When to start using nutrients

[panel350]

Hi All, This will be my second Outdoor grow , I have 4 strains on the go I had 100% in germ VERY happy!! I'm using Canna Pro as media with some Pearlite added and Ph'ed water at 6.4 ,in the photo they are at day 18 since germ,early spring so average of 15c and 8-10 at night ,When can I start introducing some Nutes ??
Looking forward to the next few months!!
Cheers Kev

 

[Old Salt]

I start with a weak nutrient solution when the third set of leaves start to appear, not counting the cotyledons.