Poison sumac, maybe?

It's possible there was a caterpillar on it with stinging hairs?

Now how can I be sure I get rid of this VERY nasty plant?

As for getting rid of the plant, how about putting on heavy gloves and pulling it?

Saddleback caterpillar was my first thought. Those things HURT!

It looks like a Mimosa to me. Mimosa nuttallii is the only one that would be found in your zone. Has it ever flowered?

The plant is so spindly that I would probably have noticed an 'asp' - as those nasty hairy caterpillars are called locally. And it's too late now to go check. Does not quite look like a mimosa - the leaves are not as fine - too little still to have flowers - hoping to have it OUT of the garden before it does flower! I'm going to research the sumac option. THANKS to you all!

Is there any chance of a better picture? It's really hard to make out any details of leaves, etc. However, they do look like folded mimosa type leaves.

I really don't think it is the poison sumac ... that usually lives in wetlands.