Some of them, such as purple loosestrife or wisteria, are favored by gardeners for their beauty and hardiness. But when allowed to escape into wildlands, they cause serious ecological and economic damage to many areas. Weeds take over important habitat areas for wildlife, devastating shelter and forage while reducing the diversity and quantity of native plants. When weeds do not hold and protect the soil the way native plants do, erosion increases, causing sediment in streams, which can hurt fish populations and water quality. Weeds are often less resistant to wildfire than are native plants.
Weeds also reduce land values, causing damaging economic impacts to local communities.
For example, weeds have a profound effect on ranching and agricultural operations because they can reduce production of forage and crops. Weeds are often thorny and scratchy, and grow in dense patches, which makes hiking or other recreation activities difficult if not impossible. Weeds increase the cost of maintaining recreational facilities. Weeds are a problem from coast to coast, ranging from invasions such as kudzu, an aggressive Asian vine, in the Southeast, to knapweed, a tough, rapidly invading competitor, in the Northwest.