Freezing Point of Common Annuals

Tender Annuals

• Some annuals cannot tolerate even a light frost, which occurs when air temperatures fall between 28 and 32 degrees Fahrenheit. Tender annuals may even fail to bloom during extended periods of cool, cloudy weather during the summer. These plants often grow as perennials in tropical climates but are grown as annuals in cold areas. Common tender annuals include ageratums (Ageratum houstonianum), begonias (Begonia x tuberhybrida), coleus (Coleus x hybridus), geraniums (Pelargonium hortorum), impatiens (Impatiens wallerana) and marigolds (Tagetes patula).

Semihardy Annuals

• Semihardy annuals grow best in hot weather, but they'll tolerate cooler temperatures than tender annuals, and they can survive light frosts down to 28 degrees. Semihardy annuals include carnations (Dianthus caryophyllus), gaillardias, also known as blanket flowers (Gaillardia pulchella), gazanias (Gazania rigeas), petunias (Petunia spp.), rudbeckias (Rudbeckia hirta), salvias (Salvia spp.) and sunflowers (Helianthus annuus).

Hardy Annuals

• Hardy annuals are the workhorses of the cool-weather flower garden. These annuals can tolerate a brief hard frost, which occurs when temperatures fall below 28 degrees. Many of them can be planted in early spring and some, such as cornflowers, even need a period of cold storage to induce germination. These plants not only withstand freezing temperatures, but they also thrive in areas with cool summers. For blooms from spring to fall, try alyssums (Lobularia maritime), cornflowers (Centaurea cyanus), dianthus (Dianthus chinensis), larkspurs (Consolida ambigua), pansies (Viola wittrockiana) or sweet peas (Lathyrus oderatus).

Care

• How you care for your annuals can help extend the season by a few weeks. First, start seeds indoors or buy nursery transplants to get a head start on the season. Plant tender and semitender annuals outdoors only after the last expected frost. Hardy annuals can be planted earlier. Cover annuals with cloches or floating row covers if a frost is expected. Annuals grown in the ground can tolerate more frost than those grown in containers because the ground stays warmer. A 2- to 3-inch wood chip mulch can also protect them. Move lightweight containers into the garage if a frost is expected to prolong the bloom season.