A native plant is, quite simply, a plant native
to the area in which it is planted. This word is used quite a lot in habitat restoration
and other related fields. Native plants are useful when one wants to restore an area to its natural state, and are also excellent in landscapes because they require little care and shelter wildlife. However, defining which plants are 'native' can be difficult.
You can't always figure out which plants are native by going out to a 'natural' site and looking at what's growing there. Often, many of the plants in nature are introduced, weedy plants which don't really belong there and have been brought in from other areas. These are not considered natives and are often pests, crowding out the true natives. In America, a native plant is generally considered to be one which was there before the Europeans colonized the continent. This ignores the fact that Native Americans moved plants around too, but it is impossible to tell what they moved anyway. In other parts of the world it is even more complicated
Another issue is the scale of 'nativeness'. Often in California, for instance, a native plant is just considered to be a plant from California. This gets silly because California is a huge place. A cactus is not native to the redwood forest, nor is a palm tree native to the High Sierras. If you plant them in these places, they will probably die. However, it is often impossible to tell exactly what was growing at a site 400 years ago. Some areas are extremely variable on a small scale due to variations in moisture, temperature, and elevation, especially in mountainous areas.
In general, the concept of native plants is complicated. However, native plants are a useful and often neglected addition to landscapes, and the confusion shouldnt discourage one from using these plants.