What is an Annual Flower:
An annual grows from seed and blooms and sets seed and then dies in just one growing season. Petunia and marigold are examples of flowers widely grown as annuals. Annuals need to be replanted each spring. Most annuals bloom continuously from spring through fall.
What is a Perennial Flower:
A perennial flower lives for three or more seasons. It may or may not be mature enough to bloom the first year from seed. (Hint: P is for Permanent and for Perennial). Perennials will need periodic rejuvenation and/or replacement, typically every three to five years. Most perennials bloom for only a short period -- a week or two or three -- once a year.
What is a Biennial Flower:
A biennial grows vegetatively its first year, lives over the winter, then finally blooms in the second season. Once it has bloomed and set seed, it dies. Foxgloves and hollyhocks are usually biennial. Gardening is never simple. There are always exceptions.
This is an interesting term and varies in usage depending on your climate. It describes plants that are perennial in warmer climates but can be grown as annuals in colder climates. These plants can be categorized with annuals because they will bloom the first year from seed. They are termed half hardy because although they can handle frost, they can't survive extremely cold winter weather. If you live in a cold winter climate, you would not realistically expect these to come back the next year. But if you live in an area with mild winters, they may be perennial for you. Osteospermum is an example of a half hardy annual, as is snapdragon (Antirrhinum). And in a mid-way location, you may find they survive and overwinter successfully for you during especially mild years -- but not all years. Half Hardy Annual:
Frost Tender Annual or Tender Perennial:
You may see this term used to indicate a perennial plant grown as an annual because it is killed by frost. If you live in a frost free climate, this plant would be perennial for you. If you live where it gets cold enough to frost, you will lose it!