[1] It is native to Eastern Europe and Western Asia, specifically the nations of Turkey, Syria, Bulgaria, Greece, Romania, Ukraine, and southern Russia.

Diffuse knapweed is considered an invasive species in some parts of North America, having established itself in many areas of the continent.

Areas with large amounts of shade or high levels of water discourage diffuse knapweed growth.

By 1998 diffuse knapweed had naturalized over 26,640 square kilometres (10,290 sq mi) in the western US, and was increasing its range at a rate of 18% annually.

A study in 1973 concluded that ranches lost approximately US$20/km2 (8 cents per acre) of diffuse knapweed due to decreased grazing area.

[4] Research based on simulation models have shown that for biocontrol agents to be effective, they must kill their host, otherwise plants can compensate by having increased seedling survival.

[4] Physical control of diffuse knapweed primarily comprises cutting, digging or burning to remove the plants.

Digging this removes both the portion above ground and the root of diffuse knapweed and has shown to be very effective; if the plant is properly disposed of, it can neither regrow nor spread its seeds.

However, precautions must be taken to first ensure that the fire is properly contained and that a new plant community is established to prevent the reintroduction of diffuse knapweed.

Areas such as fallow land, ditches, rangelands, residential and industrial districts and roadsides are all disturbed habitats where diffuse knapweed frequently establishes.

Additionally, the removal of foliage and other ground cover increases the likelihood that seeds will come in contact with the soil and germinate.

[citation needed] The largest impact of humans on diffuse knapweed is efforts in controlling and eradicating its invasive populations.

Besides reducing the spread of diffuse knapweed, efforts are also providing selective pressure against the individuals that cannot withstand a certain method of control.

However, the most importance must be attributed to the ERH because diffuse knapweed, while a very effective invasive species in its novel environment, is non-invasive and doesn't establish monocultures in its native range.

[citation needed] To demonstrate that the ERH applies to diffuse knapweed, it is essential to show that the absence of natural enemies has a significant positive effect on its success.

[citation needed] Another aspect of diffuse knapweed's success relies on the effect of its allelopathic chemicals in its novel environment.

[citation needed] Besides the advantages that diffuse knapweed gains from the ERH and allelopathy, it also possesses several characteristically invasive traits.

One factor leading to the superior resource competition of diffuse knapweed is its ability to exist in drought conditions.

A higher density of knapweed will not only increase the concentration of allelopathic chemicals in the soil but will also restrict the nutrients available to native plants.

[citation needed] Diffuse knapweed is successful in its novel range primarily because the organisms and conditions that prevent it from becoming invasive in its native environment are absent.