I'm pretty sure flowering groups refer to groups of varieties or species that bloom at the same time. In a broad sense there are the early bloomers, which include most deciduous fruit
trees, which bloom early
enough to be vulnerable to late freezes and so ameliorating this danger can be part of the design of where to place them; and the later bloomers, mostly fruits originating in warmer climates which bloom at leafing out time or later (mulberry, persimmon, most nut trees are examples), which are thus less sensitive to late frosts. On a finer scale, varieties within a species also bloom at somewhat different times, so much so that the earliest varieties of
apple to bloom, for instance, won't be good pollinizers for the latest varieties to bloom.....something to take into account when selecting varieties if cross-pollination is desirable. If you have limited space, for instance for only two trees, you need to be sure they are compatible in every way. It took me a long time to thus determine, and then find sources for, the two chestnuts I ended up planting for this very reason.
