Most weeds establish and thrive in places where the soil has been disturbed in some way. When soil is exposed by machinery (e.g. cultivation or grading), burning of vegetation, herbicide application, over grazing or the simple action of a tree falling over to expose an area of exposed soil around the roots leaves the soil open for establishment of colonising plants. In an intact and healthy ecosystem (no introduced species or processes) native colonising species are able to take advantage of the exposed soil and revegetate the area. This is a natural process and these colonising species with highly mobile seed and short life spans give way to more permanent longer-lived species. Many weed species are very effective colonising species that have modes of spreading seed (blown in the wind, sticking to the fur of animals etc.) to also take advantage of areas of disturbed soil to take root and proliferate.
Key to the successful eradication of a weed infestation and the prevention of future re-infestation is for other more desirable plants to occupy the space that could otherwise be available for undesirable weed species. If the soil and water resources of a given site are fully utilised by for example a patch of native grass, there is no space or the essential resources of nutrients and water available for an undesirable species like Chilean needle grass (Nassella neesiana ) to establish. If an area of infestation has been controlled by herbicide or tillage, the long term eradication and success of control efforts is highly dependent on the establishment of a more desirable species to occupy the site and therefore render it unavailable for re-infestation.
Nutrient and site preference
Certain species have a competitive advantage over other species due to their tolerance of salt or the ability to thrive in areas of depleted nutrients or low water availability. Spiny Rush (Juncus acutus) is a good example in that it can thrive in areas that are too salty for many other species enabling the total domination of low lying salt effected land over time. Spiny Rush does not necessarily prefer such areas but can out compete other species due to its salt tolerance. Once dominant in an area, Spiny Rush, due to the huge amount of seed produced will spread beyond this area of low competition into more fertile and productive soil.
The low nutrient content of soil can also permit weed species to dominate due to being able to thrive in areas that do not have high enough nutrient levels for other species. Onion Grass (Romulea rosea) is a prime example, being able to thrive in areas of low fertility where more desirable pasture species cannot. The addition of fertilizer to such an area permits other species to also thrive and compete with the Onion Grass for space and resources. The addition of a specific herbicide control in this situation can result in a dramatic turnaround in species composition and productivity.