SOME OF THE DISADVANTAGES

Weeds are plants that grow where people do not want them. People have spread plants beyond their native ranges and some of these introduced plants become invasive, damaging existing ecosystems by displacing native species. Invasive plants cause billions of dollars in crop losses annually by displacing crop plants, they increase the cost of production and the use of chemical means to control them affects the environment.
Plants may cause harm to people. Plants that produce windblown pollen invoke allergic reactions in people who suffer from hay fever. A wide variety of plants are poisonous. Several plants cause skin irritations when touched, such as poison ivy. Certain plants contain psychotropic chemicals, which are extracted and ingested or smoked, including tobacco, cannabis (marijuana), cocaine and opium, causing damage to health or even death.[21][22] Both illegal and legal drugs derived from plants have negative effects on the economy, affecting worker productivity and law enforcement costs.[23][24] Some plants cause allergic reactions in people and animals when ingested, while other plants cause food intolerances that negatively affect health.