This might initially sound ridiculous, but it works! So, we all end up with cardboard boxes, we buy things that come in boxes, so invariably these cardboard boxes are "free", can't get more economically friendly than that.
Well, here's what to do with them, Eco-friendly/Green solution: weed mat! You read right weed mat. It works. First spray (not plants, this will kill them too), spray weeds with 3 parts vinegar & 1 part lemon juice (or salt), next put down carboard (once you have removed all tape), it is biodegradable, free & effective! Win, win, win.