I get them out when the first true set of leaves are just starting to appear.
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Some will disagree with me on the following point however - when I get them out, very gently mind you, I pretty much remove all skerrics of the jiffy material and them plant in a seed raising mix. I do this because I find the Jiffy pellets take probably three solid waterings before they start to become hydrophobic, and you don't want any material like that around the roots. I have not lost a single seedling in the process, and the growth spurt shortly thereafter is really noticeable.
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My two cents.
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Regards,
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Tim