Kelly
Nice greenhouse hubby is building for you! If you use mulch, put down a weed barrier first. You will probably have to replace the mulch periodically as it decomposes. One negative aspect of mulch inside a greenhouse is it is more prone to bacteria and fungus. I used very fine gravel (screenings) covered with flagstones since I wanted to retain as much heat as I could, but I live in the Pocono Mountains in PA, and with the exception of just a couple of months, our nights can be quite chilly.
Don't hesitate to ask questions; we all had questions (and still do!) when we got the greenhouse bug.
Jim
