Herbarium, University of California, Berkeley
The history of plant introductions, now seen around the world, began after the development of agriculture and civilization about 10,000 years ago. Two groups of plants were deliberately carried by the human immigrants for their usefulness. The earliest were agricultural plants. Second were ornamental plants, these came much later. Through all of the centuries of migrations of people and plants there was an accompanying third group, the weeds, those were carried accidentally. Weeds will be defined and placed into several categories, as benign, aggressive, and noxious. Migrations of people and plants into California and the changes which they brought began with arrival of the Spanish in 1769 at San Diego. During the following two and one-quarter centuries, the number of immigrant plants into California brought changes to the state's vegetation patterns and added many non-native plants to the flora. These changes, the result of people's migration and activities, make it apparent that people have become the great distributor of plants around the world. I will close by discussing some of the plants introduced into California.