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I just got an order today shipped by freight line. We buy burlap from dayton burlap and bag . Here is there web site http://daybag.com/
If you are using 10 x 20 trays and you keep the burlap moist you would not need soil for growing. That would make the mess of cleaning for you very easy. I would put plastic over the tray until you see the first leaves showing and then take it off and allow to open air grow but you would have to keep the burlap moist (not flooded). I would keep an end of the tray visible for about an inch and water the tray up to the ribs that the burlap is sitting on and the water would wick up into the burlap keeping the canopy dry .But you could water threw the canopy if you wish.