Burned leaf tips

Hey, I just started growing indoor since about 2-3 weeks ago.
I'm growing 5 plants in a 1m/1m/1.80m growbox.
3 of my 5 plants (Amnesia Haze) are growing really well, but the other 2 (AK-47) are growing really slow.
They also show some burned tips on their leaves and i don't know what causes this? I'm growing underneath Viper Spectra LED 600W about 70cm's away. I don't know if it's lightburn because the other three are growing really well..
Can anyone help me identify the problem?

Thanks

It has been 30+ degrees C a few days tho. Could this be the cause?

CrDolan said:

It has been 30+ degrees C a few days tho. Could this be the cause?

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They would all be a bit stunted if it was temp related.

CrDolan said:

... I'm growing underneath Viper Spectra LED 600W about 70cm's away ...

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70 cm's is 27" ... hmmm. That does seem a bit too close for those little ones under the Viparspectra 600W Reflectors. The Reflectors have 5 IR diodes and IR can fry young plants if the lights are too close.

I use those same 600 W lights for my veg cycle with both the VEG and BLOOM lights on. But I won't put my clones underneath them until I see one node fully opened, that opened node is greening up nicely and the next node is poking through eager to join the party. Until that time, I have them under a 23W CFL about 4" above the plants.

And when they do go under those LED's, I start at 36" (90 cms) and watch daily for new growth. Once new growth is happening, the lights are gradually lowered about 2" per day until they are 20" above the girls. By then, the plants are hearty enough for those LED's.

It could indeed be the cause if i think about it. The 3 that've grown healthy were early, Got out of the earth about 3-4 days earlier. I've put them the first few days outside in the sun because it was about 27-30 degrees outside and i hadn't gotten my lamp yet.

So the other three were about 3-4 days older than the other two that i put under the lights. Could that be the case? Because they weren't strong enough?

I've put my lamp on 90cm's for now, i'll see if they recover or not.

Thanks for the response

I'm having doubts about putting my lamp higher to 90 cm's again. Won't it slow down the plants that have grown good? I've put it inbetween at 80.

CrDolan said:

I'm having doubts about putting my lamp higher to 90 cm's again. Won't it slow down the plants that have grown good? I've put it inbetween at 80.

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The ones that looks the best can handle the current light intensity. What you could do is put that them on a box (something about 8" tall) so that you can keep it the same distance away as it is now.

Good idea. It's the first time i'm growing so still learning a lot everyday.
In the early stage, do you put them all under one 23W CFL lamp or multiple lamps?
I also bought some Canna Terra Vega, should i already start giving them the food?