I don't know what you have around your house, but what I have are pine needles. Those and broken pine branches would look good and cover the soil.

Do you have any leaves around your yard? That might also work.

How about wet newspapers? They will stay flat when wet and when you're ready to plant, just chop them up into the soil. They retain water nicely and worms love 'em. Happy Spring and Happy Growing.

Debra

Wet cardboard and/or wet brown paper bags would work too. Depending on the size of the window boxes, a brown paper bag might fit nicely laid on its side, filled with newspaper or cardboard or leaves or pine needles.....

Claire

Great ideas. I knew you would come up with something.

I think I will go with brown paper grocery bags cut to fit.
Will put a rock on it to keep it from blowing off until it gets wet from rain.

If you have any bark mulch, you can scoop up a bit of it to cover the top layer of soil.

Spanish moss (available at garden and craft stores) is good too. And when you're planting, you can tuck it around to hide bare soil and any empty spots and also let it drape over the edge here and there if you like that look.