As some food for thought, the phrasing I've always read is that it's illegal to trade/ship/transport noxious weeds (NW's) across state lines - the way I read & understand it, doesn't that mean that if you have a plant that's brought within state borders before its added to the noxious weed list, aren't you essentially free to cultivate it as you please (as long as you don't intentionally introduce/naturalize it into the wild) and ship/trade it within your state borders?
From a more legal-ese perspective (i.e. prepare for long-winded blather), punishing you for possession of a live plant obtained before its recognition as a noxious weed would be, essentially, a violation of the ban on ex post facto laws (U.S. Constitution, Article I, Section 9) - or, in other words, you can't be guilty of committing a criminal act if the act wasn't considered criminal when you committed it.
Furthermore, since the U.S. Bill of Rights (Amendment V) protects citizens from being deprived of life, liberty, or property (i.e. a plant you bought) without due process of law (i.e. as a result of a criminal or civil court verdict), and further prevents the confiscation of private property for public use without "just compensation" (this is usually the basis for imminent domain situations, but the prevention of spreading an invasive species is, very arguably, a "public use" of a privately owned posession), the state/federal government theoretically can't confiscate or destroy your legally owned plants without paying you their fair market value.
Wow, that was probably way too technical and all, but in a basic list format:
1. You can't be punished for buying plants that are legally tradeable, even if they're outlawed later on.
2. The government isn't allowed to take your stuff without paying you for it.
3. You have the right to do as you see fit with legally-obtained (non-animal) property (except in scenarios where possession is, in itself, illegal - i.e. narcotics laws) while it's on your private land.
4. A ban on trading NW's across state borders doesn't appear, IMO, to stop you from trading it within state borders.
5. Thus, once you buy/receive/collect a plant, even if it's later declared a NW, there would appear to be no reason you can't grow it, as long as you don't send/sell it someplace you're not allowed to.
I could be wrong about that, in which case, please point me to any contradictory legal reference so I can read it and reevaluate my understanding, but from what I see, that's the situation as it stands in U.S. law. (But then again, I'm not a lawyer - just a fairly smart college kid - so take what I've said with whatever grains of salt you deem appropriate.)
On another note, for the life of me I don't really see the point in declaring anything as invasive/noxious on a national level. Given the wide range of habitats, climates, etc. in this nation, really these things should be left up to states to decide IMO. I see no reason that trading H. polysperma between, say, CT (where I live) and RI or NY (since they have the same approximate climate) needs to be illegal - as Dielectric said, it would never winter over here and therefore can't establish itself invasively. The same can be said about piranha - even if one were released, it's not like it would survive more than a few months, nor would it be able to breed enough in one season to have a swarm "attack" someone (I won't even go into how that stereotype is so much nonsense) - so why does the gov't feel the need to outlaw selling them?
And my third (and final) point to make, in the case of state invasive/noxious plants, the existence of laws preventing people from collecting them where they've invaded (as mudboots mentioned) is really, really, downright stupid as far as I'm concerned. If the government considers them a problem 'cause they're not native, wouldn't it be a great control method to encourage as much private harvesting as they can - especially for commercial, out-of-state trade purposes? I mean, from any reasonable perspective, all that happens to collected invasive specimens is that they end up leaving the wild population - whether they're kept contained indoors or sold to people in another state, the end result is that they can't spread/propagate in state lands if they're all removed. Then the state gov't doesn't even have to pay anyone to get rid of them o.0' It seems like a win-win situation no matter how I look at it. People get to obtain plants they want, commercial harvesters can make money, and the plants aren't the public's problem anymore. Maybe pass a few regulations to prevent the disruption/disturbance of native species in the same habitats, or whatever, but by all means they should practically be begging us to help get rid of their invasive crypts and hygros and whatever.
...Bottom line is that bureaucracies are really, really stupid sometimes. And now I'll shut up and go find something else to ramble about.
-Amanda
