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Defoliation is a term which is used to describe the removal or loss of leaves. People usually use it to mean “total defoliation,” meaning that all of the leaves on a plant are lost. Loss of leaves happens on an annual basis for deciduous trees, and it can also be artificially triggered by people, for a variety of reasons. In some cases, defoliation can be accomplished by hand, while in other instances, it may be orchestrated with irrigation control, and in some situations, it may be accomplished with chemicals.
One of the most common reasons to trigger leaf loss is to make it easier to harvest a crop. Defoliation makes the usable part of a crop more visible and easier to see. It also reduces losses caused by insects, moisture, and other problems, and facilitates the use of automated harvesting equipment, sparing the expense associated with hand harvesting. This type of defoliation is usually managed with controlled irrigation practices which encourage plants to drop their leaves.
Defoliation may also be done to control an outbreak of disease. Some pest infestations can only be controlled by removing leaves and hoping that the plants survive, and removal of leaves can also be used to control the spread of fungi and other disease causing organisms. This type of defoliation may be treated as a last resort, when other options have failed.
Chemical defoliation has historically been a weapon of warfare. Dropping defoliants onto enemy territory accomplishes several goals which can further a military objective. The first is the destruction of food crops, which can cause social unrest and lead to pressure to end the war. The second is the deprivation of cover; with no leaves to hide behind or under, enemy soldiers will be easier to identify. This tactic can also be demoralizing, which undermines public support for the war.
In bonsai management, defoliation may be used as a tool to control the leafing patterns of some cultivars. This technique is limited to specific cultivars, and the plant must be healthy before defoliation. In this case, each leaf is snipped off with scissors, with the pedicel of each leaf being left in place to provide nourishment until the bonsai recovers. Before defoliating bonsai, it is critical to make sure that a plant is of an appropriate species, and that the time is right for this practice: plants need to be extremely healthy, and it needs to be the right time of year, usually late spring or early summer.