Our native plants grew here prior to European contact. California's native plants evolved here over a very long period, and are the plants which the first Californians knew and depended on for their livelihood. These plants have co-evolved with animals, fungi and microbes to form a complex network of relationships. They are the foundation of our native ecosystems, or natural communities.

Specimens, seeds and drawings of new world plants were taken to Europe by early explorers over many years. Thus, American plants were included in ongoing botanical studies of the world's flora. The science of paleobotany allows scientists to compare fossil records with modern plants to understand which plants are native to an area. While there is general agreement among plant scientists on which California plants are native, qustions arise. Example: Coast ice plant, Carpobrotus chilensis, once thought to be native here, but now widely regarded as an alien.

Our native plants, having evolved here, are ideally suited to perform, such ecological services as manufacturing oxygen and filtering impurities from our water. These plants also do the best job of providing food and shelter for native wild animals. Plants are a cornerstone of biological diversity. Biodiversity is vital to humans, because our survival depends on the earth and its life forms. Native plants are used in the development of new foods, medicines and industrial products. Commercial strawberries were developed using our coast strawberry, Fragaria chiloensis. The pacific yew, Taxus brevifolia, yielded Taxol, an anti-cancer drug. Native plants are also an essential element in the natural beauty for which California is famous.

When non-native plants spread and become established in wild areas, they are referred to as "naturalized." Some, such as forget-me-nots and English daisies, are widespread, yet fairly harmless. But others take over natural areas and out-compete or smother native plants. They can do this because the natural pests, diseases or weather conditions which kept the plants in check in their homeland are absent here. These weeds deprive our wild animals of food and shelter, since native animals cannot make much use of them. Weeds also do damage in waterways, clogging water flow and choking out aquatic plants essential to wildlife. While immigrant people bring needed cultural and ethnic diversity, alien plants and animals can reduce biological diversity! Many weeds belong to the grass, pea and daisy families, with jubata grass, broom and Cape ivy as well known examples.