That depends on the plant. Cool season annuals such as snapdragons, pansies, lobelia, Euphorbia Diamond Frost and alyssum will thrive when nights are as low as 40 degrees. Most annuals (petunias, geraniums, begonias) like nights above 50 degrees but warmth loving annuals such as coleus, marigolds, basil, tomatoes and won\'t be happy unless nights are as warm as 55 degrees. Putting plants in pots and keeping them close to the house and out of the wind will keep them warmer at night. Most annuals in your area can be planting the beginning of May. Keep growing, Marianne Binetti