Your Top 5 T5 Fluorescent Growing Questions Answered

Your Top 5 T5 Fluorescent Growing Questions Answered

Now, we all know what we need a strong grow light to get your plants where they need to be, but with increasing costs of energy out there we’ve been getting lots of questions asking if growers can use T5 grow lights to grow from seed to harvest, and all the caveats in between. That’s why we’re bringing you the top 5 questions we get when it comes to growing flowers and fruit with T5’s.

1) “So how do T5’s work and how is it that they don’t generate as much heat as HPS’s?”


This question cuts right at the heart of the “T5= less energy and heat” conundrum.  Let’s unpack this question and get a better idea of how these bulbs work:

Now just like HID’s, T5 fluorescent bulbs use electricity and a chemical reaction to give you light. However, that’s where the similarities stop.

T5 grow lights have a small bit of mercury and gas (usually argon) inside of themselves to generate light. They’re ignited with a small current of electricity, but unlike HID’s that give off a really big, bright light, the light that’s generated from this chemical reaction is mostly made of UV wavelengths which we usually can’t detect with our own eyes. So how does that produce light we can see?

Ever notice how HID grow bulbs are clear but T5 grow lights are coated all white inside? That coating is made out of a phosphor that reacts with the UV rays to give you light. Depending on the combination of phosphors used, you’ll get the white or yellowish light you’re used to seeing.

T5’s don’t need to heat up very much to give you light, and the energy it takes to heat up that material to give you light is lower than most other sources of light.

2 ) “Do I need different colored T5 lights? There are yellowish and blueish bulbs like HID’s, but there are also blue and red bulbs, too. Which ones do I use?”

All plants love a full spectrum of light when they grow, so you’ll definitely need both a vegging and a flowering light to give you a good harvest. For the vegging stage of growth, you’ll want to use a T5 fluorescent grow light with a 6400k spectrum of light– that’s going to give you a white/blueish spectrum. When it’s time to flower you’re going to go with a 2700k bulb to give you a yellowish/white spectrum of light that stimulates flower growth.

Now, in the past, most growers needed to either increase the amount of 2700k lights over their plants or switch to an HID to finish the flowering stage, but that’s not necessarily the case anymore. Because plants need lots energy they may not get to create flowers and fruit, companies started putting out T5’s with different phosphors (the coating inside the bulb that helps give off light) that mimic specific wavelengths of light instead of a broad spectrum- just like LED’s!

These colored T5 fluorescent bulbs do what LED’s do: fill gaps in wavelengths that the 6400k veg and 2700k flower left. That means more direct red and other red spectrums in the flowering spectrum, more direct blue and supplemental blue in the vegging spectrum, and even more UV with the full spectrum bulbs they offer. Just like LED’s of multiple spectrums, multi-spectrum T5’s are great for filling in gaps that other T5’s leave behind.

So which T5’s do you use? We say to use as many spectrums as you can afford. If you can only afford 2700k and 6400k T5 grow lights stick with the basics- they’ll get you where you need to go. But if you can spring for multi-spectrum T5 bulbs they’ll definitely improve the quality of your T5 harvests and take your plants even further

3) “Is there UV in T5 bulbs? If not, are there UV T5 bulbs and will it fit in the same reflector?”

When it comes to trichome and resin production, lots of people are skeptical of T5’s ability to get the job done.

That’s a great question because as it turns out the wavelengths of light created in T5 fluorescent lights is actually UV. So to answer the question quickly: do T5’s have UV spectrum in them? Yes, they do. Does that UV spectrum necessarily get to your plants? Now that’s a different story

We’ve mentioned earlier that most lights give off UV wavelengths, and for the most part that’s true for T5’s. However, the amount of UV from a normal HID grow light is minimal. Now, take that and add the fact that T5’s are coated in phosphors that activate when UV hits them, and you’re getting extremely little to no UV’s coming from T5’s.

This is another reason growers don’t typically love growing with only T5’s: hardly any UV comes from them, which means your plants won’t get nearly as strong or have nearly as many resins as they would with UV. But that doesn’t mean you can’t get UV to your plants through your T5.

On the contrary: there are actually T5 grow light bulbs that give you high amounts of UV. Instead of a heavy coating of phosphors inside of the bulb, there’s a light coating, allowing lots of the UV-A and UV-B wavelengths to leave the bulb. These are known as either “UV bulbs” or “Full Spectrum+UV bulbs” and they’ll definitely help give you quality harvests.

4) “How high to I hang T5 lights above my plants?”

