I think it's a great idea to build a hoophouse over your hugelkultur, or any kind of structure that captures heat, such as cold frames, tunnels, etc. After the considerable investment of time and energy in developing your hugel bed, it makes sense to do something to help capture the heat generated by decomposition. Most of my hugel beds are buried underground. I've tried a few different approaches toward the goal of capturing their heat. One was an underground cold frame, with about 2 feet of hugel under it and hugel all around it. You can check this one out in the two-part blog post, starting at http://woodforfood.blogspot.com/2014/04/underground-cold-frame-part-i.html
. It worked really well in that my swiss chard overwintered here in North Central WA State! The downside is that I think I made it too deep and it doesn't get much direct sun all the way in the bottom of the pit. If I did it again, I'd make it longer in the direction of the path of the sun, so that there was more real estate beyond the shadow of the south wall. It was a good learning process. Even a traditional cold frame on top of a hugel bed is a good idea; this week I put one on top of my horizontal spruce hugel bed (http://woodforfood.blogspot.com/2015/02/cold-frame-fun.html
). Now I'm working on building simple, larger covered frames for some of the other beds and can post again when I have those done. Season extension is important where we live and hugelkultur can help make it happen.
How did it go with your hoophouse Paulo?