When to thin out Poppy seedlings

My Poppy seeds have sprouted and are looking quite pleased with themselves about that . I'm amazed at how quickly they've grown. Its been a long time since I grew anything from seed. They are in mushroom trays on kitchen wìndowsill. When is the best time to thin them out, please?

 

Poppy's are mostly direct sown I think, so not easy to say about how to thin, but best perhaps to do it earlier rather than late.

 

I have done that also, but on Anglesey I bought Poppy plug plants ( native, not Pavars ), so thought I'd sow in trays as well as a back up plan.

 

May vary according to variety but from long distant past, I recall that they have a tap root; its critical not to damage this so transplant before it becomes predominant ,into deeply cultivated soil and water intensively.