A cold frame
can be used to harden off seedlings, but it will still need daily tending.
On a sunny day, the temperature inside a sealed cold frame can soar,
damaging and even killing plants. So you’ll need to open the top
when necessary to provide air circulation. In late afternoon, close
the top to hold in the day’s heat. If the night is predicted to
be particularly chilly, you can cover the cold frame with a blanket.
