The nativeflora of the United States includes about 17,000 species of vascular plants, plus tens of thousands of additional species of other plants and plant-like organisms such as algae, lichens and other fungi, and mosses. About 3,800 additional non-native species of vascular plants are recorded as established outside of cultivation in the U.S., as well as a much smaller number of non-native non-vascular plants and plant relatives. The United States possesses one of the most diverse temperate floras in the world, comparable only to that of China.[1]
Several biogeographic factors contribute to the richness and diversity of the U.S. flora. While most of the United States has a temperateclimate, Alaska has vast arctic areas, the southern part of Florida is tropical, as well as Hawaii (including high mountains), and the U.S. territories in the Caribbean and the Pacific, and alpine summits are present on many western mountains, as well as a few in the Northeast. The U.S. coastline borders three oceans: The Atlantic (and Gulf of Mexico), the Arctic, and the Pacific. Finally, the U.S. shares long borders with Canada and Mexico, and is relatively close to the Bahamas, Cuba and other Caribbean islands, and easternmost Asia. There are also rainforests as well as some of the driest deserts in the world.
Some of the native U.S. plants, such as Franklinia alatamaha, have demonstrably become extinct or extinct in the wild; others, such as Micranthemum micranthemoides, have not been seen in decades, but may still be extant. Thousands of other native U.S. vascular plants are considered rare, threatened, or endangered, either globally (rangewide) or within particular states.