Vegetables are categorized by their type: root vegetables are edible plant roots; bulb vegetables are edible bulbs and leafy vegetables are edible leaves. Vegetables are further differentiated by when they are planted; however, these two categories are not mutually exclusive. For example, some root vegetables are cool-season vegetables and others must be planted after the last frost.
There are a wide assortment of bulb vegetables. Garlic matures into a cluster of bulbets. Scallions and leeks are grown more often than onions, according to the "Reader's Digest Illustrated Guide to Gardening," because home-grown scallions and leeks are more flavorful. Shallots are similar to garlic in appearance but have a more delicate flavor.
There are several different types of leaf vegetables with the most recognizable being lettuce. Lettuce varieties include crispheads, which form a solid head with white, densely packed inner leaves; loose heads, such as bibbs or butterheads with thick outer leaves covering a yellow or white center; romaine, which are tall, upright heads with long, thick leaves; and looseleaf that do not form heads, according to "Gardening" by the National Gardening Association. Kale is a hardy, leafy vegetable that tolerates fall frost. Collard looks like an elongated cabbage.
Root vegetables are edible plant roots. Turnips are a cool weather crop that can be harvested any time during the growing season. Sweet potatoes are a warm season root vegetable that has a 150-day harvest cycle, according to "Fruits and Vegetables." Radishes are available in different colors including red and white, and red and black. Carrots are warm weather annuals. Beets are fast-growing and as a cool-season vegetable, can be planted in early spring and then into the fall.