In article , Phil L
writes
I put one of those min/max thermometers in the window for a few days to
check and it seems the lowest is 66F and the highest around 74F is this warm
enough or should I turn on the radiator? - bearing in mind that this will
*greatly* increase the temperatures through the day as the radiator is
directly under the window.
I would say that it is rather over warm, but it depends on the amount of
light reaching your seedlings, also on humidity to some extent. If the
seedlings begin to grow tall and leggy, either lower the temperature or
increase the light. We germinate ours at 20C/66F, then grow them on at
cooler levels.

Also tomorrow I'm planting onion seeds, is this too late?

No.
--
Alan & Joan Gould - North Lincs.