Here’s a recent email I got from Bob regrading this…
The spray I told you about is KDL, short for potassium (K) dextrose-lactose. You should order it directly from Agro-K Corp. in Minneapolis. Their toll-free number is 1-800-328-2418, and they are one hour ahead of the Mountain time zone. You can tell them that I recommended you to them. The cost of a one-gallon jug should still be $22 although you will have to pay for shipping as well. The rate I use is 3 oz KDL + 3 oz. household vinegar/gallon of water. You should only spray the tank mix lightly on your apricot trees; do not spray them to runoff as you might with other products. As I mentioned, at full bloom this should give you protection for 7-10 days if the weather is mild or 10-14 days if the temperatures are cool (daytime highs under 50F). Based on my experience here, the rate I quoted above should give 7-8 Fahrenheit degrees of protection at full bloom, or 3-4 Fahrenheit degrees of protection when the fruitlets are in the shuck. Once the fruitlets emerge from the shucks, under-tree sprinkling is the only thing that will protect them.
At red calyx stage (which is before the “popcorn” stage), the 10% and 90% mortality temperatures for 1/2 hour are 22F and 9F respectively; at full bloom, 27F and 22F respectively; in the shuck, 28 and 24F respectively; and once the fruitlets are out in the open, 28F and 25F. This is based on research at Washington State University back in the 1980s with commercial apricot cultivars. My experience in Selah and here shows that Puget Gold, Tomcot, Jerseycot, and the Harrow apricots except Harcot are above average in their frost tolerance relative to commercial cultivars such as Goldrich, Rival, Perfection, and maybe Robada.
I do not think it would be a good idea for you to have more than one of the cold-hardy apricots (Westcot, Morden 604, and Debbie’s Gold) as a pollenizer for Apache. A better focus for your selection of apricots would be those that are slow to respond to accumulation of heat units during the winter and early spring. Those specifically are Zard, Hoyt Montrose, Corbett, Alfred, Sugar Pearls, Montrose, and Jimmy Thomas (the apricot whose mother tree is 1 mile west of here). Their being later-blooming apricots will help somewhat in getting them through frosts. There is a good match between bloom time of Sugar Pearls and that of Harlayne and Jerseycot. Orange Red is average in its bloom time and response to fluctuating winter temperatures.
Be advised that Goldrich, Rival, Perfection, Apache, Orange Red, Harlayne, and Sugar Pearls require cross-pollination. Tomcot and Robada are partially self-fruitful. All the other apricots on my list are alluded to above are self-fruitful."