Seedlings under T5's: how close and what temp?

Hey guys. My first batch of indoor grown just coming up. I did have a false start and had to wipe a whole crop because I switched from T12 to T5 and got a little over zealous with how close the lights were to the seedlings (about 2 inches). They all immediately sunburned and I couldn't save them. So I germinated a new batch of seeds with the paper towel plates method and they're all just starting to put out their first set of new leaves. Got the light about 3.5 inches away and the temp is reading about 78'F. They seem to like it.
I'm planning to use plant training techniques to create short bushy plants with lots of tops and don't want them to start stretching to early, so my question is how close should the T5's be to the tops to compromise between keeping them from stretching and not letting them get too hot or sunburned.
Anybody have any advice?
-growtender

I'm no expert but the last time I started from seed I had the Maxlum 4' 8 bulb at the top of the dome cover. Once they were ready to go in my top feed buckets (week later) the light was kept at 6" above the canopy. Make sure you keep that canopy even because the T5 does not penetrate well when it's further up.

Also, I am counting 16 plants. Do you have another T5, or a bigger one? I only say that because I have 12 under a 2 bulb and when they were growing quickly, the T5 was no longer efficient. They streched like a gymnast!

Eventually you can put those t5s right on top of them, like an inch or two away. But you will have to harden them to the light, so start about 10 inches away and slowly sneek up on them. Week or two and they should be mature enough to handle it. A little air movement keeps what little heat they produce, at bay.

GreenLantern513 said:

Also, I am counting 16 plants. Do you have another T5, or a bigger one? I only say that because I have 12 under a 2 bulb and when they were growing quickly, the T5 was no longer efficient. They streched like a gymnast!

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I do plan on keeping them trimmed evenly, mainlining and manifolding essentially. I have some supplementary lights I will use once the need arises, but pretty quickly they'll be heading to the flowering room under an HPS. Only aiming to do get to 12-18" of height before switching over.

Oh and don't have the feeding tubes going directly on the stem of the plants. They will rot.

chino7564 said:

Oh and don't have the feeding tubes going directly on the stem of the plants. They will rot.

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just repeating that in case you missed it the 1st time....some/most people put the feeding tubes on a timer...I don't but again, make sure the tube isn't constantly wetting the original rootball (grow plug) AND the emission is away from the center/stem

• Eventually you can put those t5s right on top of them, like an inch or two away. But you will have to harden them to the light, so start about 10 inches away and slowly sneek up on them​

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  yup ^^^ but one exception: I shouldn't advocate this, but what I do from get go is place a 23W CFL pretty darn close to the seedling; almost from get go...I've not had any issues yet, in fact my seedlings come out pretty stocky .....
  you can have issue with seeds planted too shallow as the leaves will deform at 1st or sometime even, the seeding will sorta "self top" in that it has a hard time shucking the bean shell...but again, I've gotten away with it and start off with some pretty robust, stocky plants (I got 2ea right now and should post pics) ​
  
  
  
  

Hey growtender! I'm using 2 2ft 4 tubes T5's and I started them at a 6" above the soil. About 36 hours later i lowered them to 4" above the tallest seedling and they are loving it! Ive read that 2" is the closest you can get without burns (but that didnt work this time haha). I think your good at 3.5"!

Trippy.

I think it may have to do with temperature. A t5 high output gives off heat. In my 60 degree room I grew my seedlings from day one with one t5 h.o. just above them but not touching them. No burn, in fact they did very well without stretching. But when I put it in the grow tent had to take it out because the temp in there was higher and burn became an issue. I'm just learning these things but it seems that they need to be up close to plants for best effect. But to do that the ambient temp needs to be quite cool because they can raise the temps 10 degrees in a small enclosed space. This was just two 4 ft high output t5 in a 4x4 tent.

mangojuice said:

I think it may have to do with temperature. A t5 high output gives off heat. In my 60 degree room I grew my seedlings from day one with one t5 h.o. just above them but not touching them. No burn, in fact they did very well without stretching. But when I put it in the grow tent had to take it out because the temp in there was higher and burn became an issue. I'm just learning these things but it seems that they need to be up close to plants for best effect. But to do that the ambient temp needs to be quite cool because they can raise the temps 10 degrees in a small enclosed space. This was just two 4 ft high output t5 in a 4x4 tent.

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Testing the lights you have is the best solution imo.Also leaving your hand under them ( palm down ) works as well.If it's too hot for your hand,it's too hot for them.I'm no expert as well but i'm constantly asking for advice..and this is what i';ve learnt .Is there a general rule about ''stretching'' on plants ? When and how do you notice it ?

I use a laser temp gun. The back of my hand seems to do better with heat than the plants with my last batch of seedlings, two grew much quicker than the others and when I raised the light for them the other two seedlings began stretching. They put more growth into the stem that it looks like four little leaves on an extra tall toothpick. The weight of the leaves will knock the little guy over. One of mine stretched so much she did fall over. Not a prob, the extra long stem can be.buried so the plant has some sturdiness and then correcting the distance to the light will stop anymore stretching. temp and heat go together. They seem to do better at temps below 80 and above 70 so the distance to light and heat output would account for the different answers. I can't put the t5 into my grow tent anywhere near the distance to the plants as I can outside the tent where it is cooler. Happy growing

I posted these in another thread but here goes again, IMHO seedlings should look more like this and not those thin spindly things many users post (IMO at 2-5 days old, most strains look similar more or less) View attachment 2878328 this girl is a lil under 2weeks and is directly under twin 23W daylight CFL's

I can't post pics now but hoping next month I can. I have two freebie seedlings thatare one week olds right now that are 7 and 8 inches wide and in thirds set of true leaves w/ fourth set showing. Lights have been the biggest learning curve once I switched to a familiar soil. Still learning with the led and plasma lights but the t5's have shown their worth with seedlings in my cool room.