Actually, when I owned the blueberry farm (a brief 5 year foray into hard work) the Ag agents used "Growing Degree Days" as the main descriptor of where we were in the growing season. It measured the accumulation a time at or above a set temperature. For example blueberries would start blooming at some number days base 50F. Meaning significant growth only happened above 50F, and the AgDept weather stations would report the progress for that location at the different base temperatures. One could then track the total accumulated time above a chosen set temperature at a given location. Base 40 F, base 50 F, base 60 F, were the charts made available in Michigan. They'd be updated daily from early spring through to hard freeze in November. Grape growers used base 60 F to time fruit ripening.
Apple and most fruit varieties in wholesale catalogs will have degree day listings for flowering and fruit ripening or harvest if they are serious about catering to real farmers and commercial orchards.
These degree days were also very useful in predicting when pests would emerge and timing reapplication intervals for pesticide spraying. The goal was to spray to kill adults, then reapply timed to kill juveniles that hatched from eggs missed by the initial spray. The goal is to spray just before the next generation is able to lay eggs. This can be once every two or three weeks in cool weather or once every three days in hot weather. The degree day tables have been developed for economic pests like the apple coddling moth, the spotted wing fruit fly (SWD) and the blueberry maggot. They are each at different temperature-day accumulation rates for predicting best timing of spray application.
So farming has become sophisticated around pest control, and measuring impact of climate. USDA cold hardiness zones are only part of the picture.
There is not enough money in bonsai for a USDA specialty group, though the landscape nursery industry is pretty closely related to what we do as far as bulking trees up with field growing.
