For alpha 19 I would regularly plant my farm on the top level of my base, complete with wood bar walls and a high wood bar roof. I have tried the same for alpha 20 and have found that although I can plant the seeds, many will not grow. There does not appear to be any obvious reason to me, why some will grow in certain positions while others won't. They are all walled in with bars and all have bars across the top of them. Is this a bug or have TFP done this to make such gardens unviable? The greenhouse was aesthetically very pleasing in alpha 19, so I'm hoping it is a bug that they will fix....