Aggressive and enthusiastic may be debatable points, but invasive has a very specific meaning. The major difference between invasive and aggressive seems to be the plant’s ability to establish itself in the wild and disrupt the native ecosystem.
According to Federal Laws and Regulations Executive Order 13112, “"Invasive species" means an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.” That’s still pretty broad. We can probably all agree that purple loosestrife is harmful to the environment, but you’d have a tough case getting easterners to forsake the English ivy that’s become such a problem in the Northwest.
A reader asked me recently why bamboo is considered invasive when it is so easily killed by stepping on the young seedlings when they come up. If we all did that, bamboo probably wouldn’t be invasive. But non-native plants with no natural predators that are able to establish themselves, can change the environment for the worse, as is believed to be the case with purple loosestrife.
The good news comes in the form of the “tens rule”, put forth by Mark Williamson and Allistair Fitter in 1996. According to the “tens rule”, 1 in 10 imported plants will eventually be found in the wild; of these 1 in 10 will establish a self-sustaining population; of these, 1 in 10 will become a pest or invasive. So for every 1,000 imported plant species, 100 may escape into the wild, 10 may establish themselves there , but only 1 will become a pest. Of course, there are always exceptions.
OK. So what’s a gardener to do? Learn to identify the invasive species in your area. Check before you plant something newly introduced. And if you should find an invasive on your property, remove it. The National Arboretum has some more tips and cautions on dealing with invasive plants.