The 1,000­-foot-­long terrace, or garden plateau, was literally hewed from the side of the mountain with slave labor between 1806 and 1809. Supported by a massive stone wall that stood over twelve feet in its highest section, it was described by one visitor as a "hanging garden." The 2­acre garden was divided into twenty­four large "squares" or plots that were numbered and, at least in 1812, arranged according to which part of the plant was being harvested –; whether "fruits" (tomatoes, beans), "roots" (beets, carrots), or "leaves" (lettuce, cabbage). The vegetable garden was a also kind of laboratory where Jefferson could experiment with imported squashes and broccoli from Italy, beans and salsify collected by the Lewis and Clark expedition, figs from France, and peppers from Mexico. Although he would grow as many as twenty varieties of bean and fifteen types of English pea, his use of the scientific method selectively eliminated inferior types: "I am curious to select one or two of the best species or variety of every garden vegetable, and to reject all others from the garden to avoid the dangers of mixing or degeneracy." The site and situation of the garden enabled Jefferson to extend the growing season into the winter months and provided an amenable microclimate for tender vegetables such as the French artichoke. Because of favorable air drainage on a small mountaintop, late spring frosts are rare, and the first freezing temperatures in the fall rarely occur before Thanksgiving. A particularly warm environment was created in the Northwest Borders by radiating warmth from the grassy bank of the slope below Mulberry Row. Jefferson used this border to plant tomatoes, cucumbers, and peas very early in the season. In an annual contest with his neighbors to see who would first bring peas to the table, this setting gave Jefferson a decided advantage.