What are weeds?

A weed is a plant that grows in a place where you do not want to grow. Generally, this concept applies to the species that grow on crops.
The weeds have spread globally, associated to crops that characterize so many of the weed species are introduced Navarra, from other places.
Some are part of the flora of Navarra long, while others have very recently joined, like weeds from the rice fields.
Weeds are characterized by their high dispersal ability, very persistent and very competitive. Decrease crop yield, interfere with agricultural structures such as water pipes, or in the process of harvesting and marketing.

Near the weed concept is invasive plant: it is an exotic species with great expandability and reaching alter natural ecosystems of a region, often displacing native flora. Some of these invasive species are weeds. They can be from plants grown as ornamental plants or be accidentally present in other crops.
Some are parasitic weeds such as Cuscuta, or broomrape (Orobanche sp.); however, are not all parasitic weeds, and some appear to not restrain their host, as with Cytinus ruber, a parasitic cistus white flowers (Cistus albidus).