(a) Gisela 6 rootstock (b) Colt rootstock ( c) Sweetheart on Colt in the foreground, Sweetheart on Gisela 6 in the background (d) photo © a week later.
See reference here:
I didn’t know that the rootstock could influence chill hours! Just thought it might have an effect on precocity. According to this reference from Australia, of the dwarfing rootstock choices — Colt is not so great.
Also they comment on using evaporative cooling to try to counter the chill negating effect of +60 degree days. Basically they run overhead misters. This seems easy enough to try for the backyard…
Interesting, there is also a bit in there that suggests mineral oil is a dormancy-breaker. So I wonder if that is problem for us cold-climate folks applying dormant oil at dormant stage?
Probably not, provided you haven’t hit enough chill portions for your cultivar to break rest. In other words only apply your hort. oils in early dormancy.
Here’s a few data points from my experience. Almost 10 years ago I planted Minnie Royal & Royal Lee – both on Colt rootstock in the western Rancho Penasquitos area of San Diego, USDA 9b/10a (near Black Mtn Middle School). The growth habit of the trees was semi-dwarf, as you might expect a pit fruit on Citation in our climate; i.e., semi-vigorous. Two years after planting I harvested 7 gallons of cherries from the combined trees and thereafter 10-15 gallons – leaving many more gallons for the birds because we couldn’t keep up with the production of the trees.
Further, I never sprayed with horticultural oil, but rather Liqui-Cop (copper ammonium) for winter spray and occasionally with Evergreen EC to control pests during the summer.
… And yet you just gave both trees the axe in your current house…
For a completely unrelated reason: they’re being replaced by a single 6GM25 – freeing up space for an additional fruit/nut tree.