Just stumbled across this thread while looking for hemlock seeds (I'm growing a poison garden, in case you're wondering)
Yup, that definitely looks like hemlock. It can be quite unpredictable in the size and shape that it grows. For example, outside my parent's house in Suffolk, England, there is a lovely bush, plush and green with lovely white flowers. A quarter mile up the road, there is a small forest of it by a field entrance. So you can never tell by the growing habits as such.
Cow Parsley (not sure if that grows in the US) usually grows only to a height of 2ft max. QAL can grow to 6ft.
There are three good ways of telling if what you've got there is hemlock
1 - The stems will have red or purple blood splatter. It's not actually blood, of course, but it looks like an axe murder has taken place nearby and the blood has spattered all over the stems.
2 - In hot weather, hemlock smells like a mouse cage, or stale popcorn
3 - Crushing the leaves/stem will give off this potent mousey-popcorny smell
4 - Take a chunk out of a stem and eat it*
. In ten minutes, if your extremities are getting numb and the feeling is moving slowly but surely towards your torso......then yup, its hemlock (and you're probably about to die)
* - seriously, DO NOT DO THIS, if it is hemlock, it WILL kill you.
However, don't be too horrible to the plant. Cultivated properly, it can be a very attractive looking shrub. Like I said, I want one in my poison garden. It looks nice, grown properly.
It is invasive but again, with proper cultivation and maintenance, it can be controlled. How many people have mint plants on their gardens? Loads, and that plant is even more invasive than hemlock.
It's smell is unlikely to make it desirable to small children to pick and eat. Animals avoid it for this very reason.
And think of this....90% of the plants you have in your garden are in some way toxic...some more than others. Of that 90%, 30% of those will be deadly.
That's not to say don't be careful around it. It will die off in the Fall (Autumn...wherever in the US you are) but the stems are still highly toxic, as are the seeds. Sprouting shoots of hemlock are easily killed with weedkiller...you gotta love the irony.
If you are intent of getting rid of it, be careful. Glove up. Inhalation of the pollen might make you a bit nauseous, but it won't kill you. What you need to worry about is getting any of the juice into open wounds. the juice isn't absorbed by the skin, but get any into cuts/grazes/sores/any other open access to the bloodstream and it could have the same effects as ingesting it by eating.
It is safe to touch it. Sniffing it is OK but I wouldn't recommend taking great big lungfulls of it like you might with a rose, Mainly because it doesn't smell that great.
Mostly, don't hate it. It's not trying to kill you. It didn't pop up out of the ground in your garden thinking "I'm deadly poisonous. I'm going to kill that person, muhuhuhhahahahahaaa!". It's a defence mechanism to stop deer and stuff eating it. Much like a saguaro has big spiky sharp spines.