Because the definition of weed is not standardized
(Randall 1997), I used four different weeds lists for the
data analyses. There is an order of magnitude difference in
the number of USA weed species among the four lists. The
largest weeds list (3,038 species) is from the Synthesis of
the North American Flora (Kartesz and Meacham 1999)
which defines weeds as “plants that generally grow in
disturbed areas where they are unwanted and unappreciated”.
The second weeds list (1,336 species) is a list of
Invasive Plants of the US from the PLANTS Database
(USDA, NRCS 2002). This list was compiled from weeds
lists from 12 sources (see http://plants.usda.gov/cgi_bin/
topics.cgi?earl=noxious.cgi). The third weeds list (385
species) is a collection of plant species that are either on
the USA Federal Noxious Weed List or State Noxious
Weed Lists (Kartesz and Meacham 1999). The last weeds
list (311 species) is from Daehler (1998) and represents