Hi Maxwin,
From what I've read, it would seem that you only need to pull apart a small fragment and you'll have a nightmare on your hands in a short space of time.
http://www.wes.army.mil/el/aqua/apis/mechanical/html/ludwigi 0.html
http://www.wes.army.mil/el/aqua/apis/plants/html/ludwigi1.ht ml
There is apparently a lot of confusion about which are natives and which are foreign invaders in the US. Take a look here.
http://www.nwcb.wa.gov/weed_info/primrose.html
And to quote the above site:
"Water primrose was placed on the 1997 Monitor List to gather information on distribution and plant biology. Water primrose is known and sold as an ornamental pond plant. In King County, this plant was found growing in an outdoor nursery pond. A single cutting in 1997 resulted in this plant taking over that pond. The King County Weed Board requested that this plant be placed on the 2000 state noxious weed list. San Juan County reported finding this species for sale in 1999. The nursery owner agreed to not sell it since it was being proposed for the 2000 state noxious weed list."
I don't think it's hardy in your zone, but I wouldn't recommend that you share it with anyone in any of the shaded states listed here.
http://plants.usda.gov/cgi_bin/plant_profile.cgi?symbol=LUUR
You might also want to ask these folks if they can grow it.
http://forums.gardenweb.com/forums/rmgard/
Hope that helps.
Newt