Thanks Rick,
I think I got my answers but this makes me ask more questions :)
Only problem is we have really long and hot summers here.
I'd say June is already hot, and so are July, August, September.
15-20 C is probably May... but then a long hot summer comes.
Would beginning of May be OK for these DH ?
I will monitor these pots as they will be on the balcony which is all closed with windows and on the north side, so direct sun only for a couple hours in the morning (in summer).
And in winter, the temp will probably be a few degrees above outside but still very cold.
