For gardeners, a weed is a plant not valued for use or beauty. For farmers a weed is a major pest problem of cropland and pastures that cause unsustainable economic losses through the reduction of farm crop yields. Weeds are exotic and alien in a garden or a pasture or a field. The word exotic meaning "belonging to another country" comes in English from the Middle French word exotique which comes directly from the Latin word exoticus. The Latin word itself comes from the Greek word exotikos which meaning "foreign". The Greek word is literally translated as "from the outside," derived from the Greek word exo that expresses the idea of "outside". The Online Etymology Dictionary refers to the sense of "unusual, strange" as being first recorded in English around 1620 from notion of "alien, outlandish." A weed in a field or a garden is, then, a plant from outside the system which is foreign to the specified processes of the landscape in question be it a farm field or a garden.
It is, therefore, but a short journey beyond the garden to the wilderness and the realms of Rousseauian nature, indeed, the very word garden is derived from the Indo-European language family's root idea of an enclosure or defined enclosed controlled and managed space. On the other hand, our present ideas of nature include a return to an unbounded, limitless world and an escape from a fenced-in yard. It does not take much to observe the few hitchhiking plants that accompany our flight to nature and freedom. We come not only with our philosophical baggage but also with our companion species together with which we impacting the very pristine unmanaged environment we seek.
It is an easy step from garden weed to invasive plant. An invasive species is any species, according to the US Executive Order 13112, that is “an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health." The term invasive species is further clarified and defined as “a species that is non-native to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health.”
The successful immigrant species is the one that establishes thrives and reproduces in such numbers as to alter the expected outputs, harvest, yields, resources or services of the local landscape or ecosystem. This success and alteration reduces the benefits of the existing system and in extreme cases reduces the inherent biological interactions to a point where the complex emergent patterns of the local environment can no longer sustain itself. This is true in a crop (monoculture) of corn sustained by the strong direct interactions of the famer and his technologies or in the 'unmanaged' wild places connected by large numbers of strong and weak biological interactions from richness of diverse species.
The control and management of unwanted plants in our garden and farms requires an endless dedication to weeding. It is not enough to weed once and then think you are finished for the life of the landscape or crop. Weeding is the first thing a gardener does and the last thing a farmer plans to do the next day. This tireless dedication to the removal of hardy resilient highly competitive plants from our landscapes is the unseen background work of the gardener and farmer. To cultivate a successful garden the weeds must be attended to everyday of every year.
Why, then, should it be different for our natural areas? Why do we set about to remove an invasive species without a plan to return the following day year after year to assess the landscape? What makes us think that after we have removed an invasive plant, everything will return to a self managing system that is not in need of our constant attention? The answer is in our old ideas that there still exist wildernesses that are un-impacted by the works of mankind. Our fragmented natural areas have been reduced to gardens which demand our constant weeding, for as fast as we remove one overbearing newcomer, a new invasive species will arrive to take its place. As gardeners of the wild places we must weed the land and in doing so interact with those species we wish to encourage and yes to cultivate. What we call natural areas are highly complex gardens that who have lost their ability to sustain themselves against the fragmentation of area, dramatic changes in air quality and temperature, as well as increased human visitation and ground compaction.
As stewards of the wild places we will have to manage these ecosystems as well as the novel ones in which we live. We will have to decide policy and actions for managed fields, cultivated landscapes and wildernesses. Those places we decide not to manage will be left to our companion species which have evolved with us since the coming of agriculture. Invasive species will change the ecosystems creating new patterns with which humans will interact. Weeds are the special plant species that are best suited to take advantage of the constant chronic disturbance of the land and the water which arise from the daily actions of 7 billion people. In a very real sense human and their companion species are re-engineering the planet through their constant plowing of the fields that are the ecosystems of the earth. We need to weed the garden that supports us each and every day.
 Beck KG, Zimmerman K, Schardt JD, Stone J, Lukens RR, Reichard S, Randall J, Cangelosi AA, Cooper D, and Thompson JP. 2008. Invasive Species Defined in a Policy Context: Recommendations from the Federal Invasive Species Advisory Committee. Invasive Plant Science and Management 1(4):414–421. Weed Science Society of America. from Subcommittee of the Invasive Species Advisory Committee (ISAC). April 27, 2006. Invasive Species Definition Clarification and Guidance White Paper. 11pp. [accessed September 11, 2012] http://www.invasivespecies.gov/global/ISAC/ISAC_documents/ISAC%20Definititions%20White%20Paper%20%20-%20FINAL%20VERSION.pdf