Whenever you ask how high to hang your T5 grow light you usually hear “you can hang T5’s as close as you can to the plant because it doesn’t generate a lot of heat”- but that’s a cheap answer. There’s got to be a definitive answer, right? Well, there is 10-12” above the plants at the highest.


The power of a T5 grow light can be felt 0-12” above the plants. Above 12” and the light will start losing a significant amount of its effect on your plants. Now, where this gets tricky is when you have plants that like to get tall.

Clones, seedlings, spices, herbs: all these types of plants love T5’s because they’re powerful enough to give them a good dose of light, but not so strong that they’ll fry. But once those clones and seedlings start to grow into taller plants they’ll start to lose some of the light they were getting before, which will significantly reduce growth if you don’t know how to combat it. How’s that done? Training.

In order to get your plants to flower and grow fruit, they’ll need lots of light energy- a no-brainer, right? But if that light’s only useful up to 12” above the plant you’ll need to make sure that the plant is getting as much of that light to as much of itself as possible. That’s where training comes in handy:

T5 growers that want to use T5’s from start to finish will often LST, Bonsai, or Sea of Green their plants. These techniques will help keep plants and their canopies to grow out instead of up, which will give the plant more of a squat, bushy look than the tall, tree-style look you’re used to. When plants start to grow unexpectedly tall, growers will lower their lights to focus light energy on the canopy of their plants while trimming the undergrowth of their plants to direct growth to fruits and flowers above.

5) “Is there a difference between T5’s and CFL’s? Can’t I grow with CFL’s?”

Though there are a few differences, T5 grow lights and CFL’s work in essentially the same way: a tube filled with a little argon gas and mercury is charged with electricity causing a reaction between UV’s and the coating of the glass and you get light. You can definitely grow with CFL’s (we’ve actually mentioned that a few times in the past), and with CFL’s you actually have a wider range of mounting choices over T5’s. However, those advantages come at a cost.

Now a big difference between the two styles of lights is that CFL’s need to be plugged into lamp sockets. That will let you virtually mount any bulb anywhere in your grow room: in between plants, above them, under them, on the sides, wherever! But they don’t kick out very much power, which means you’ll likely need a lot of CFL bulbs around your room. Even though they don’t use lots of power, each of those sockets will probably need its own power cord and depending on how many lights you need all those cords can become a fire hazard to work around.

All those lights can also be tricky to hang safely, too. As you may know, not all parts of a plant will grow in uniform so if you’re running multiple lights get ready to look around your grow room and adjust them as needed.

That’s the great thing about T5 grow lights: uniformity. No mess, no hanging multiple bulbs around your room. All you do is install the tubes, hang, put your plants under the light, and hit the switch.

Share this:

3 thoughts on “Your Top 5 T5 Fluorescent Growing Questions Answered”

  1. I always go through your all post . I really very much like your all posts. Keep the coming it helps a beginner like me

  2. One issue that you did not address is the heat generated by T5 bulbs. I have a 4 ft 4 bulb T5HO fixture that is hanging 12 inches from my seedlings. When all 4 of the T5 bulbs are on they simply fry the seedlings. I have measured the temperature increase when the lights are turned on and it soars at least 5-6 degrees. To keep the canopy from frying I need to raise the light above 12 inches (perhaps as high as 18 or more). This may decrease the benefit that the plants get from the light. For this reason I have not been completely satisfied with T5 lighting. I should mention that this is So. Cal summer conditions so the lights are in a room which is already at 77 degrees. With all 4 lights on the temp goes up to 84.

    • Great observation, Alan, because while we do mention that there is heat coming from T5’s that doesn’t necessarily hard mature plants too much, any high output light (whether HID, T5, or even LED) can be too much for seedlings. Mature leaves, branches, and roots can withstand warm temperatures, but clones and seedlings have a very hard time taking all the heat high output lights give them. That’s why we recommend one of two things:
      1) Decrease the intensity of your lights by lowering the intensity of the bulbs. For example, if you’re using 54w T5’s to grow, consider switching to 24w’s and cutting the output down by more than half. That will allow you to lower the lights within 12″ of your seedlings without burning them.
      2) Decrease the intensity of your lights by only using 2x of those bulbs at a time. Most T5 fixtures will allow you to run fewer bulbs than they’re capable of handling without messing up the fixture itself, so if you have a 4x bulb fixture consider removing two of the bulbs and only using the remaining two to grow (be sure to space them out so that the light isn’t intensely focused on to your plant).

Leave a Comment