When do you change lights from fluorescent to hid/ hps lights? When to add nutrients? My plants at 2 to 4 inches tall.
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i would wait until i had three or four nodes before doing either @claxon they are young. thats just me, hopefully a pro will chime in and give you more exact info… be patient
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Hi ! @claxon , welcome to ILGM forum 
. @BIGE is right, they are too young for the HID / HPS light and the nutrients.
Is it possible to give us more information, like how powerful is both of your sets of lights, your type of growing medium soil or hydroponic, if it’s soil which compagny and other information like that just by filling the follow grow support ticket
This “Support Ticket” into your forum post.
Answer these simple questions the best you can.
If you do not know, or do not use something; Just say so = NA
Strain; Type, Bag seed, or NA
Soil in pots, Hydroponic, or Coco?
System type?
PH of runoff or solution in reservoir?
What is strength of nutrient mix? EC, or TDS
Indoor or Outdoor
Light system, size?
Temps; Day, Night
Humidity; Day, Night
Ventilation system; Yes, No, Size
AC, Humidifier, De-humidifier,
Co2; Yes, No
Add anything else you feel would help us give you a most informed answer. Feel free to elaborate, but short, to the point questions and facts will help us help you smile
And if you could provide photos, it will be helpful too 
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Hps is for flower and now star feeding at 1/4 amounts increasing to 1/2 at end of vegation stage
It’s usually sets of leaves, not height, when asking about nutrients.
The first set, called “cotyledon” are rounded and these leaves help the baby live until the regular leaves grow in…usually by the 4th set of regular or “real” leaves the plant is ready to get a light feeding, starting with half the recommended dosage to see how the plant reacts…increasing the dosage with subsequent feedings.
A set of leaves is a pair of leaves on the stalk.
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