For the "M" , (or whatever) designation, that means ....'for it's class'. So, an SDB that's "M",would bloom in the middle area of all the SDB's. It IS a bit confusing , or can be! MDB's bloom 1st, then SDB's, followed by the IB's, MTB's usually open with the later IB's , and the BB's, and TB's bloom together. That said, in certain climates / conditions The "blooming order" of the classes *may* not always "follow suit", because some classes don't do as well in some areas.
And those bloom-time designations were put there by the hybridizer, according to what he / she saw in THEIR garden....which may...or may nor...reflect what happens in YOUR garden. All -in -all, it's just a "guide," and not an exact "date".
