A study reported at prairieecologist.com looked at the growth of bull thistle (Cirsium vulgare) and smooth brome (Bromus inermis) in 24 experimental prairie plots. Half of the 3/4 acre squares were planted with 8 grass and 7 wildflower species. The other half were planted with 100 species. They found that the more diverse test plots contained less of these invading plants.
Bull thistle and other invasive thistle species are difficult to control due to the thousands of light feathery seeds the wind disperses over the field. In many states like Kansas, the persistent presence of them in your field can lead to a daily fine. Controlling them requires early spraying of the flat rosettes and later pulling or digging them out before they go to seed. Anything that reduces their spread is welcome.
A separate pilot project studied these areas for the presence of poison hemlock. Walking transects and counting the number of plants one meter on either side of the line, they came up with the rather impressive figures illustrated below.
|Poison hemlock stand|
The other obvious message is the importance of maintaining diverse species where they occur naturally. While "nature abhors a vacuum", most invasive plants love one.