Hi all, I'm several years into deep mulching some gardens - 12 inches deep. Here is what I wished I knew years ago, since I failed at many things to learn this:
First year - don't even try smaller annuals. Go with
1.) potatoes and sweet potatoes. Using a pitchfork, plunge it into the woodchips - pry it back and slip a potato in between the ground and the woodchips. The sweet potatoes did especially well.
2.) Bush beans. Just plant them an inch deep into the woodchips. For me, they grew right down through all the wood chips and we had a great harvest.
3.) Big vigorous plants like squash and watermelon. Instead of making a row, just dig out a small circle. In some cases, I put a paper cup with the bottom torn out around the seed so when a dog/cat/kid kicked the woodchips back in the hole the little seedling was protected.
4.) Add some nitrogen fertilizer. Some people say there is no problem with the woodchips stealing the nitrogen, but I'm not so sure - at least in the very top layer of soil. After the first year, this wasn't necessary though.
5.) Areas where I added Garden Giant mushroom spawn, the woodchips decomposed literally twice as fast - and I got mushrooms!
Second year - it's easier so basically the same as above but mixing in some smaller annuals.
Third year - I was raking back the top layer of woodchips and planing lettuce, kale, cilantro and anything else I wanted.
Fourth year - I'll be adding a 2 inch layer of winter woodchips this year. Now I don't want them to break down - because I want them to suppress weeds and weeds are growing in the compost that was once woodchips. I'm guessing a top layer of very little leaves - just the wood part of the chips will take care of this.
As far as the thistle - it keeps coming back because of it's strong root system, often with horizontal runners connecting multiple plants. From what I can see, if it's already in the ground it loves the woodchips. We let ours grow to 2-3 inches high and pulled it. After about 3-4 pulls, it quit and hasn't come back since.