I live in Illinois and just moved in to a new house with a big brick flower box that was full of weeds. Is it too late to plant flowers? It's a large space, so I need something that is on the larger side and that will grow fast.

I don't think so, but you could find out on a flower package it usually has the times where you should plant. Maybe you should just get plants that have allready bloomed insted of seeds. Happy Gardening

it is almost never too late to plant flowers. there is plenty of time left where you are to plant flowers especially if you plant them from starters by buying them in flats.