Annual flowers are divided into two groups, hardy and tender. Hardy annual flowers can usually withstand temperatures down to 26 degrees F. (some varieties as low as 20 degrees) without being killed. Sometimes flowers or leaves may be damaged, but plants will recover. The top portion of most perennial and biennial flowers can also withstand similar temperatures. Tender annual flowers may be severely damaged by temperatures of 31 or 32 degrees. In some garden books (especially those written in Britain), tender annuals are referred to as half-hardy. Annual flowers are considered hardy, if they can survive 26 degrees or lower. They are tender if they are killed by temperatures between 26 and 32 degrees.
Some popular hardy cool weather annual flowers are pansy, viola, poppy, primula, petunia, alyssum, carnation, dianthus, godetia, nasturtium, cosmos, lobelia, calendula, snapdragon, verbena, and sweet pea. Tender annual flowers include impatiens, marigold, salvia, vinca, zinnia, aster, dahlia, nicotiana, ageratum, celosia, coleus, begonia and geranium. Perennial and hardy annual flowers can usually be planted two to five weeks before the average last frost date. They can withstand the frosty nights which occur early in the spring. Tender flowers should not be planted until after the average last frost date. Even then, late frosts may occur, and plants may need protection. The average last frost date for most of the Pacific Northwest is from early to late April.
If you want to move or divide perennials it is best to do it early or if they are early spring bloomers, wait until after they bloom. Summer blooming bulbs such as dahlias, gladiolus, lilies, cannas, and callas can be planted by late March or April. Begonia bulbs are best planted in May.