Vegetation refers to the amount of plants, herbs, trees, bushes and other wildlife in the world. Out of the wastelands explored in the Fallout series (eg. New California), vegetation is rare and exists in the rare form of cacti and small weeds. This is because of the nuclear winters and nuclear bomb dropping making the sands of the earth an inhospitable place.
In New California, vegetation is rare due to the amount of nuclear missiles dropped. This makes any herbs, trees or wildlife found precious to some, though worthless to others. However, by the time of Fallout 2 vegetation and wildlife have become common with many plants and herbs becoming available. However, they are by no means healthy as some will attack wanderers and some are poisonous.
Areas like Shady Sands and Vault City, however, have managed to reintroduce wildlife such as bushes, trees and grass to the wasteland. Vault City did so with their G.E.C.K; how the New California Republic managed to do so is currently unknown.
The only pure water source in New California is the Colorado River, which after flowing from the Mojave Wasteland, enters Baja California and California, both part of New California and both part of the NCR. A reason why plants may have started growing more abundantly in New California in the recent past is due to the discovery of the Colorado River and a source of pure water.
The Capital Wasteland is a wasteland that stretches from Raven Rock to Rivet City and also comprises of the ruins of Washington D.C. However, the bulk of the Capital Wasteland, which is largely inhospitable, does contain some plants and a large forest grove which is home to the Treeminders, has now gained access to the Potomac River which potentially can be purified. With this, the Brotherhood of Steel will likely add more vegetation or let it grow on its own. The Capital Wasteland seems to have a mix of vegetation between New California and the Mojave Wasteland.
The Pitt, however, is seemingly incurable with no traces of vegetation whatsoever and with water in its river that seems to be far from pure, though a similar treatment as done on the Potomac would make it somewhat less mutated.
While areas like Point Lookout may have large quantities of vegetation, most of it is mutated and unsafe for wanderers, travelers and those looking to make communities.
Due to no bombs being dropped directly at the Mojave Wasteland, plants are not uncommon. A wide variety of cacti, flowers and herbs live around the Mojave Wasteland. In New Vegas itself, there is a wide variety of different plants as well as farms (sharecropper farms) located on the outside. In fact, in some areas, (like Vault 22) plants vigorously thrive.
The Mojave Wasteland contains the Colorado River which then further flows into New California. A reason why plants grow more abundantly in the Mojave Wasteland than any other wasteland is partly due to House, the NCR and the Legion's access to the source of pure water. Due to this, the Mojave Wasteland is the most ecologically developed wasteland.