Dog violet (Viola conspersa), a common weed.

A weed is a plant that people think is bad, because it is growing in the wrong place. Trees are not often called weeds, only smaller plants.

A person may grow a plant because it has beautiful flowers, but other people may think that this plant is a weed for some reason. This may be for one of these reasons:

• it has pollen that makes them sneeze,
• it spreads seeds which will grow quickly,
• the roots go too far under the ground and can cause damage to houses or pipes,
• the roots spread quickly,
• the plant has poisonous berries,
• the plant has liquid (sap) in it which can hurt a person's skin,

If someone is farming a plant and it then moves outside the farm, the plant may then be called a weed.