edit 07.10.20: the picture on the left shows the fibres after they have been though the break and hackle, not right after the retting.
Water retting (white)
I harvested the flax on time and filled a basin with rainwater. I immerged the flax fully and left it in there for approximately 14 days, rinsed it and dried it. The flax can quickly overret, especially if outside temperature rises and the basin water gets hot. This is also what happened for my flax here: The fibres were very brittle and rather short, which means the microorganisms have already begun breaking down the fibres. Lesson: Take it out of the basin sooner!
Attention: water retting got forbidden in the past because the waste water killed marine life.
Dew retting (creme)
We harvested on time and placed the plants on fresh cut grass. We left them there for 4 weeks and turned the plants three times so they would ret equally. In a week, where we had always 30+°c and no rain, I would pour some water over the plants to accelerate the retting (because I was running short on time). In general the retting result was okay. The fibres were still strong yet from time to time a few shives were still stuck to the fibres.
Natural retting (grey)
I just basically didn’t harvest this field until all stems would have turned grey. They basically retted by themselves just by standing there 3 months longer than the regular harvest.