On Slate Mountain in eastern Eldorado County, during a backpacking trip at about 4000 feet I find two blackberry species -- species from Europe growing as a weed in America's disturbed areas. One is often known as the Himalayan Blackberry, despite its European ancestry. If you Google "Himalayan Blackberry" you'll find more websites describing techniques for removing it as a noxious weed than praising it. It's RUBUS DISCOLOR, seen at http://ww1.clunet.edu/wf/chap/flowers/fwr-989.htm
This is the same species I wrote about last September when I gushed "I have never enjoyed such wonderful blackberry picking!" And it's true: Its abundant berries are delicious.
The species is easily distinguishable from our natives in that usually its flowers and fruits number over ten per cluster (natives usually have less than ten), its canes scramble up to 40 feet over bushes and into trees (much longer than native canes), and the fruits ripen far later in the season than those of our native species.
The second-most commonly encountered blackberry species seen during my walk likewise was a European invasive, and likewise very different from our native species. Mainly, its leaves were twice-compound, not once-divided into three or five leaflets like our natives, plus the leaflets were deeply cut, or "lacerate," as the botanists say. One English name for it is Evergreen Blackberry. It's RUBUS LACINIATUS and you can see its unblackberrylike leaves at http://www.cnr.vt.edu/dendro/dendrology/syllabus/factsheet.cfm?ID=251
The Evergreen Blackberry didn't seem to be doing anyone any good, but I'm still impressed with the Himalayan Blackberry. Could it be that sometimes, rarely, these invasives aren't so bad? If someone out there has further insight I'd be glad to hear from them.