I have seen bittercress form a solid carpet or sod in a garden, obliterating all plants but the rose bushes. I our yard it will displace the lawn (moss) if left unchecked. Also, seeds into pots and will out compete seedlings in flats of winter sown species.
We don't have a problem with thistles or poison ivy, and what little poison ivy we do find I can pull by hand without repercussions. Lucky that way.
Some day Jill, stilt grass will invade your yard. On a weeds course field trip we were shown a section of Patapsco State Park where it had completely eliminated understory growth, suppressing even tree seedlings.
