I don't know how cold is your area Litha, you can place it in your green house now but if it is going to be hard frost, take them indoor again please and put them back in the green house the next day. If the night temperature don't fall to freezing, they should be ok to stay there. If your seedlings are way too leggy, you can always resow. I know the feeling of killing seedlings as I tend to try to keep them all (I hate thinning seedlings) but it is really up to you.

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