Is it okay to place my flattened cardboard next to the green cart?

Yes! Smaller (flattened) cardboard items can go right in the cart (so break down those boxes). But if you have a heap of cardboard, here’s what to do: break it down, cut it into pieces less than 3' x 3' (about the length of a guitar), and stack it next to your cart at the curb. Remember to chuck any non-recyclable packing material into the regular blue trash cart.