The exact temp depends on wind chill and soil temperature and most importantly how \"hardened off\" the plants are which means did they just come from a warm greenhouse or have they spent some time under cover but outdoors at night. I would cover them with a sheet if the night temp falls below 40 degrees. You will know if they get too cold because the foliage will take on a silver or dull green look. I am guilty of planting too early so now I try to wait until all danger of frost and cold nights have passed. Keep growing, Marianne Binetti