Landscape Architecture is defined as the science and art of modifying land areas by organizing natural, cultivated, or constructed elements according to a comprehensive, aesthetic plan. The rules that exist in the world of landscaping are virtually non-existent because of the differing possibilities and challenges that each piece of land creates, dealing with contour, climate, etc...
Formerly called landscape gardening, the concept of landscape architecture has been around since the beginning of time, too early in the world's history to even document. However, Egyptians as early as the 3rd millennium B.C. are known to have built walled gardens and ponds in the areas surrounding their homes. In time, these ponds and gardens grew into large, landscaped areas that included orderly rows of fruitful trees and many different species of ornamental plants.

In ancient Mesopotamia, King Nebuchadnezzar II had the Hanging Gardens built in the corner of the palace complex in Babylon to please his wife, who had a love for a mountainous surrounding. Recognized today as one of the seven wonders of the world, the Hanging Gardens includes full size trees built on earth-covered terraces, which are raised on stone vaults. Throughout the rest of the world, the trend continued to grow as civilizations realized the many benefits of landscaping. (An artists rendering of what the Hanging Gardens may have looked like when they were first built is shown below).

Of course, landscape architecture varies from place to place in style, usefulness, and creation. Many of these changes are based on the difference in climates and conditions from one region to the next. Early Muslim landscaping, for instance, centered around the desert oasis and often consisted of gardens built on a grassy platform, surrounded by watery plants and shrubs. Following this trend, it is safe to say that early civilizations landscaped according to the most luxurious and attractive thing that would sustain that particular climate at any given time.
During the medieval time of Europe, many castles included smaller gardens that were protected from the outside elements by walls and overhanging portions of ceiling. Castles featured herb gardens near the kitchen, and another separate garden for the lord and lady to have to themselves. Many monasteries of the time were divided up into different sections for vegetables, herbs, flowers, and fruits.
As time progressed into the Renaissance, the art of landscape architecture moved to include more of a residential setting in addition to the surrounding plants. This was most evident in Italy, where a single architect would design the house and the landscaping around the residence to ensure a harmonious blend between the two. Landscapes and houses alike became very symmetrical, featuring centrally located paths and walkways, as well as geometric flower beds and, whenever possible, areas where one could view the beautiful countryside. It was also during this time of the Renaissance that gardens began to include many spouting fountains, some of very generic design, and others of significant figures, such as the one shown below.
In the 17th century, France replace Italy as the leading nation in landscape architecture when Louis XIV created the palace of Versailles, a vast piece of land that included miles of gardens, fountains, trees, and shrubberies. This garden set the standard for kings and other leader's gardens in the near future. A picture of a garden in Versailles can be seen below.

Later, in
the 18th century, the style of landscape architecture changed once again to a
more freely flowing design. As opposed to the earlier trend of symmetry
and order, landscaping was now created to mimic the natural forms of the
wilderness. Shrubs and trees were place not in rows, but rather in random
places around the land. Rivers and ponds flowed freely wherever the land
took them. This romantic style was introduced to North American by Thomas
Jefferson, who employed the style at his home in Virginia.
In the 20th century and into the 21st, landscape architecture
has shifted once again to include not only individual residence projects, but
entire community projects and even state and national ones as well.
Landscaping also has been broadened and innovated due to environmental
concerns. The field now contains aspects that are both cost effective as
well as environmentally friendly. The art of landscape architecture will
surely continue to grow, mature, and adapt as time goes on, much like we have
seen in the past.