If the planter is small enough, if it gets too cold out at night, you could always bring it inside. That's what I'm doing with my containers.

Yep, I was thinking I would throw a blanket over it rather than try moving it. It's plenty heavy. I guess that leads to a more specific question... What's the cold tolerance of those specific annuals? Do I only need to worry about actual frost? My goal is for them to thrive, not just survive.

An actual frost can turn impatiens to mush, but they can tolerate the evening temps we have been getting lately (40+), lets all hope that the last frost has already occurred cuz I can feel the urge to plant swelling up and may not be able to hold out much longer ;-)