<p>In 1993, an estimated $4 billion was spent to control the spread of over 1400 <a href="http://www.finegardening.com/plants/articles/invasive-plants.aspx" title="Link to article">non-native pest plants</a> nationwide. Most of this was spent killing agricultural and rangeland weeds, but much of it went to fighting the 450 or so exotic pests originally brought here as ornamental garden plants from Europe, Asia, Africa, and Australia. Many of these plants are still being sold in nurseries today.</p>