From Academic Kids
Botanically, an annual plant is a plant that usually germinates, flowers and dies in one year. True annuals will only live longer than a year if they are prevented from setting seed. Some seedless plants can also be considered annuals even though they do not flower.
In gardening, annual often refers to a plant grown outdoors in the spring and summer and surviving just for one growing season. Many food plants are, or are grown as, annuals, including most domesticated grains. Some perennials and biennials are grown in gardens as annuals for convenience, particularly if they are not considered cold hardy for the local climate. Carrot, celery and parsley are true biennials that are usually grown as annual crops for their edible roots, petioles and leaves, respectively. Tomato, sweet potato and bell pepper are tender perennials usually grown as annuals.
Ornamental annuals are often called bedding plants. Annuals are often used in gardens to provide splashes of color, as they tend to have a longer season of bloom than hardy herbaceous perennials. Some tender perennials commonly grown as annuals are impatiens, wax begonia, snapdragon, Pelargonium, coleus and petunia. Some biennials that can be grown as annuals are pansy and hollyhock.
One seed-to-seed life cycle for an annual can occur in as little as a month in some species, though most last several months. Wisconsin fast plant can go from seed-to-seed in about five weeks under a bank of fluorescent lamps in a school classroom. Many desert annuals are termed ephemerals because their seed-to-seed life cycle is only a few weeks. They spend most of the year as seeds to survive dry conditions.
Summer annuals sprout, flower and die within the same spring/summer/fall. The lawn weed, crabgrass, is a summer annual.
Winter annuals are plants that germinate in the late fall, grow through the winter, and flower and die in the spring. Winter annuals grow low to the ground, where they are usually sheltered from the coldest days by snow cover, and make use of warm periods for growth when the snow melts. Some common winter annuals include henbit, deadnettle, chickweed, and winter cress. Winter annuals are important ecologically, as they provide vegetative cover that prevents soil erosion during periods when no other cover exists.
Although many people consider winter annuals to be weeds in gardens, this viewpoint is not necessary as most of them die back once other plants leaf out in the spring. When their time does not overlap with other plants, they do not compete for the same resources. Even though they do not compete directly with cultivated plants, sometimes winter annuals are considered a pest in commercial agriculture, because they can be hosts for insect pests which attack the crops being cultivated. Ironically, the property that they prevent the soil from drying out can also be problematic for commercial agriculture.
- Caring for Annuals (http://www.ehow.com/how_16118_care-annuals.html)