to medium places. They produce small amounts of heat and are very efficient. Autoflowering plants usually stay small and that is perfect for fluorescent tube growing as you need to get those plants as close to light bulbs as possible to get the maximum light intensity. Usually, the plant canopy should be a few centimeters from the bulb! You can also mount these fluorescent lights on the side walls to get more light to side branches and lower branches and get an even bud formation all over the plant not just at the top of the plants!

Fluorescent tubes are also good for larger SUPER autoflower plants and regular marijuana strains as you can mount them together with different light sources and get better yields and denser buds. This method of putting Fluorescent tubes at the sides of the marijuana plants is well known in the growing community as it has been done for a number of years and putting these T5 lights at the sides of the grow room with a HID or LED light at the top will give you the best results!


  1. As soon as the seed pops how far should your t5 ho be to the new plant? Should it stay the same distance through the grow, or will it ever be better to give it more or less distance to the tops of the plant?

    • Hi Brad,
      You should keep those little seedligs as close to those T5 lights as possible troughout the entire life cycle of the plant. In the first few days I usually like to keep my plants a little bit farther away from those light sources so those seedlings would not dry out as fast. But as the first and second true fan leaves start to appear you should move them closer and closer to your light. The exact distance is hard to know because every grow room has different humidity and heat connditions and those T5 light can be either NO, HO or VHO and each has its own heat rating.

  2. Kenneth Chan on

    Is it a good idea to use window light for auto flowering cannabis? And until at night use one of the lamps you mention about?


    • Hi, yes you can grow your plants on a windowsill but they will grow slower than in a direct sunlight or undera powerful grow light.

  3. I was thinking of using 4 bulb, 4 foot T5 lights for 5 auto-fem plants. I would run the lights while the plants were in front of a six foot sliding glass door which faces south-east in the northern part of the US. What do you think about the combination of T5 lights and natural light?

    • Hi brent,
      I think that it would be a great combination but don’t get those new super autos as 5 super auto pants would be too much for a 4 feet T5 fixture, but 5 smaller autoflowers should do great under it.

    • Hi, so I had a pretty successful grow last season but feel there is room for improvement. My latest question involves how many auto flowering plants should I put under a 4 tube, 4 foot long t5 light? Thank you.

      • That depends on the strain and pot size that you choose as well as the overall environment in your grow room. You can have two massive plants or even 6 smaller plants. That is why there is no one correct answer and you will have to test it yourself.

  4. Hi Brent,

    Great site. I have 3 2 bulb T5 6400K lights over my seedlings did I read right that you recommend mixing 1 6400 and 1 2700 to cover all spectrum growing ALH-F

  5. I’m actually getting ready to start my first inside grow. I just started growing outside a year ago and really enjoyed it and it turned out nice and now I wanna keep going…. I have some questions that I would love to be able to chat with a experience grower about. 🙂

  6. Pencilhead on

    My 3000k lights are yellow would 2 of these and 2 6500k work or would red work better

  7. Is it possible that I can get too much light I am thinking about putting the T5 tube on the side and but 1 LED above itch planet but I’m afraid that my planet will get over light

  8. Newbdude on

    I plan to grow on a balcony that receives morning and afternoon sun all year round. Yes, its near the equator. Reason is for security and stealth most of all.

    My plan is to leave it outside the whole time but when sun goes down, I will turn on T5 lights i plan to purchase and just fix it on the extended roof/ shade ceiling. The timer should switch it off a few hours b4 sunrise. There will be an estimated 5-6 plants in 5 gallon fabric pots. I read of moving the plants in and out but it is tiresome and could cause stress.

    Will this result in a better harvest? What lights should i use and how many? Will probably try to wrap a foil hat around the tubes for better focus on the plants but nothing too much to arouse suspicion.

  9. I have two questions how much bulbs i need to leave turned on ? i have Hydroplanet T5 4ft 8lamp Fluorescent 6500K Ho Bulbs and second question when plant is in flowering stage do i need to change all 6500K bulbs to 3000K Red – 5000 Lumens ?

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