The deer population in many parts of the U.S. has increased enormously in recent years as humans have populated rural areas and killed or driven away animals that were deer predators. In many areas, deer no longer have any predators and are less and less nervous about human contact. At the same time, animal-rights groups have succeeded in limiting or prohibiting hunting.
In the early 1900s, there were an estimated 500,000 white-tailed deer in the United States. Today there are more than 20 million, and the numbers are rising rapidly. Birthrates are responsive to the lack of adequate forage, but this effect does not happen quickly. As a result, in many suburban and formerly rural areas, the deer population has exceeded its healthy limits, resulting in stunting of offspring and increased damage to household landscaping and commercial plantings. Vehicle accidents involving deer have also increased.
There are two kinds of deer repellent: those that depend on unpleasant odor, and those that use unpleasant taste. According to Cornell Cooperative Extension, in New York state, "Hinder, an ammonium soap-based repellent, and Deer-Off, a product that incorporates putrescent egg solids, are the only repellents currently approved for use on garden vegetables and fruit-bearing trees during the growing season." Most other states have similar restrictions.
Some gardeners use home remedies that include bloodmeal, various kinds of urine (coyote urine is a favorite), human hair, Irish Spring soap and hot pepper spray. Dogs left loose on a property can deter deer from visiting the area.
Repellents are usually applied as sprays, often with the addition of a spreader-sticker ingredient. Some repellents are supplied dry, in small cloth bags for hanging on or near threatened plants. All repellents are subject to dilution by weathering and have to be replaced on a monthly basis or following rainfall.
Commercial-grade electric fencing, properly installed and maintained, can be moderately effective against deer. It is important that appropriate energizers are used and that the fence is installed correctly. Panicked deer will jump over or through an electric fence. An 8-foot electrified mesh or a system of double fences---with a 3-foot space between electrified wires set at different heights---has been shown to work unless feeding pressure is very high.
In areas where it is permitted, hunting is effective in controlling deer populations. Trophy hunting is less effective, since it is the females (does) that are responsible for population increase. Population control by selective hunting of does will reduce the population but will not prevent all damage to ornamental and agricultural crops. Birth-control programs are expensive and have not been shown to be effective.
Deterrents, repellents, dogs, electric fencing and hunting can all be moderately effective, at least for a while. A total physical barrier, in the form of an 8-foot-high woven wire fence, tightly strung on 10-foot-high posts with a single visible wire on top of the posts, remains the only method that has been found to be completely effective against deer.