Would it be too hot to grow spinach, romaine lettuce, and swiss chard in the greenhouse?
It depends on your greenhouse and climate. But probably as a rule, if its a typical hobby greenhouse without sophisticated environmental controls such as pad and fan cooling or air conditioning, the answer would probably be "yes".
in the summer - yes! Perfect for winter though - plant around sept/october or whenever they will be full grown by about dec 1.....and they will keep through spring....
I plan on planting my fall crop on the first of september. I'll have lettuce through thanksgiving. I grow in low tunnels.