A tap root, also spelled "taproot," is a large, thick root that generally grows straight down from a plant in order to collect water and minerals from deep in the soil. In most cases, small, fibrous projections grow horizontally outward from the large tap root. Many types of common plants have tap roots, including dandelions, carrots, turnips, and some types of trees. In contrast to tap roots, fibrous root systems are composed of a multitude of small, branching roots that grow outward from the plant and tend not to go deep into the ground. Grass, clovers, and marigold plants are common examples of plants with fibrous root systems.
Many commonly eaten vegetables are actually roots that grow underground as opposed to the other types of vegetables that are actually leaves or stems of plants. A carrot, for example, has a tap root — its large orange root is eaten while the rest of the carrot is not. Similarly, the roots of parsnips, beets, radishes, and turnips are all eaten. In some cases, as with carrots and parsnips, the whole tap root is eaten. In others, such as beets and radishes, it is common to eat the bulbous section at the top of the tap root but not the rest of the root.
Tap roots tend to grow very deep into the ground and often have many small branching tendrils that extend from the main root, so they can be quite difficult to uproot. As such, removing weeds or transplanting other plants that have such roots can be quiet difficult. Failure to destroy a weed's tap root, for instance, generally means that the plant will just grow back within a few days. Common weeds with tap roots include dandelions and plantains. Removing such plants is difficult, and one must often dig a wide circle around the plant in order to uproot it.
A young tree is likely to possess a taproot, but most trees tend to develop shallow fibrous root systems as they age. Hickory trees and some other types of trees do maintain tap roots even as they age, however, and such roots can grow quite massive and quite deep. The branching roots of most trees actually help them to stay upright in spite of wind, rain, and erosion. For such conditions to successfully topple a tree, they would need to cause substantial damage to the soil at least several feet (a couple meters) around a given tree.