Flower gardeners often use the term hardiness when discussing flowers and shrubs. Hardiness refers to the plants ability to survive degrees of winter cold often expressed in terms of geographical zones.
Hardy
Hardy annuals are those flowers whose seeds can withstand temperatures below freezing (32°F) and may even require frigid conditions to break dormancy and sprout. Hardy annuals can easily weather any number of late-winter and early-spring freezes and thaws.
Tender or Greenhouse
Tender, or greenhouse, annuals originate in tropical climates and require very warm conditions to flourish. These annuals will not survive a frost nor tolerate cool soil.
Half-Hardy
Half-hardy annuals are somewhere in between the two extremes of Hardy Annuals and Tender Annuals. Typically, half-hardy annuals should not be sown or transplanted to the garden after the last frost even if the soil is not thoroughly warm.
Locate flower seeds based on the hardiness of the plant.