Fruits and vegetables can grow either above ground or underground. Vegetables grown underground still grow stems and leaves above ground, but are mainly used for the edible parts pulled from the ground. Vegetables and fruit grown underground can often be planted in conjunction with bush or vine vegetables since the edible portions grow in opposite directions, such as carrots and tomatoes, or onions and peas.
Look for a variety of vegetables in this category. Root vegetables are edible roots of plants which store more food than is required for them to grow; this causes the roots to expand and grow. Examples of root vegetables include carrots, beets, parsnips, rutabagas and radishes. Root vegetables grow in cool climates later in the season. Plant root vegetables in loose soil and full sun to ensure the best growth.
Bulb vegetables are used the same way you use root vegetables. The best known bulb vegetable is the onion; related vegetables include garlic, chives and scallions. Bulb vegetables have a very strong flavor and smell when fully grown. Look for bulbs to be made of tight thin layers of skin, often iridescent. The leaves are tightly folded and attached to a short stem. When the greens above ground begin turning brown, this is a sign that the bulb is mature and ready to harvest. Use bulb vegetables for soups and salads when you want a stronger flavor.
The best known tuber vegetable is the potato; potatoes to come in many varieties. Plant tubers where there is ample room or on a hill because these vegetables grow as a vine and can reach up to 3 feet in length or height. Star-shaped flowers of white, lavender, light blue or pink petals with a yellow center are followed by small green fruit; these are the signs that the vegetable is ready to harvest. Only use the tuber (root) from these plants because the remainder is poisonous.
Peanuts are classified as a legume, and therefore a fruit. This makes the peanut the only fruit found growing underground. The peanut plant flowers above ground, but produces fruit below ground. The plants produce delicate flowers at the base of the plant which self-pollinate, then lose the petals; the ovary enlarges and produces a peg which turns downward and grows into the ground. The peanut then forms underground in a hard shell. Up to 40 peanut pods per plant grow over four or five months.