I occasionally find garlic mustard (Alliaria petiolata) growing in my yard. It's not surprising because it's everywhere.
I keep my eyes open for it, since it's very controllable if you don't let it go to seed. It's a biennial, so I try to make sure none of them make it to their second year, when they can create an enormous seed bank—a stash of seeds in the ground that can germinate for many years.
Where there would otherwise be trilliums, garlic mustard is invading our local nature center
Garlic mustard is alleopathic, exuding a chemical inhibiting the germination of seeds of other plants. I've also read that it interferes with the interaction between trees and mycorrhizal fungi—bad news for our forests.
Garlic mustard may also have an unholy alliance with earthworms, especially problematic in natural areas. The earthworms may "pave the way" for garlic mustard by digesting forest leaf litter (the duff layer), thus creating the bare ground where it thrives.
This dame's rocket (Hesperis matronalis) showed up in my yard one summer. It's alarming to see how easy it is for these invasive plants to travel. Being in a more urban type of suburb, we're pretty far from any location where you'd expect to see this growing.
Unfortunately, this is another of those plants with "pretty" flowers so people are reluctant to get rid of it. But we show no mercy.
It's also very similar to the native phlox (Phlox paniculata), which also grows along roadsides.
For comparison, this is the NATIVE phlox: five petals
How to tell them apart? This trick helps me remember:
Dame's rocket has four petals and "dame," has four letters;
phlox has five petals and "phlox" has five letters.
I had quite a few common daylilies (Hemerocallis fulva) since they were given to me free, and they filled in empty spaces—common excuses. In this case, it's odd that I planted them because these roadside orange daylilies aren't even very attractive.
Back in my ornamental gardening days, I moved on to 'named varieties' (i.e. cultivars) of daylilies, sometimes paying $25 for one plant.
Of course, I diligently divided these so I ended up with quite a few daylilies. Knowing how much I had paid for these, it was hard to compost them, but I've composted most of them. I still have a few of these cultivars, but I've managed to get rid of most of the roadside type, though they still lurk in corners. It's hard to get all the roots out, but I'm working on it.
Bishop's weed (Aegopodium podagraria) is the plant I'm most worried about. It has been creeping under the fence for the last few years, and I can't seem to stop it. This is one that jeopardizes my own yard more than it does any natural areas since I don't believe it spreads by seeds, berries, or wind, just underground, and we aren't close to natural areas.
Even worse, we discovered a new patch that apparently had come in with some soil. Since it was in an out of the way spot at the edge of our yard behind some bushes, it grew to the point of actually flowering before we discovered it. We're keeping an eye on that patch and will cover the mound of soil with black plastic.
What is most alarming is that not only is it still sold, but some university extension services even recommend it as a groundcover!! Sadly, despite the fact that there are some very knowledgeable people associated with these agencies, I always have to take the official recommendations with a big grain of salt since they have dual allegiances to industry and (perhaps less so) to the environment.