I've just noticed you want to grow the lettuce in your greenhouse @LeadFarmer. You might find it too hot for them and that can make them bolt. As @Obelixx was saying, she has them in a shadier bed, and even here, I try to keep them reasonably shaded.
I only have them inside when I sow initially, ie at this time of year. I've never had them undercover apart from that. I don't know if others grow them inside through the main season.
I agree - I grow lettuce outside and shaded in the summer.
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I agree - I grow lettuce outside and shaded in the summer.